type_err_ctxt_ext.rs

// ignore-tidy-filelength :(

use super::on_unimplemented::{AppendConstMessage, OnUnimplementedNote, TypeErrCtxtExt as _};
use super::suggestions::{get_explanation_based_on_obligation, TypeErrCtxtExt as _};
use crate::errors::{
    AsyncClosureNotFn, ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch,
};
use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use crate::infer::InferCtxtExt as _;
use crate::infer::{self, InferCtxt};
use crate::traits::error_reporting::infer_ctxt_ext::InferCtxtExt;
use crate::traits::error_reporting::{ambiguity, ambiguity::Ambiguity::*};
use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
use crate::traits::specialize::to_pretty_impl_header;
use crate::traits::NormalizeExt;
use crate::traits::{
    elaborate, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation,
    ObligationCause, ObligationCauseCode, ObligationCtxt, Overflow, PredicateObligation,
    SelectionError, SignatureMismatch, TraitNotObjectSafe,
};
use core::ops::ControlFlow;
use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
use rustc_errors::codes::*;
use rustc_errors::{pluralize, struct_span_code_err, Applicability, MultiSpan, StringPart};
use rustc_errors::{Diag, EmissionGuarantee, ErrorGuaranteed, FatalError, StashKey};
use rustc_hir as hir;
use rustc_hir::def::{DefKind, Namespace, Res};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::intravisit::Visitor;
use rustc_hir::{GenericParam, Item, Node};
use rustc_infer::infer::error_reporting::TypeErrCtxt;
use rustc_infer::infer::{InferOk, TypeTrace};
use rustc_middle::traits::select::OverflowError;
use rustc_middle::traits::SignatureMismatchData;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::error::{ExpectedFound, TypeError};
use rustc_middle::ty::fold::{BottomUpFolder, TypeFolder, TypeSuperFoldable};
use rustc_middle::ty::print::{with_forced_trimmed_paths, FmtPrinter, Print};
use rustc_middle::ty::{
    self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
    TypeVisitable, TypeVisitableExt,
};
use rustc_session::config::DumpSolverProofTree;
use rustc_session::Limit;
use rustc_span::def_id::LOCAL_CRATE;
use rustc_span::symbol::sym;
use rustc_span::{BytePos, ExpnKind, Span, Symbol, DUMMY_SP};
use std::borrow::Cow;
use std::fmt;
use std::iter;

use super::{
    dump_proof_tree, ArgKind, CandidateSimilarity, FindExprBySpan, FindTypeParam,
    GetSafeTransmuteErrorAndReason, HasNumericInferVisitor, ImplCandidate, UnsatisfiedConst,
};

pub use rustc_infer::traits::error_reporting::*;

pub enum OverflowCause<'tcx> {
    DeeplyNormalize(ty::AliasTy<'tcx>),
    TraitSolver(ty::Predicate<'tcx>),
}

pub fn suggest_new_overflow_limit<'tcx, G: EmissionGuarantee>(
    tcx: TyCtxt<'tcx>,
    err: &mut Diag<'_, G>,
) {
    let suggested_limit = match tcx.recursion_limit() {
        Limit(0) => Limit(2),
        limit => limit * 2,
    };
    err.help(format!(
        "consider increasing the recursion limit by adding a \
         `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
        suggested_limit,
        tcx.crate_name(LOCAL_CRATE),
    ));
}

#[extension(pub trait TypeErrCtxtExt<'tcx>)]
impl<'tcx> TypeErrCtxt<'_, 'tcx> {
    fn report_fulfillment_errors(
        &self,
        mut errors: Vec<FulfillmentError<'tcx>>,
    ) -> ErrorGuaranteed {
        self.sub_relations
            .borrow_mut()
            .add_constraints(self, errors.iter().map(|e| e.obligation.predicate));

        #[derive(Debug)]
        struct ErrorDescriptor<'tcx> {
            predicate: ty::Predicate<'tcx>,
            index: Option<usize>, // None if this is an old error
        }

        let mut error_map: FxIndexMap<_, Vec<_>> = self
            .reported_trait_errors
            .borrow()
            .iter()
            .map(|(&span, predicates)| {
                (
                    span,
                    predicates
                        .0
                        .iter()
                        .map(|&predicate| ErrorDescriptor { predicate, index: None })
                        .collect(),
                )
            })
            .collect();

        // Ensure `T: Sized` and `T: WF` obligations come last. This lets us display diagnostics
        // with more relevant type information and hide redundant E0282 errors.
        errors.sort_by_key(|e| match e.obligation.predicate.kind().skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred))
                if Some(pred.def_id()) == self.tcx.lang_items().sized_trait() =>
            {
                1
            }
            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => 3,
            ty::PredicateKind::Coerce(_) => 2,
            _ => 0,
        });

        for (index, error) in errors.iter().enumerate() {
            // We want to ignore desugarings here: spans are equivalent even
            // if one is the result of a desugaring and the other is not.
            let mut span = error.obligation.cause.span;
            let expn_data = span.ctxt().outer_expn_data();
            if let ExpnKind::Desugaring(_) = expn_data.kind {
                span = expn_data.call_site;
            }

            error_map.entry(span).or_default().push(ErrorDescriptor {
                predicate: error.obligation.predicate,
                index: Some(index),
            });
        }

        // We do this in 2 passes because we want to display errors in order, though
        // maybe it *is* better to sort errors by span or something.
        let mut is_suppressed = vec![false; errors.len()];
        for (_, error_set) in error_map.iter() {
            // We want to suppress "duplicate" errors with the same span.
            for error in error_set {
                if let Some(index) = error.index {
                    // Suppress errors that are either:
                    // 1) strictly implied by another error.
                    // 2) implied by an error with a smaller index.
                    for error2 in error_set {
                        if error2.index.is_some_and(|index2| is_suppressed[index2]) {
                            // Avoid errors being suppressed by already-suppressed
                            // errors, to prevent all errors from being suppressed
                            // at once.
                            continue;
                        }

                        if self.error_implies(error2.predicate, error.predicate)
                            && !(error2.index >= error.index
                                && self.error_implies(error.predicate, error2.predicate))
                        {
                            info!("skipping {:?} (implied by {:?})", error, error2);
                            is_suppressed[index] = true;
                            break;
                        }
                    }
                }
            }
        }

        let mut reported = None;

        for from_expansion in [false, true] {
            for (error, suppressed) in iter::zip(&errors, &is_suppressed) {
                if !suppressed && error.obligation.cause.span.from_expansion() == from_expansion {
                    let guar = self.report_fulfillment_error(error);
                    reported = Some(guar);
                    // We want to ignore desugarings here: spans are equivalent even
                    // if one is the result of a desugaring and the other is not.
                    let mut span = error.obligation.cause.span;
                    let expn_data = span.ctxt().outer_expn_data();
                    if let ExpnKind::Desugaring(_) = expn_data.kind {
                        span = expn_data.call_site;
                    }
                    self.reported_trait_errors
                        .borrow_mut()
                        .entry(span)
                        .or_insert_with(|| (vec![], guar))
                        .0
                        .push(error.obligation.predicate);
                }
            }
        }

        // It could be that we don't report an error because we have seen an `ErrorReported` from
        // another source. We should probably be able to fix most of these, but some are delayed
        // bugs that get a proper error after this function.
        reported.unwrap_or_else(|| self.dcx().delayed_bug("failed to report fulfillment errors"))
    }

    /// Reports that an overflow has occurred and halts compilation. We
    /// halt compilation unconditionally because it is important that
    /// overflows never be masked -- they basically represent computations
    /// whose result could not be truly determined and thus we can't say
    /// if the program type checks or not -- and they are unusual
    /// occurrences in any case.
    fn report_overflow_error(
        &self,
        cause: OverflowCause<'tcx>,
        span: Span,
        suggest_increasing_limit: bool,
        mutate: impl FnOnce(&mut Diag<'_>),
    ) -> ! {
        let mut err = self.build_overflow_error(cause, span, suggest_increasing_limit);
        mutate(&mut err);
        err.emit();
        FatalError.raise();
    }

    fn build_overflow_error(
        &self,
        cause: OverflowCause<'tcx>,
        span: Span,
        suggest_increasing_limit: bool,
    ) -> Diag<'tcx> {
        fn with_short_path<'tcx, T>(tcx: TyCtxt<'tcx>, value: T) -> String
        where
            T: fmt::Display + Print<'tcx, FmtPrinter<'tcx, 'tcx>>,
        {
            let s = value.to_string();
            if s.len() > 50 {
                // We don't need to save the type to a file, we will be talking about this type already
                // in a separate note when we explain the obligation, so it will be available that way.
                let mut cx: FmtPrinter<'_, '_> =
                    FmtPrinter::new_with_limit(tcx, Namespace::TypeNS, rustc_session::Limit(6));
                value.print(&mut cx).unwrap();
                cx.into_buffer()
            } else {
                s
            }
        }

        let mut err = match cause {
            OverflowCause::DeeplyNormalize(alias_ty) => {
                let alias_ty = self.resolve_vars_if_possible(alias_ty);
                let kind = alias_ty.opt_kind(self.tcx).map_or("alias", |k| k.descr());
                let alias_str = with_short_path(self.tcx, alias_ty);
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0275,
                    "overflow normalizing the {kind} `{alias_str}`",
                )
            }
            OverflowCause::TraitSolver(predicate) => {
                let predicate = self.resolve_vars_if_possible(predicate);
                match predicate.kind().skip_binder() {
                    ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ })
                    | ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
                        struct_span_code_err!(
                            self.dcx(),
                            span,
                            E0275,
                            "overflow assigning `{a}` to `{b}`",
                        )
                    }
                    _ => {
                        let pred_str = with_short_path(self.tcx, predicate);
                        struct_span_code_err!(
                            self.dcx(),
                            span,
                            E0275,
                            "overflow evaluating the requirement `{pred_str}`",
                        )
                    }
                }
            }
        };

        if suggest_increasing_limit {
            suggest_new_overflow_limit(self.tcx, &mut err);
        }

        err
    }

    /// Reports that an overflow has occurred and halts compilation. We
    /// halt compilation unconditionally because it is important that
    /// overflows never be masked -- they basically represent computations
    /// whose result could not be truly determined and thus we can't say
    /// if the program type checks or not -- and they are unusual
    /// occurrences in any case.
    fn report_overflow_obligation<T>(
        &self,
        obligation: &Obligation<'tcx, T>,
        suggest_increasing_limit: bool,
    ) -> !
    where
        T: ToPredicate<'tcx> + Clone,
    {
        let predicate = obligation.predicate.clone().to_predicate(self.tcx);
        let predicate = self.resolve_vars_if_possible(predicate);
        self.report_overflow_error(
            OverflowCause::TraitSolver(predicate),
            obligation.cause.span,
            suggest_increasing_limit,
            |err| {
                self.note_obligation_cause_code(
                    obligation.cause.body_id,
                    err,
                    predicate,
                    obligation.param_env,
                    obligation.cause.code(),
                    &mut vec![],
                    &mut Default::default(),
                );
            },
        );
    }

    /// Reports that a cycle was detected which led to overflow and halts
    /// compilation. This is equivalent to `report_overflow_obligation` except
    /// that we can give a more helpful error message (and, in particular,
    /// we do not suggest increasing the overflow limit, which is not
    /// going to help).
    fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
        let cycle = self.resolve_vars_if_possible(cycle.to_owned());
        assert!(!cycle.is_empty());

        debug!(?cycle, "report_overflow_error_cycle");

        // The 'deepest' obligation is most likely to have a useful
        // cause 'backtrace'
        self.report_overflow_obligation(
            cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(),
            false,
        );
    }

    fn report_overflow_no_abort(
        &self,
        obligation: PredicateObligation<'tcx>,
        suggest_increasing_limit: bool,
    ) -> ErrorGuaranteed {
        let obligation = self.resolve_vars_if_possible(obligation);
        let mut err = self.build_overflow_error(
            OverflowCause::TraitSolver(obligation.predicate),
            obligation.cause.span,
            suggest_increasing_limit,
        );
        self.note_obligation_cause(&mut err, &obligation);
        self.point_at_returns_when_relevant(&mut err, &obligation);
        err.emit()
    }

    /// The `root_obligation` parameter should be the `root_obligation` field
    /// from a `FulfillmentError`. If no `FulfillmentError` is available,
    /// then it should be the same as `obligation`.
    fn report_selection_error(
        &self,
        mut obligation: PredicateObligation<'tcx>,
        root_obligation: &PredicateObligation<'tcx>,
        error: &SelectionError<'tcx>,
    ) -> ErrorGuaranteed {
        let tcx = self.tcx;

        if tcx.sess.opts.unstable_opts.next_solver.map(|c| c.dump_tree).unwrap_or_default()
            == DumpSolverProofTree::OnError
        {
            dump_proof_tree(root_obligation, self.infcx);
        }

        let mut span = obligation.cause.span;

        let mut err = match *error {
            SelectionError::Unimplemented => {
                // If this obligation was generated as a result of well-formedness checking, see if we
                // can get a better error message by performing HIR-based well-formedness checking.
                if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
                    root_obligation.cause.code().peel_derives()
                    && !obligation.predicate.has_non_region_infer()
                {
                    if let Some(cause) = self
                        .tcx
                        .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
                    {
                        obligation.cause = cause.clone();
                        span = obligation.cause.span;
                    }
                }

                if let ObligationCauseCode::CompareImplItemObligation {
                    impl_item_def_id,
                    trait_item_def_id,
                    kind: _,
                } = *obligation.cause.code()
                {
                    debug!("ObligationCauseCode::CompareImplItemObligation");
                    return self.report_extra_impl_obligation(
                        span,
                        impl_item_def_id,
                        trait_item_def_id,
                        &format!("`{}`", obligation.predicate),
                    )
                    .emit()
                }

                // Report a const-param specific error
                if let ObligationCauseCode::ConstParam(ty) = *obligation.cause.code().peel_derives()
                {
                    return self.report_const_param_not_wf(ty, &obligation).emit();
                }

                let bound_predicate = obligation.predicate.kind();
                match bound_predicate.skip_binder() {
                    ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
                        let trait_predicate = bound_predicate.rebind(trait_predicate);
                        let trait_predicate = self.resolve_vars_if_possible(trait_predicate);

                        // Let's use the root obligation as the main message, when we care about the
                        // most general case ("X doesn't implement Pattern<'_>") over the case that
                        // happened to fail ("char doesn't implement Fn(&mut char)").
                        //
                        // We rely on a few heuristics to identify cases where this root
                        // obligation is more important than the leaf obligation:
                        let (main_trait_predicate, o) = if let ty::PredicateKind::Clause(
                            ty::ClauseKind::Trait(root_pred)
                        ) = root_obligation.predicate.kind().skip_binder()
                            && !trait_predicate.self_ty().skip_binder().has_escaping_bound_vars()
                            && !root_pred.self_ty().has_escaping_bound_vars()
                            // The type of the leaf predicate is (roughly) the same as the type
                            // from the root predicate, as a proxy for "we care about the root"
                            // FIXME: this doesn't account for trivial derefs, but works as a first
                            // approximation.
                            && (
                                // `T: Trait` && `&&T: OtherTrait`, we want `OtherTrait`
                                self.can_eq(
                                    obligation.param_env,
                                    trait_predicate.self_ty().skip_binder(),
                                    root_pred.self_ty().peel_refs(),
                                )
                                // `&str: Iterator` && `&str: IntoIterator`, we want `IntoIterator`
                                || self.can_eq(
                                    obligation.param_env,
                                    trait_predicate.self_ty().skip_binder(),
                                    root_pred.self_ty(),
                                )
                            )
                            // The leaf trait and the root trait are different, so as to avoid
                            // talking about `&mut T: Trait` and instead remain talking about
                            // `T: Trait` instead
                            && trait_predicate.def_id() != root_pred.def_id()
                            // The root trait is not `Unsize`, as to avoid talking about it in
                            // `tests/ui/coercion/coerce-issue-49593-box-never.rs`.
                            && Some(root_pred.def_id()) != self.tcx.lang_items().unsize_trait()
                        {
                            (
                                self.resolve_vars_if_possible(
                                    root_obligation.predicate.kind().rebind(root_pred),
                                ),
                                root_obligation,
                            )
                        } else {
                            (trait_predicate, &obligation)
                        };
                        let trait_ref = main_trait_predicate.to_poly_trait_ref();

                        if let Some(guar) = self.emit_specialized_closure_kind_error(
                            &obligation,
                            trait_ref,
                        ) {
                            return guar;
                        }

                        // FIXME(effects)
                        let predicate_is_const = false;

                        if let Err(guar) = trait_predicate.error_reported()
                        {
                            return guar;
                        }
                        // Silence redundant errors on binding acccess that are already
                        // reported on the binding definition (#56607).
                        if let Err(guar) = self.fn_arg_obligation(&obligation) {
                            return guar;
                        }
                        let mut file = None;
                        let (post_message, pre_message, type_def) = self
                            .get_parent_trait_ref(obligation.cause.code())
                            .map(|(t, s)| {
                                let t = self.tcx.short_ty_string(t, &mut file);
                                (
                                    format!(" in `{t}`"),
                                    format!("within `{t}`, "),
                                    s.map(|s| (format!("within this `{t}`"), s)),
                                )
                            })
                            .unwrap_or_default();
                        let file_note = file.as_ref().map(|file| format!(
                            "the full trait has been written to '{}'",
                            file.display(),
                        ));

                        let mut long_ty_file = None;

                        let OnUnimplementedNote {
                            message,
                            label,
                            notes,
                            parent_label,
                            append_const_msg,
                        } = self.on_unimplemented_note(trait_ref, o, &mut long_ty_file);
                        let have_alt_message = message.is_some() || label.is_some();
                        let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
                        let is_unsize =
                            Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
                        let (message, notes, append_const_msg) = if is_try_conversion {
                            (
                                Some(format!(
                                    "`?` couldn't convert the error to `{}`",
                                    trait_ref.skip_binder().self_ty(),
                                )),
                                vec![
                                    "the question mark operation (`?`) implicitly performs a \
                                     conversion on the error value using the `From` trait"
                                        .to_owned(),
                                ],
                                Some(AppendConstMessage::Default),
                            )
                        } else {
                            (message, notes, append_const_msg)
                        };

                        let err_msg = self.get_standard_error_message(
                            &main_trait_predicate,
                            message,
                            predicate_is_const,
                            append_const_msg,
                            post_message,
                        );

                        let (err_msg, safe_transmute_explanation) = if Some(trait_ref.def_id())
                            == self.tcx.lang_items().transmute_trait()
                        {
                            // Recompute the safe transmute reason and use that for the error reporting
                            match self.get_safe_transmute_error_and_reason(
                                obligation.clone(),
                                trait_ref,
                                span,
                            ) {
                                GetSafeTransmuteErrorAndReason::Silent => {
                                    return self.dcx().span_delayed_bug(
                                        span, "silent safe transmute error"
                                    );
                                }
                                GetSafeTransmuteErrorAndReason::Error {
                                    err_msg,
                                    safe_transmute_explanation,
                                } => (err_msg, Some(safe_transmute_explanation)),
                            }
                        } else {
                            (err_msg, None)
                        };

                        let mut err = struct_span_code_err!(self.dcx(), span, E0277, "{}", err_msg);

                        if let Some(long_ty_file) = long_ty_file {
                            err.note(format!(
                                "the full name for the type has been written to '{}'",
                                long_ty_file.display(),
                            ));
                            err.note("consider using `--verbose` to print the full type name to the console");
                        }
                        let mut suggested = false;
                        if is_try_conversion {
                            suggested = self.try_conversion_context(&obligation, trait_ref.skip_binder(), &mut err);
                        }

                        if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
                            err.span_label(
                                ret_span,
                                format!(
                                    "expected `{}` because of this",
                                    trait_ref.skip_binder().self_ty()
                                ),
                            );
                        }

                        if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
                            self.add_tuple_trait_message(
                                obligation.cause.code().peel_derives(),
                                &mut err,
                            );
                        }

                        if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
                            && predicate_is_const
                        {
                            err.note("`~const Drop` was renamed to `~const Destruct`");
                            err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
                        }

                        let explanation = get_explanation_based_on_obligation(
                            self.tcx,
                            &obligation,
                            trait_ref,
                            &trait_predicate,
                            pre_message,
                        );

                        self.check_for_binding_assigned_block_without_tail_expression(
                            &obligation,
                            &mut err,
                            trait_predicate,
                        );
                        if self.suggest_add_reference_to_arg(
                            &obligation,
                            &mut err,
                            trait_predicate,
                            have_alt_message,
                        ) {
                            self.note_obligation_cause(&mut err, &obligation);
                            return err.emit();
                        }

                        file_note.map(|note| err.note(note));
                        if let Some(s) = label {
                            // If it has a custom `#[rustc_on_unimplemented]`
                            // error message, let's display it as the label!
                            err.span_label(span, s);
                            if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
                                // When the self type is a type param We don't need to "the trait
                                // `std::marker::Sized` is not implemented for `T`" as we will point
                                // at the type param with a label to suggest constraining it.
                                err.help(explanation);
                            }
                        } else if let Some(custom_explanation) = safe_transmute_explanation {
                            err.span_label(span, custom_explanation);
                        } else {
                            err.span_label(span, explanation);
                        }

                        if let ObligationCauseCode::Coercion { source, target } =
                            *obligation.cause.code().peel_derives()
                        {
                            if Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
                                self.suggest_borrowing_for_object_cast(
                                    &mut err,
                                    root_obligation,
                                    source,
                                    target,
                                );
                            }
                        }

                        let UnsatisfiedConst(unsatisfied_const) = self
                            .maybe_add_note_for_unsatisfied_const(
                                &trait_predicate,
                                &mut err,
                                span,
                            );

                        if let Some((msg, span)) = type_def {
                            err.span_label(span, msg);
                        }
                        for note in notes {
                            // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
                            err.note(note);
                        }
                        if let Some(s) = parent_label {
                            let body = obligation.cause.body_id;
                            err.span_label(tcx.def_span(body), s);
                        }

                        self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
                        self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
                        suggested |= self.suggest_dereferences(&obligation, &mut err, trait_predicate);
                        suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
                        let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
                        suggested = if let &[cand] = &impl_candidates[..] {
                            let cand = cand.trait_ref;
                            if let (ty::FnPtr(_), ty::FnDef(..)) =
                                (cand.self_ty().kind(), trait_ref.self_ty().skip_binder().kind())
                            {
                                err.span_suggestion(
                                    span.shrink_to_hi(),
                                    format!(
                                        "the trait `{}` is implemented for fn pointer `{}`, try casting using `as`",
                                        cand.print_trait_sugared(),
                                        cand.self_ty(),
                                    ),
                                    format!(" as {}", cand.self_ty()),
                                    Applicability::MaybeIncorrect,
                                );
                                true
                            } else {
                                false
                            }
                        } else {
                            false
                        } || suggested;
                        suggested |=
                            self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
                        suggested |= self.suggest_semicolon_removal(
                            &obligation,
                            &mut err,
                            span,
                            trait_predicate,
                        );
                        self.note_version_mismatch(&mut err, &trait_ref);
                        self.suggest_remove_await(&obligation, &mut err);
                        self.suggest_derive(&obligation, &mut err, trait_predicate);

                        if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
                            self.suggest_await_before_try(
                                &mut err,
                                &obligation,
                                trait_predicate,
                                span,
                            );
                        }

                        if self.suggest_add_clone_to_arg(&obligation, &mut err, trait_predicate) {
                            return err.emit();
                        }

                        if self.suggest_impl_trait(&mut err, &obligation, trait_predicate) {
                            return err.emit();
                        }

                        if is_unsize {
                            // If the obligation failed due to a missing implementation of the
                            // `Unsize` trait, give a pointer to why that might be the case
                            err.note(
                                "all implementations of `Unsize` are provided \
                                automatically by the compiler, see \
                                <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
                                for more information",
                            );
                        }

                        let is_fn_trait = tcx.is_fn_trait(trait_ref.def_id());
                        let is_target_feature_fn = if let ty::FnDef(def_id, _) =
                            *trait_ref.skip_binder().self_ty().kind()
                        {
                            !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
                        } else {
                            false
                        };
                        if is_fn_trait && is_target_feature_fn {
                            err.note(
                                "`#[target_feature]` functions do not implement the `Fn` traits",
                            );
                        }

                        self.try_to_add_help_message(
                            &obligation,
                            trait_ref,
                            &trait_predicate,
                            &mut err,
                            span,
                            is_fn_trait,
                            suggested,
                            unsatisfied_const,
                        );

                        // Changing mutability doesn't make a difference to whether we have
                        // an `Unsize` impl (Fixes ICE in #71036)
                        if !is_unsize {
                            self.suggest_change_mut(&obligation, &mut err, trait_predicate);
                        }

                        // If this error is due to `!: Trait` not implemented but `(): Trait` is
                        // implemented, and fallback has occurred, then it could be due to a
                        // variable that used to fallback to `()` now falling back to `!`. Issue a
                        // note informing about the change in behaviour.
                        if trait_predicate.skip_binder().self_ty().is_never()
                            && self.fallback_has_occurred
                        {
                            let predicate = trait_predicate.map_bound(|trait_pred| {
                                trait_pred.with_self_ty(self.tcx, Ty::new_unit(self.tcx))
                            });
                            let unit_obligation = obligation.with(tcx, predicate);
                            if self.predicate_may_hold(&unit_obligation) {
                                err.note(
                                    "this error might have been caused by changes to \
                                    Rust's type-inference algorithm (see issue #48950 \
                                    <https://github.com/rust-lang/rust/issues/48950> \
                                    for more information)",
                                );
                                err.help("did you intend to use the type `()` here instead?");
                            }
                        }

                        self.explain_hrtb_projection(&mut err, trait_predicate, obligation.param_env, &obligation.cause);
                        self.suggest_desugaring_async_fn_in_trait(&mut err, trait_ref);

                        // Return early if the trait is Debug or Display and the invocation
                        // originates within a standard library macro, because the output
                        // is otherwise overwhelming and unhelpful (see #85844 for an
                        // example).

                        let in_std_macro =
                            match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
                                Some(macro_def_id) => {
                                    let crate_name = tcx.crate_name(macro_def_id.krate);
                                    crate_name == sym::std || crate_name == sym::core
                                }
                                None => false,
                            };

                        if in_std_macro
                            && matches!(
                                self.tcx.get_diagnostic_name(trait_ref.def_id()),
                                Some(sym::Debug | sym::Display)
                            )
                        {
                            return err.emit();
                        }



                        err
                    }

                    ty::PredicateKind::Subtype(predicate) => {
                        // Errors for Subtype predicates show up as
                        // `FulfillmentErrorCode::SubtypeError`,
                        // not selection error.
                        span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
                    }

                    ty::PredicateKind::Coerce(predicate) => {
                        // Errors for Coerce predicates show up as
                        // `FulfillmentErrorCode::SubtypeError`,
                        // not selection error.
                        span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
                    }

                    ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(..))
                    | ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(..)) => {
                        span_bug!(
                            span,
                            "outlives clauses should not error outside borrowck. obligation: `{:?}`",
                            obligation
                        )
                    }

                    ty::PredicateKind::Clause(ty::ClauseKind::Projection(..)) => {
                        span_bug!(
                            span,
                            "projection clauses should be implied from elsewhere. obligation: `{:?}`",
                            obligation
                        )
                    }

                    ty::PredicateKind::ObjectSafe(trait_def_id) => {
                        let violations = self.tcx.object_safety_violations(trait_def_id);
                        report_object_safety_error(self.tcx, span, None, trait_def_id, violations)
                    }

                    ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(ty)) => {
                        let ty = self.resolve_vars_if_possible(ty);
                        if self.next_trait_solver() {
                            // FIXME: we'll need a better message which takes into account
                            // which bounds actually failed to hold.
                            self.dcx().struct_span_err(
                                span,
                                format!("the type `{ty}` is not well-formed"),
                            )
                        } else {
                            // WF predicates cannot themselves make
                            // errors. They can only block due to
                            // ambiguity; otherwise, they always
                            // degenerate into other obligations
                            // (which may fail).
                            span_bug!(span, "WF predicate not satisfied for {:?}", ty);
                        }
                    }

                    ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..)) => {
                        // Errors for `ConstEvaluatable` predicates show up as
                        // `SelectionError::ConstEvalFailure`,
                        // not `Unimplemented`.
                        span_bug!(
                            span,
                            "const-evaluatable requirement gave wrong error: `{:?}`",
                            obligation
                        )
                    }

                    ty::PredicateKind::ConstEquate(..) => {
                        // Errors for `ConstEquate` predicates show up as
                        // `SelectionError::ConstEvalFailure`,
                        // not `Unimplemented`.
                        span_bug!(
                            span,
                            "const-equate requirement gave wrong error: `{:?}`",
                            obligation
                        )
                    }

                    ty::PredicateKind::Ambiguous => span_bug!(span, "ambiguous"),

                    ty::PredicateKind::NormalizesTo(..) => span_bug!(
                        span,
                        "NormalizesTo predicate should never be the predicate cause of a SelectionError"
                    ),

                    ty::PredicateKind::AliasRelate(..) => span_bug!(
                        span,
                        "AliasRelate predicate should never be the predicate cause of a SelectionError"
                    ),

                    ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
                        let mut diag = self.dcx().struct_span_err(
                            span,
                            format!("the constant `{ct}` is not of type `{ty}`"),
                        );
                        self.note_type_err(
                            &mut diag,
                            &obligation.cause,
                            None,
                            None,
                            TypeError::Sorts(ty::error::ExpectedFound::new(true, ty, ct.ty())),
                            false,
                            false,
                        );
                        diag
                    }
                }
            }

            SignatureMismatch(box SignatureMismatchData {
                found_trait_ref,
                expected_trait_ref,
                terr: terr @ TypeError::CyclicTy(_),
            }) => self.report_cyclic_signature_error(
                &obligation,
                found_trait_ref,
                expected_trait_ref,
                terr,
            ),
            SignatureMismatch(box SignatureMismatchData {
                found_trait_ref,
                expected_trait_ref,
                terr: _,
            }) => {
                match self.report_signature_mismatch_error(
                    &obligation,
                    span,
                    found_trait_ref,
                    expected_trait_ref,
                ) {
                    Ok(err) => err,
                    Err(guar) => return guar,
                }
            }

            SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id) => return self.report_opaque_type_auto_trait_leakage(
                &obligation,
                def_id,
            ),

            TraitNotObjectSafe(did) => {
                let violations = self.tcx.object_safety_violations(did);
                report_object_safety_error(self.tcx, span, None, did, violations)
            }

            SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
                bug!(
                    "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
                )
            }
            SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
                match self.report_not_const_evaluatable_error(&obligation, span) {
                    Ok(err) => err,
                    Err(guar) => return guar,
                }
            }

            // Already reported in the query.
            SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(guar)) |
            // Already reported.
            Overflow(OverflowError::Error(guar)) => return guar,

            Overflow(_) => {
                bug!("overflow should be handled before the `report_selection_error` path");
            }
        };

        self.note_obligation_cause(&mut err, &obligation);
        self.point_at_returns_when_relevant(&mut err, &obligation);
        err.emit()
    }

    fn emit_specialized_closure_kind_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        mut trait_ref: ty::PolyTraitRef<'tcx>,
    ) -> Option<ErrorGuaranteed> {
        // If `AsyncFnKindHelper` is not implemented, that means that the closure kind
        // doesn't extend the goal kind. This is worth reporting, but we can only do so
        // if we actually know which closure this goal comes from, so look at the cause
        // to see if we can extract that information.
        if Some(trait_ref.def_id()) == self.tcx.lang_items().async_fn_kind_helper()
            && let Some(found_kind) = trait_ref.skip_binder().args.type_at(0).to_opt_closure_kind()
            && let Some(expected_kind) =
                trait_ref.skip_binder().args.type_at(1).to_opt_closure_kind()
            && !found_kind.extends(expected_kind)
        {
            if let Some((_, Some(parent))) = obligation.cause.code().parent() {
                // If we have a derived obligation, then the parent will be a `AsyncFn*` goal.
                trait_ref = parent.to_poly_trait_ref();
            } else if let &ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } =
                obligation.cause.code()
                && let Some(typeck_results) = &self.typeck_results
                && let ty::Closure(closure_def_id, _) | ty::CoroutineClosure(closure_def_id, _) =
                    *typeck_results.node_type(arg_hir_id).kind()
            {
                // Otherwise, extract the closure kind from the obligation.
                let mut err = self.report_closure_error(
                    &obligation,
                    closure_def_id,
                    found_kind,
                    expected_kind,
                    "async ",
                );
                self.note_obligation_cause(&mut err, &obligation);
                self.point_at_returns_when_relevant(&mut err, &obligation);
                return Some(err.emit());
            }
        }

        let self_ty = trait_ref.self_ty().skip_binder();

        if let Some(expected_kind) = self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id()) {
            let (closure_def_id, found_args, by_ref_captures) = match *self_ty.kind() {
                ty::Closure(def_id, args) => {
                    (def_id, args.as_closure().sig().map_bound(|sig| sig.inputs()[0]), None)
                }
                ty::CoroutineClosure(def_id, args) => (
                    def_id,
                    args.as_coroutine_closure()
                        .coroutine_closure_sig()
                        .map_bound(|sig| sig.tupled_inputs_ty),
                    Some(args.as_coroutine_closure().coroutine_captures_by_ref_ty()),
                ),
                _ => return None,
            };

            let expected_args = trait_ref.map_bound(|trait_ref| trait_ref.args.type_at(1));

            // Verify that the arguments are compatible. If the signature is
            // mismatched, then we have a totally different error to report.
            if self.enter_forall(found_args, |found_args| {
                self.enter_forall(expected_args, |expected_args| {
                    !self.can_sub(obligation.param_env, expected_args, found_args)
                })
            }) {
                return None;
            }

            if let Some(found_kind) = self.closure_kind(self_ty)
                && !found_kind.extends(expected_kind)
            {
                let mut err = self.report_closure_error(
                    &obligation,
                    closure_def_id,
                    found_kind,
                    expected_kind,
                    "",
                );
                self.note_obligation_cause(&mut err, &obligation);
                self.point_at_returns_when_relevant(&mut err, &obligation);
                return Some(err.emit());
            }

            // If the closure has captures, then perhaps the reason that the trait
            // is unimplemented is because async closures don't implement `Fn`/`FnMut`
            // if they have captures.
            if let Some(by_ref_captures) = by_ref_captures
                && let ty::FnPtr(sig) = by_ref_captures.kind()
                && !sig.skip_binder().output().is_unit()
            {
                let mut err = self.tcx.dcx().create_err(AsyncClosureNotFn {
                    span: self.tcx.def_span(closure_def_id),
                    kind: expected_kind.as_str(),
                });
                self.note_obligation_cause(&mut err, &obligation);
                self.point_at_returns_when_relevant(&mut err, &obligation);
                return Some(err.emit());
            }
        }
        None
    }

    fn fn_arg_obligation(
        &self,
        obligation: &PredicateObligation<'tcx>,
    ) -> Result<(), ErrorGuaranteed> {
        if let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } =
            obligation.cause.code()
            && let Node::Expr(arg) = self.tcx.hir_node(*arg_hir_id)
            && let arg = arg.peel_borrows()
            && let hir::ExprKind::Path(hir::QPath::Resolved(
                None,
                hir::Path { res: hir::def::Res::Local(hir_id), .. },
            )) = arg.kind
            && let Node::Pat(pat) = self.tcx.hir_node(*hir_id)
            && let Some((preds, guar)) = self.reported_trait_errors.borrow().get(&pat.span)
            && preds.contains(&obligation.predicate)
        {
            return Err(*guar);
        }
        Ok(())
    }

    /// When the `E` of the resulting `Result<T, E>` in an expression `foo().bar().baz()?`,
    /// identify thoe method chain sub-expressions that could or could not have been annotated
    /// with `?`.
    fn try_conversion_context(
        &self,
        obligation: &PredicateObligation<'tcx>,
        trait_ref: ty::TraitRef<'tcx>,
        err: &mut Diag<'_>,
    ) -> bool {
        let span = obligation.cause.span;
        struct V {
            search_span: Span,
        }
        impl<'v> Visitor<'v> for V {
            type Result = ControlFlow<&'v hir::Expr<'v>>;
            fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) -> Self::Result {
                if let hir::ExprKind::Match(expr, _arms, hir::MatchSource::TryDesugar(_)) = ex.kind
                    && ex.span.with_lo(ex.span.hi() - BytePos(1)).source_equal(self.search_span)
                    && let hir::ExprKind::Call(_, [expr, ..]) = expr.kind
                {
                    ControlFlow::Break(expr)
                } else {
                    hir::intravisit::walk_expr(self, ex)
                }
            }
        }
        let hir_id = self.tcx.local_def_id_to_hir_id(obligation.cause.body_id);
        let body_id = match self.tcx.hir_node(hir_id) {
            hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. }) => body_id,
            _ => return false,
        };
        let ControlFlow::Break(expr) =
            (V { search_span: span }).visit_body(self.tcx.hir().body(*body_id))
        else {
            return false;
        };
        let Some(typeck) = &self.typeck_results else {
            return false;
        };
        let Some((ObligationCauseCode::QuestionMark, Some(y))) = obligation.cause.code().parent()
        else {
            return false;
        };
        if !self.tcx.is_diagnostic_item(sym::FromResidual, y.def_id()) {
            return false;
        }
        let self_ty = trait_ref.self_ty();
        let found_ty = trait_ref.args.get(1).and_then(|a| a.as_type());

        let mut prev_ty = self.resolve_vars_if_possible(
            typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
        );

        // We always look at the `E` type, because that's the only one affected by `?`. If the
        // incorrect `Result<T, E>` is because of the `T`, we'll get an E0308 on the whole
        // expression, after the `?` has "unwrapped" the `T`.
        let get_e_type = |prev_ty: Ty<'tcx>| -> Option<Ty<'tcx>> {
            let ty::Adt(def, args) = prev_ty.kind() else {
                return None;
            };
            let Some(arg) = args.get(1) else {
                return None;
            };
            if !self.tcx.is_diagnostic_item(sym::Result, def.did()) {
                return None;
            }
            arg.as_type()
        };

        let mut suggested = false;
        let mut chain = vec![];

        // The following logic is simlar to `point_at_chain`, but that's focused on associated types
        let mut expr = expr;
        while let hir::ExprKind::MethodCall(path_segment, rcvr_expr, args, span) = expr.kind {
            // Point at every method call in the chain with the `Result` type.
            // let foo = bar.iter().map(mapper)?;
            //               ------ -----------
            expr = rcvr_expr;
            chain.push((span, prev_ty));

            let next_ty = self.resolve_vars_if_possible(
                typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
            );

            let is_diagnostic_item = |symbol: Symbol, ty: Ty<'tcx>| {
                let ty::Adt(def, _) = ty.kind() else {
                    return false;
                };
                self.tcx.is_diagnostic_item(symbol, def.did())
            };
            // For each method in the chain, see if this is `Result::map_err` or
            // `Option::ok_or_else` and if it is, see if the closure passed to it has an incorrect
            // trailing `;`.
            if let Some(ty) = get_e_type(prev_ty)
                && let Some(found_ty) = found_ty
                // Ideally we would instead use `FnCtxt::lookup_method_for_diagnostic` for 100%
                // accurate check, but we are in the wrong stage to do that and looking for
                // `Result::map_err` by checking the Self type and the path segment is enough.
                // sym::ok_or_else
                && (
                    ( // Result::map_err
                        path_segment.ident.name == sym::map_err
                            && is_diagnostic_item(sym::Result, next_ty)
                    ) || ( // Option::ok_or_else
                        path_segment.ident.name == sym::ok_or_else
                            && is_diagnostic_item(sym::Option, next_ty)
                    )
                )
                // Found `Result<_, ()>?`
                && let ty::Tuple(tys) = found_ty.kind()
                && tys.is_empty()
                // The current method call returns `Result<_, ()>`
                && self.can_eq(obligation.param_env, ty, found_ty)
                // There's a single argument in the method call and it is a closure
                && args.len() == 1
                && let Some(arg) = args.get(0)
                && let hir::ExprKind::Closure(closure) = arg.kind
                // The closure has a block for its body with no tail expression
                && let body = self.tcx.hir().body(closure.body)
                && let hir::ExprKind::Block(block, _) = body.value.kind
                && let None = block.expr
                // The last statement is of a type that can be converted to the return error type
                && let [.., stmt] = block.stmts
                && let hir::StmtKind::Semi(expr) = stmt.kind
                && let expr_ty = self.resolve_vars_if_possible(
                    typeck.expr_ty_adjusted_opt(expr)
                        .unwrap_or(Ty::new_misc_error(self.tcx)),
                )
                && self
                    .infcx
                    .type_implements_trait(
                        self.tcx.get_diagnostic_item(sym::From).unwrap(),
                        [self_ty, expr_ty],
                        obligation.param_env,
                    )
                    .must_apply_modulo_regions()
            {
                suggested = true;
                err.span_suggestion_short(
                    stmt.span.with_lo(expr.span.hi()),
                    "remove this semicolon",
                    String::new(),
                    Applicability::MachineApplicable,
                );
            }

            prev_ty = next_ty;

            if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
                && let hir::Path { res: hir::def::Res::Local(hir_id), .. } = path
                && let hir::Node::Pat(binding) = self.tcx.hir_node(*hir_id)
            {
                let parent = self.tcx.parent_hir_node(binding.hir_id);
                // We've reached the root of the method call chain...
                if let hir::Node::Local(local) = parent
                    && let Some(binding_expr) = local.init
                {
                    // ...and it is a binding. Get the binding creation and continue the chain.
                    expr = binding_expr;
                }
                if let hir::Node::Param(_param) = parent {
                    // ...and it is a an fn argument.
                    break;
                }
            }
        }
        // `expr` is now the "root" expression of the method call chain, which can be any
        // expression kind, like a method call or a path. If this expression is `Result<T, E>` as
        // well, then we also point at it.
        prev_ty = self.resolve_vars_if_possible(
            typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
        );
        chain.push((expr.span, prev_ty));

        let mut prev = None;
        for (span, err_ty) in chain.into_iter().rev() {
            let err_ty = get_e_type(err_ty);
            let err_ty = match (err_ty, prev) {
                (Some(err_ty), Some(prev)) if !self.can_eq(obligation.param_env, err_ty, prev) => {
                    err_ty
                }
                (Some(err_ty), None) => err_ty,
                _ => {
                    prev = err_ty;
                    continue;
                }
            };
            if self
                .infcx
                .type_implements_trait(
                    self.tcx.get_diagnostic_item(sym::From).unwrap(),
                    [self_ty, err_ty],
                    obligation.param_env,
                )
                .must_apply_modulo_regions()
            {
                if !suggested {
                    err.span_label(span, format!("this has type `Result<_, {err_ty}>`"));
                }
            } else {
                err.span_label(
                    span,
                    format!(
                        "this can't be annotated with `?` because it has type `Result<_, {err_ty}>`",
                    ),
                );
            }
            prev = Some(err_ty);
        }
        suggested
    }

    fn report_const_param_not_wf(
        &self,
        ty: Ty<'tcx>,
        obligation: &PredicateObligation<'tcx>,
    ) -> Diag<'tcx> {
        let span = obligation.cause.span;

        let mut diag = match ty.kind() {
            _ if ty.has_param() => {
                span_bug!(span, "const param tys cannot mention other generic parameters");
            }
            ty::Float(_) => {
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0741,
                    "`{ty}` is forbidden as the type of a const generic parameter",
                )
            }
            ty::FnPtr(_) => {
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0741,
                    "using function pointers as const generic parameters is forbidden",
                )
            }
            ty::RawPtr(_) => {
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0741,
                    "using raw pointers as const generic parameters is forbidden",
                )
            }
            ty::Adt(def, _) => {
                // We should probably see if we're *allowed* to derive `ConstParamTy` on the type...
                let mut diag = struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0741,
                    "`{ty}` must implement `ConstParamTy` to be used as the type of a const generic parameter",
                );
                // Only suggest derive if this isn't a derived obligation,
                // and the struct is local.
                if let Some(span) = self.tcx.hir().span_if_local(def.did())
                    && obligation.cause.code().parent().is_none()
                {
                    if ty.is_structural_eq_shallow(self.tcx) {
                        diag.span_suggestion(
                            span,
                            "add `#[derive(ConstParamTy)]` to the struct",
                            "#[derive(ConstParamTy)]\n",
                            Applicability::MachineApplicable,
                        );
                    } else {
                        // FIXME(adt_const_params): We should check there's not already an
                        // overlapping `Eq`/`PartialEq` impl.
                        diag.span_suggestion(
                            span,
                            "add `#[derive(ConstParamTy, PartialEq, Eq)]` to the struct",
                            "#[derive(ConstParamTy, PartialEq, Eq)]\n",
                            Applicability::MachineApplicable,
                        );
                    }
                }
                diag
            }
            _ => {
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0741,
                    "`{ty}` can't be used as a const parameter type",
                )
            }
        };

        let mut code = obligation.cause.code();
        let mut pred = obligation.predicate.to_opt_poly_trait_pred();
        while let Some((next_code, next_pred)) = code.parent() {
            if let Some(pred) = pred {
                self.enter_forall(pred, |pred| {
                    diag.note(format!(
                        "`{}` must implement `{}`, but it does not",
                        pred.self_ty(),
                        pred.print_modifiers_and_trait_path()
                    ));
                })
            }
            code = next_code;
            pred = next_pred;
        }

        diag
    }
}

#[extension(pub(super) trait InferCtxtPrivExt<'tcx>)]
impl<'tcx> TypeErrCtxt<'_, 'tcx> {
    fn can_match_trait(
        &self,
        goal: ty::TraitPredicate<'tcx>,
        assumption: ty::PolyTraitPredicate<'tcx>,
    ) -> bool {
        if goal.polarity != assumption.polarity() {
            return false;
        }

        let trait_goal = goal.trait_ref;
        let trait_assumption = self.instantiate_binder_with_fresh_vars(
            DUMMY_SP,
            infer::BoundRegionConversionTime::HigherRankedType,
            assumption.to_poly_trait_ref(),
        );

        self.can_eq(ty::ParamEnv::empty(), trait_goal, trait_assumption)
    }

    fn can_match_projection(
        &self,
        goal: ty::ProjectionPredicate<'tcx>,
        assumption: ty::PolyProjectionPredicate<'tcx>,
    ) -> bool {
        let assumption = self.instantiate_binder_with_fresh_vars(
            DUMMY_SP,
            infer::BoundRegionConversionTime::HigherRankedType,
            assumption,
        );

        let param_env = ty::ParamEnv::empty();
        self.can_eq(param_env, goal.projection_ty, assumption.projection_ty)
            && self.can_eq(param_env, goal.term, assumption.term)
    }

    // returns if `cond` not occurring implies that `error` does not occur - i.e., that
    // `error` occurring implies that `cond` occurs.
    #[instrument(level = "debug", skip(self), ret)]
    fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
        if cond == error {
            return true;
        }

        if let Some(error) = error.to_opt_poly_trait_pred() {
            self.enter_forall(error, |error| {
                elaborate(self.tcx, std::iter::once(cond))
                    .filter_map(|implied| implied.to_opt_poly_trait_pred())
                    .any(|implied| self.can_match_trait(error, implied))
            })
        } else if let Some(error) = error.to_opt_poly_projection_pred() {
            self.enter_forall(error, |error| {
                elaborate(self.tcx, std::iter::once(cond))
                    .filter_map(|implied| implied.to_opt_poly_projection_pred())
                    .any(|implied| self.can_match_projection(error, implied))
            })
        } else {
            false
        }
    }

    #[instrument(skip(self), level = "debug")]
    fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>) -> ErrorGuaranteed {
        if self.tcx.sess.opts.unstable_opts.next_solver.map(|c| c.dump_tree).unwrap_or_default()
            == DumpSolverProofTree::OnError
        {
            dump_proof_tree(&error.root_obligation, self.infcx);
        }

        match error.code {
            FulfillmentErrorCode::SelectionError(ref selection_error) => self
                .report_selection_error(
                    error.obligation.clone(),
                    &error.root_obligation,
                    selection_error,
                ),
            FulfillmentErrorCode::ProjectionError(ref e) => {
                self.report_projection_error(&error.obligation, e)
            }
            FulfillmentErrorCode::Ambiguity { overflow: None } => {
                self.maybe_report_ambiguity(&error.obligation)
            }
            FulfillmentErrorCode::Ambiguity { overflow: Some(suggest_increasing_limit) } => {
                self.report_overflow_no_abort(error.obligation.clone(), suggest_increasing_limit)
            }
            FulfillmentErrorCode::SubtypeError(ref expected_found, ref err) => self
                .report_mismatched_types(
                    &error.obligation.cause,
                    expected_found.expected,
                    expected_found.found,
                    *err,
                )
                .emit(),
            FulfillmentErrorCode::ConstEquateError(ref expected_found, ref err) => {
                let mut diag = self.report_mismatched_consts(
                    &error.obligation.cause,
                    expected_found.expected,
                    expected_found.found,
                    *err,
                );
                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
                if let ObligationCauseCode::BindingObligation(..)
                | ObligationCauseCode::ItemObligation(..)
                | ObligationCauseCode::ExprBindingObligation(..)
                | ObligationCauseCode::ExprItemObligation(..) = code
                {
                    self.note_obligation_cause_code(
                        error.obligation.cause.body_id,
                        &mut diag,
                        error.obligation.predicate,
                        error.obligation.param_env,
                        code,
                        &mut vec![],
                        &mut Default::default(),
                    );
                }
                diag.emit()
            }
            FulfillmentErrorCode::Cycle(ref cycle) => self.report_overflow_obligation_cycle(cycle),
        }
    }

    #[instrument(level = "debug", skip_all)]
    fn report_projection_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        error: &MismatchedProjectionTypes<'tcx>,
    ) -> ErrorGuaranteed {
        let predicate = self.resolve_vars_if_possible(obligation.predicate);

        if let Err(e) = predicate.error_reported() {
            return e;
        }

        self.probe(|_| {
            let ocx = ObligationCtxt::new(self);

            // try to find the mismatched types to report the error with.
            //
            // this can fail if the problem was higher-ranked, in which
            // cause I have no idea for a good error message.
            let bound_predicate = predicate.kind();
            let (values, err) = if let ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) =
                bound_predicate.skip_binder()
            {
                let data = self.instantiate_binder_with_fresh_vars(
                    obligation.cause.span,
                    infer::BoundRegionConversionTime::HigherRankedType,
                    bound_predicate.rebind(data),
                );
                let unnormalized_term = match data.term.unpack() {
                    ty::TermKind::Ty(_) => Ty::new_projection(
                        self.tcx,
                        data.projection_ty.def_id,
                        data.projection_ty.args,
                    )
                    .into(),
                    ty::TermKind::Const(ct) => ty::Const::new_unevaluated(
                        self.tcx,
                        ty::UnevaluatedConst {
                            def: data.projection_ty.def_id,
                            args: data.projection_ty.args,
                        },
                        ct.ty(),
                    )
                    .into(),
                };
                // FIXME(-Znext-solver): For diagnostic purposes, it would be nice
                // to deeply normalize this type.
                let normalized_term =
                    ocx.normalize(&obligation.cause, obligation.param_env, unnormalized_term);

                debug!(?obligation.cause, ?obligation.param_env);

                debug!(?normalized_term, data.ty = ?data.term);

                let is_normalized_term_expected = !matches!(
                    obligation.cause.code().peel_derives(),
                    ObligationCauseCode::ItemObligation(_)
                        | ObligationCauseCode::BindingObligation(_, _)
                        | ObligationCauseCode::ExprItemObligation(..)
                        | ObligationCauseCode::ExprBindingObligation(..)
                        | ObligationCauseCode::Coercion { .. }
                );

                let (expected, actual) = if is_normalized_term_expected {
                    (normalized_term, data.term)
                } else {
                    (data.term, normalized_term)
                };

                // constrain inference variables a bit more to nested obligations from normalize so
                // we can have more helpful errors.
                //
                // we intentionally drop errors from normalization here,
                // since the normalization is just done to improve the error message.
                let _ = ocx.select_where_possible();

                if let Err(new_err) =
                    ocx.eq(&obligation.cause, obligation.param_env, expected, actual)
                {
                    (Some((data, is_normalized_term_expected, normalized_term, data.term)), new_err)
                } else {
                    (None, error.err)
                }
            } else {
                (None, error.err)
            };

            let msg = values
                .and_then(|(predicate, _, normalized_term, expected_term)| {
                    self.maybe_detailed_projection_msg(predicate, normalized_term, expected_term)
                })
                .unwrap_or_else(|| {
                    let mut cx = FmtPrinter::new_with_limit(
                        self.tcx,
                        Namespace::TypeNS,
                        rustc_session::Limit(10),
                    );
                    with_forced_trimmed_paths!(format!("type mismatch resolving `{}`", {
                        self.resolve_vars_if_possible(predicate).print(&mut cx).unwrap();
                        cx.into_buffer()
                    }))
                });
            let mut diag = struct_span_code_err!(self.dcx(), obligation.cause.span, E0271, "{msg}");

            let secondary_span = (|| {
                let ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)) =
                    predicate.kind().skip_binder()
                else {
                    return None;
                };

                let trait_assoc_item = self.tcx.opt_associated_item(proj.projection_ty.def_id)?;
                let trait_assoc_ident = trait_assoc_item.ident(self.tcx);

                let mut associated_items = vec![];
                self.tcx.for_each_relevant_impl(
                    self.tcx.trait_of_item(proj.projection_ty.def_id)?,
                    proj.projection_ty.self_ty(),
                    |impl_def_id| {
                        associated_items.extend(
                            self.tcx
                                .associated_items(impl_def_id)
                                .in_definition_order()
                                .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident),
                        );
                    },
                );

                let [associated_item]: &[ty::AssocItem] = &associated_items[..] else {
                    return None;
                };
                match self.tcx.hir().get_if_local(associated_item.def_id) {
                    Some(
                        hir::Node::TraitItem(hir::TraitItem {
                            kind: hir::TraitItemKind::Type(_, Some(ty)),
                            ..
                        })
                        | hir::Node::ImplItem(hir::ImplItem {
                            kind: hir::ImplItemKind::Type(ty),
                            ..
                        }),
                    ) => Some((
                        ty.span,
                        with_forced_trimmed_paths!(Cow::from(format!(
                            "type mismatch resolving `{}`",
                            {
                                let mut cx = FmtPrinter::new_with_limit(
                                    self.tcx,
                                    Namespace::TypeNS,
                                    rustc_session::Limit(5),
                                );
                                self.resolve_vars_if_possible(predicate).print(&mut cx).unwrap();
                                cx.into_buffer()
                            }
                        ))),
                    )),
                    _ => None,
                }
            })();

            self.note_type_err(
                &mut diag,
                &obligation.cause,
                secondary_span,
                values.map(|(_, is_normalized_ty_expected, normalized_ty, expected_ty)| {
                    infer::ValuePairs::Terms(ExpectedFound::new(
                        is_normalized_ty_expected,
                        normalized_ty,
                        expected_ty,
                    ))
                }),
                err,
                true,
                false,
            );
            self.note_obligation_cause(&mut diag, obligation);
            diag.emit()
        })
    }

    fn maybe_detailed_projection_msg(
        &self,
        pred: ty::ProjectionPredicate<'tcx>,
        normalized_ty: ty::Term<'tcx>,
        expected_ty: ty::Term<'tcx>,
    ) -> Option<String> {
        let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
        let self_ty = pred.projection_ty.self_ty();

        with_forced_trimmed_paths! {
            if Some(pred.projection_ty.def_id) == self.tcx.lang_items().fn_once_output() {
                let fn_kind = self_ty.prefix_string(self.tcx);
                let item = match self_ty.kind() {
                    ty::FnDef(def, _) => self.tcx.item_name(*def).to_string(),
                    _ => self_ty.to_string(),
                };
                Some(format!(
                    "expected `{item}` to be a {fn_kind} that returns `{expected_ty}`, but it \
                     returns `{normalized_ty}`",
                ))
            } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
                Some(format!(
                    "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it \
                     resolves to `{normalized_ty}`"
                ))
            } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
                Some(format!(
                    "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it \
                     yields `{normalized_ty}`"
                ))
            } else {
                None
            }
        }
    }

    fn fuzzy_match_tys(
        &self,
        mut a: Ty<'tcx>,
        mut b: Ty<'tcx>,
        ignoring_lifetimes: bool,
    ) -> Option<CandidateSimilarity> {
        /// returns the fuzzy category of a given type, or None
        /// if the type can be equated to any type.
        fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
            match t.kind() {
                ty::Bool => Some(0),
                ty::Char => Some(1),
                ty::Str => Some(2),
                ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
                ty::Int(..)
                | ty::Uint(..)
                | ty::Float(..)
                | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
                ty::Ref(..) | ty::RawPtr(..) => Some(5),
                ty::Array(..) | ty::Slice(..) => Some(6),
                ty::FnDef(..) | ty::FnPtr(..) => Some(7),
                ty::Dynamic(..) => Some(8),
                ty::Closure(..) => Some(9),
                ty::Tuple(..) => Some(10),
                ty::Param(..) => Some(11),
                ty::Alias(ty::Projection, ..) => Some(12),
                ty::Alias(ty::Inherent, ..) => Some(13),
                ty::Alias(ty::Opaque, ..) => Some(14),
                ty::Alias(ty::Weak, ..) => Some(15),
                ty::Never => Some(16),
                ty::Adt(..) => Some(17),
                ty::Coroutine(..) => Some(18),
                ty::Foreign(..) => Some(19),
                ty::CoroutineWitness(..) => Some(20),
                ty::CoroutineClosure(..) => Some(21),
                ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
            }
        }

        let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
            loop {
                match t.kind() {
                    ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
                        t = *inner
                    }
                    _ => break t,
                }
            }
        };

        if !ignoring_lifetimes {
            a = strip_references(a);
            b = strip_references(b);
        }

        let cat_a = type_category(self.tcx, a)?;
        let cat_b = type_category(self.tcx, b)?;
        if a == b {
            Some(CandidateSimilarity::Exact { ignoring_lifetimes })
        } else if cat_a == cat_b {
            match (a.kind(), b.kind()) {
                (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
                (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
                // Matching on references results in a lot of unhelpful
                // suggestions, so let's just not do that for now.
                //
                // We still upgrade successful matches to `ignoring_lifetimes: true`
                // to prioritize that impl.
                (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
                    self.fuzzy_match_tys(a, b, true).is_some()
                }
                _ => true,
            }
            .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
        } else if ignoring_lifetimes {
            None
        } else {
            self.fuzzy_match_tys(a, b, true)
        }
    }

    fn describe_closure(&self, kind: hir::ClosureKind) -> &'static str {
        match kind {
            hir::ClosureKind::Closure => "a closure",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Coroutine(_)) => "a coroutine",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::Async,
                hir::CoroutineSource::Block,
            )) => "an async block",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::Async,
                hir::CoroutineSource::Fn,
            )) => "an async function",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::Async,
                hir::CoroutineSource::Closure,
            ))
            | hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async) => {
                "an async closure"
            }
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::AsyncGen,
                hir::CoroutineSource::Block,
            )) => "an async gen block",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::AsyncGen,
                hir::CoroutineSource::Fn,
            )) => "an async gen function",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::AsyncGen,
                hir::CoroutineSource::Closure,
            ))
            | hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::AsyncGen) => {
                "an async gen closure"
            }
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::Gen,
                hir::CoroutineSource::Block,
            )) => "a gen block",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::Gen,
                hir::CoroutineSource::Fn,
            )) => "a gen function",
            hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                hir::CoroutineDesugaring::Gen,
                hir::CoroutineSource::Closure,
            ))
            | hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Gen) => "a gen closure",
        }
    }

    fn find_similar_impl_candidates(
        &self,
        trait_pred: ty::PolyTraitPredicate<'tcx>,
    ) -> Vec<ImplCandidate<'tcx>> {
        let mut candidates: Vec<_> = self
            .tcx
            .all_impls(trait_pred.def_id())
            .filter_map(|def_id| {
                let imp = self.tcx.impl_trait_header(def_id).unwrap();
                if imp.polarity == ty::ImplPolarity::Negative
                    || !self.tcx.is_user_visible_dep(def_id.krate)
                {
                    return None;
                }
                let imp = imp.trait_ref.skip_binder();

                self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false).map(
                    |similarity| ImplCandidate { trait_ref: imp, similarity, impl_def_id: def_id },
                )
            })
            .collect();
        if candidates.iter().any(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. })) {
            // If any of the candidates is a perfect match, we don't want to show all of them.
            // This is particularly relevant for the case of numeric types (as they all have the
            // same category).
            candidates.retain(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. }));
        }
        candidates
    }

    fn report_similar_impl_candidates(
        &self,
        impl_candidates: &[ImplCandidate<'tcx>],
        trait_ref: ty::PolyTraitRef<'tcx>,
        body_def_id: LocalDefId,
        err: &mut Diag<'_>,
        other: bool,
        param_env: ty::ParamEnv<'tcx>,
    ) -> bool {
        // If we have a single implementation, try to unify it with the trait ref
        // that failed. This should uncover a better hint for what *is* implemented.
        if let [single] = &impl_candidates {
            if self.probe(|_| {
                let ocx = ObligationCtxt::new(self);

                self.enter_forall(trait_ref, |obligation_trait_ref| {
                    let impl_args = self.fresh_args_for_item(DUMMY_SP, single.impl_def_id);
                    let impl_trait_ref = ocx.normalize(
                        &ObligationCause::dummy(),
                        param_env,
                        ty::EarlyBinder::bind(single.trait_ref).instantiate(self.tcx, impl_args),
                    );

                    ocx.register_obligations(
                        self.tcx
                            .predicates_of(single.impl_def_id)
                            .instantiate(self.tcx, impl_args)
                            .into_iter()
                            .map(|(clause, _)| {
                                Obligation::new(
                                    self.tcx,
                                    ObligationCause::dummy(),
                                    param_env,
                                    clause,
                                )
                            }),
                    );
                    if !ocx.select_where_possible().is_empty() {
                        return false;
                    }

                    let mut terrs = vec![];
                    for (obligation_arg, impl_arg) in
                        std::iter::zip(obligation_trait_ref.args, impl_trait_ref.args)
                    {
                        if let Err(terr) =
                            ocx.eq(&ObligationCause::dummy(), param_env, impl_arg, obligation_arg)
                        {
                            terrs.push(terr);
                        }
                        if !ocx.select_where_possible().is_empty() {
                            return false;
                        }
                    }

                    // Literally nothing unified, just give up.
                    if terrs.len() == impl_trait_ref.args.len() {
                        return false;
                    }

                    let cand = self.resolve_vars_if_possible(impl_trait_ref).fold_with(
                        &mut BottomUpFolder {
                            tcx: self.tcx,
                            ty_op: |ty| ty,
                            lt_op: |lt| lt,
                            ct_op: |ct| ct.normalize(self.tcx, ty::ParamEnv::empty()),
                        },
                    );
                    if cand.references_error() {
                        return false;
                    }
                    err.highlighted_help(vec![
                        StringPart::normal(format!("the trait `{}` ", cand.print_trait_sugared())),
                        StringPart::highlighted("is"),
                        StringPart::normal(" implemented for `"),
                        StringPart::highlighted(cand.self_ty().to_string()),
                        StringPart::normal("`"),
                    ]);

                    if let [TypeError::Sorts(exp_found)] = &terrs[..] {
                        let exp_found = self.resolve_vars_if_possible(*exp_found);
                        err.help(format!(
                            "for that trait implementation, expected `{}`, found `{}`",
                            exp_found.expected, exp_found.found
                        ));
                    }

                    true
                })
            }) {
                return true;
            }
        }

        let other = if other { "other " } else { "" };
        let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diag<'_>| {
            candidates.retain(|tr| !tr.references_error());
            if candidates.is_empty() {
                return false;
            }
            if let &[cand] = &candidates[..] {
                let (desc, mention_castable) =
                    match (cand.self_ty().kind(), trait_ref.self_ty().skip_binder().kind()) {
                        (ty::FnPtr(_), ty::FnDef(..)) => {
                            (" implemented for fn pointer `", ", cast using `as`")
                        }
                        (ty::FnPtr(_), _) => (" implemented for fn pointer `", ""),
                        _ => (" implemented for `", ""),
                    };
                err.highlighted_help(vec![
                    StringPart::normal(format!("the trait `{}` ", cand.print_trait_sugared())),
                    StringPart::highlighted("is"),
                    StringPart::normal(desc),
                    StringPart::highlighted(cand.self_ty().to_string()),
                    StringPart::normal("`"),
                    StringPart::normal(mention_castable),
                ]);
                return true;
            }
            let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
            // Check if the trait is the same in all cases. If so, we'll only show the type.
            let mut traits: Vec<_> =
                candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
            traits.sort();
            traits.dedup();
            // FIXME: this could use a better heuristic, like just checking
            // that args[1..] is the same.
            let all_traits_equal = traits.len() == 1;

            let candidates: Vec<String> = candidates
                .into_iter()
                .map(|c| {
                    if all_traits_equal {
                        format!("\n  {}", c.self_ty())
                    } else {
                        format!("\n  {c}")
                    }
                })
                .collect();

            let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
            err.help(format!(
                "the following {other}types implement trait `{}`:{}{}",
                trait_ref.print_trait_sugared(),
                candidates[..end].join(""),
                if candidates.len() > 9 {
                    format!("\nand {} others", candidates.len() - 8)
                } else {
                    String::new()
                }
            ));
            true
        };

        let def_id = trait_ref.def_id();
        if impl_candidates.is_empty() {
            if self.tcx.trait_is_auto(def_id)
                || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
                || self.tcx.get_diagnostic_name(def_id).is_some()
            {
                // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
                return false;
            }
            let mut impl_candidates: Vec<_> = self
                .tcx
                .all_impls(def_id)
                // Ignore automatically derived impls and `!Trait` impls.
                .filter_map(|def_id| self.tcx.impl_trait_header(def_id))
                .filter_map(|header| {
                    (header.polarity != ty::ImplPolarity::Negative
                        || self.tcx.is_automatically_derived(def_id))
                    .then(|| header.trait_ref.instantiate_identity())
                })
                .filter(|trait_ref| {
                    let self_ty = trait_ref.self_ty();
                    // Avoid mentioning type parameters.
                    if let ty::Param(_) = self_ty.kind() {
                        false
                    }
                    // Avoid mentioning types that are private to another crate
                    else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
                        // FIXME(compiler-errors): This could be generalized, both to
                        // be more granular, and probably look past other `#[fundamental]`
                        // types, too.
                        self.tcx.visibility(def.did()).is_accessible_from(body_def_id, self.tcx)
                    } else {
                        true
                    }
                })
                .collect();

            impl_candidates.sort();
            impl_candidates.dedup();
            return report(impl_candidates, err);
        }

        // Sort impl candidates so that ordering is consistent for UI tests.
        // because the ordering of `impl_candidates` may not be deterministic:
        // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
        //
        // Prefer more similar candidates first, then sort lexicographically
        // by their normalized string representation.
        let mut impl_candidates: Vec<_> = impl_candidates
            .iter()
            .cloned()
            .map(|mut cand| {
                // Fold the consts so that they shows up as, e.g., `10`
                // instead of `core::::array::{impl#30}::{constant#0}`.
                cand.trait_ref = cand.trait_ref.fold_with(&mut BottomUpFolder {
                    tcx: self.tcx,
                    ty_op: |ty| ty,
                    lt_op: |lt| lt,
                    ct_op: |ct| ct.normalize(self.tcx, ty::ParamEnv::empty()),
                });
                cand
            })
            .collect();
        impl_candidates.sort_by_key(|cand| (cand.similarity, cand.trait_ref));
        let mut impl_candidates: Vec<_> =
            impl_candidates.into_iter().map(|cand| cand.trait_ref).collect();
        impl_candidates.dedup();

        report(impl_candidates, err)
    }

    fn report_similar_impl_candidates_for_root_obligation(
        &self,
        obligation: &PredicateObligation<'tcx>,
        trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
        body_def_id: LocalDefId,
        err: &mut Diag<'_>,
    ) {
        // This is *almost* equivalent to
        // `obligation.cause.code().peel_derives()`, but it gives us the
        // trait predicate for that corresponding root obligation. This
        // lets us get a derived obligation from a type parameter, like
        // when calling `string.strip_suffix(p)` where `p` is *not* an
        // implementer of `Pattern<'_>`.
        let mut code = obligation.cause.code();
        let mut trait_pred = trait_predicate;
        let mut peeled = false;
        while let Some((parent_code, parent_trait_pred)) = code.parent() {
            code = parent_code;
            if let Some(parent_trait_pred) = parent_trait_pred {
                trait_pred = parent_trait_pred;
                peeled = true;
            }
        }
        let def_id = trait_pred.def_id();
        // Mention *all* the `impl`s for the *top most* obligation, the
        // user might have meant to use one of them, if any found. We skip
        // auto-traits or fundamental traits that might not be exactly what
        // the user might expect to be presented with. Instead this is
        // useful for less general traits.
        if peeled
            && !self.tcx.trait_is_auto(def_id)
            && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
        {
            let trait_ref = trait_pred.to_poly_trait_ref();
            let impl_candidates = self.find_similar_impl_candidates(trait_pred);
            self.report_similar_impl_candidates(
                &impl_candidates,
                trait_ref,
                body_def_id,
                err,
                true,
                obligation.param_env,
            );
        }
    }

    /// Gets the parent trait chain start
    fn get_parent_trait_ref(
        &self,
        code: &ObligationCauseCode<'tcx>,
    ) -> Option<(Ty<'tcx>, Option<Span>)> {
        match code {
            ObligationCauseCode::BuiltinDerivedObligation(data) => {
                let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
                match self.get_parent_trait_ref(&data.parent_code) {
                    Some(t) => Some(t),
                    None => {
                        let ty = parent_trait_ref.skip_binder().self_ty();
                        let span = TyCategory::from_ty(self.tcx, ty)
                            .map(|(_, def_id)| self.tcx.def_span(def_id));
                        Some((ty, span))
                    }
                }
            }
            ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
                self.get_parent_trait_ref(parent_code)
            }
            _ => None,
        }
    }

    /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
    /// with the same path as `trait_ref`, a help message about
    /// a probable version mismatch is added to `err`
    fn note_version_mismatch(
        &self,
        err: &mut Diag<'_>,
        trait_ref: &ty::PolyTraitRef<'tcx>,
    ) -> bool {
        let get_trait_impls = |trait_def_id| {
            let mut trait_impls = vec![];
            self.tcx.for_each_relevant_impl(
                trait_def_id,
                trait_ref.skip_binder().self_ty(),
                |impl_def_id| {
                    trait_impls.push(impl_def_id);
                },
            );
            trait_impls
        };

        let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
        let traits_with_same_path: std::collections::BTreeSet<_> = self
            .tcx
            .all_traits()
            .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
            .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
            .collect();
        let mut suggested = false;
        for trait_with_same_path in traits_with_same_path {
            let trait_impls = get_trait_impls(trait_with_same_path);
            if trait_impls.is_empty() {
                continue;
            }
            let impl_spans: Vec<_> =
                trait_impls.iter().map(|impl_def_id| self.tcx.def_span(*impl_def_id)).collect();
            err.span_help(
                impl_spans,
                format!("trait impl{} with same name found", pluralize!(trait_impls.len())),
            );
            let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
            let crate_msg =
                format!("perhaps two different versions of crate `{trait_crate}` are being used?");
            err.note(crate_msg);
            suggested = true;
        }
        suggested
    }

    /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
    /// `trait_ref`.
    ///
    /// For this to work, `new_self_ty` must have no escaping bound variables.
    fn mk_trait_obligation_with_new_self_ty(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
    ) -> PredicateObligation<'tcx> {
        let trait_pred =
            trait_ref_and_ty.map_bound(|(tr, new_self_ty)| tr.with_self_ty(self.tcx, new_self_ty));

        Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
    }

    #[instrument(skip(self), level = "debug")]
    fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>) -> ErrorGuaranteed {
        // Unable to successfully determine, probably means
        // insufficient type information, but could mean
        // ambiguous impls. The latter *ought* to be a
        // coherence violation, so we don't report it here.

        let predicate = self.resolve_vars_if_possible(obligation.predicate);
        let span = obligation.cause.span;

        debug!(?predicate, obligation.cause.code = ?obligation.cause.code());

        // Ambiguity errors are often caused as fallout from earlier errors.
        // We ignore them if this `infcx` is tainted in some cases below.

        let bound_predicate = predicate.kind();
        let mut err = match bound_predicate.skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
                let trait_ref = bound_predicate.rebind(data.trait_ref);
                debug!(?trait_ref);

                if let Err(e) = predicate.error_reported() {
                    return e;
                }

                if let Err(guar) = self.tcx.ensure().coherent_trait(trait_ref.def_id()) {
                    // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for Foo` in case
                    // other `Foo` impls are incoherent.
                    return guar;
                }

                // This is kind of a hack: it frequently happens that some earlier
                // error prevents types from being fully inferred, and then we get
                // a bunch of uninteresting errors saying something like "<generic
                // #0> doesn't implement Sized". It may even be true that we
                // could just skip over all checks where the self-ty is an
                // inference variable, but I was afraid that there might be an
                // inference variable created, registered as an obligation, and
                // then never forced by writeback, and hence by skipping here we'd
                // be ignoring the fact that we don't KNOW the type works
                // out. Though even that would probably be harmless, given that
                // we're only talking about builtin traits, which are known to be
                // inhabited. We used to check for `self.tcx.sess.has_errors()` to
                // avoid inundating the user with unnecessary errors, but we now
                // check upstream for type errors and don't add the obligations to
                // begin with in those cases.
                if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
                    match self.tainted_by_errors() {
                        None => {
                            let err = self.emit_inference_failure_err(
                                obligation.cause.body_id,
                                span,
                                trait_ref.self_ty().skip_binder().into(),
                                ErrorCode::E0282,
                                false,
                            );
                            return err.stash(span, StashKey::MaybeForgetReturn).unwrap();
                        }
                        Some(e) => return e,
                    }
                }

                // Typically, this ambiguity should only happen if
                // there are unresolved type inference variables
                // (otherwise it would suggest a coherence
                // failure). But given #21974 that is not necessarily
                // the case -- we can have multiple where clauses that
                // are only distinguished by a region, which results
                // in an ambiguity even when all types are fully
                // known, since we don't dispatch based on region
                // relationships.

                // Pick the first generic parameter that still contains inference variables as the one
                // we're going to emit an error for. If there are none (see above), fall back to
                // a more general error.
                let arg = data.trait_ref.args.iter().find(|s| s.has_non_region_infer());

                let mut err = if let Some(arg) = arg {
                    self.emit_inference_failure_err(
                        obligation.cause.body_id,
                        span,
                        arg,
                        ErrorCode::E0283,
                        true,
                    )
                } else {
                    struct_span_code_err!(
                        self.dcx(),
                        span,
                        E0283,
                        "type annotations needed: cannot satisfy `{}`",
                        predicate,
                    )
                };

                let mut ambiguities = ambiguity::recompute_applicable_impls(
                    self.infcx,
                    &obligation.with(self.tcx, trait_ref),
                );
                let has_non_region_infer =
                    trait_ref.skip_binder().args.types().any(|t| !t.is_ty_or_numeric_infer());
                // It doesn't make sense to talk about applicable impls if there are more than a
                // handful of them. If there are a lot of them, but only a few of them have no type
                // params, we only show those, as they are more likely to be useful/intended.
                if ambiguities.len() > 5 {
                    let infcx = self.infcx;
                    if !ambiguities.iter().all(|option| match option {
                        DefId(did) => infcx.fresh_args_for_item(DUMMY_SP, *did).is_empty(),
                        ParamEnv(_) => true,
                    }) {
                        // If not all are blanket impls, we filter blanked impls out.
                        ambiguities.retain(|option| match option {
                            DefId(did) => infcx.fresh_args_for_item(DUMMY_SP, *did).is_empty(),
                            ParamEnv(_) => true,
                        });
                    }
                }
                if ambiguities.len() > 1 && ambiguities.len() < 10 && has_non_region_infer {
                    if let Some(e) = self.tainted_by_errors()
                        && arg.is_none()
                    {
                        // If `arg.is_none()`, then this is probably two param-env
                        // candidates or impl candidates that are equal modulo lifetimes.
                        // Therefore, if we've already emitted an error, just skip this
                        // one, since it's not particularly actionable.
                        err.cancel();
                        return e;
                    }
                    self.annotate_source_of_ambiguity(&mut err, &ambiguities, predicate);
                } else {
                    if let Some(e) = self.tainted_by_errors() {
                        err.cancel();
                        return e;
                    }
                    err.note(format!("cannot satisfy `{predicate}`"));
                    let impl_candidates = self
                        .find_similar_impl_candidates(predicate.to_opt_poly_trait_pred().unwrap());
                    if impl_candidates.len() < 40 {
                        self.report_similar_impl_candidates(
                            impl_candidates.as_slice(),
                            trait_ref,
                            obligation.cause.body_id,
                            &mut err,
                            false,
                            obligation.param_env,
                        );
                    }
                }

                if let ObligationCauseCode::ItemObligation(def_id)
                | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code()
                {
                    self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
                }

                if let Some(ty::GenericArgKind::Type(_)) = arg.map(|arg| arg.unpack())
                    && let Some(body_id) =
                        self.tcx.hir().maybe_body_owned_by(obligation.cause.body_id)
                {
                    let mut expr_finder = FindExprBySpan::new(span);
                    expr_finder.visit_expr(self.tcx.hir().body(body_id).value);

                    if let Some(hir::Expr {
                        kind:
                            hir::ExprKind::Call(
                                hir::Expr {
                                    kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)),
                                    ..
                                },
                                _,
                            )
                            | hir::ExprKind::Path(hir::QPath::Resolved(None, path)),
                        ..
                    }) = expr_finder.result
                        && let [
                            ..,
                            trait_path_segment @ hir::PathSegment {
                                res: Res::Def(DefKind::Trait, trait_id),
                                ..
                            },
                            hir::PathSegment {
                                ident: assoc_item_name,
                                res: Res::Def(_, item_id),
                                ..
                            },
                        ] = path.segments
                        && data.trait_ref.def_id == *trait_id
                        && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
                        && let None = self.tainted_by_errors()
                    {
                        let (verb, noun) = match self.tcx.associated_item(item_id).kind {
                            ty::AssocKind::Const => ("refer to the", "constant"),
                            ty::AssocKind::Fn => ("call", "function"),
                            // This is already covered by E0223, but this following single match
                            // arm doesn't hurt here.
                            ty::AssocKind::Type => ("refer to the", "type"),
                        };

                        // Replace the more general E0283 with a more specific error
                        err.cancel();
                        err = self.dcx().struct_span_err(
                            span,
                            format!(
                                "cannot {verb} associated {noun} on trait without specifying the \
                                 corresponding `impl` type",
                            ),
                        );
                        err.code(E0790);

                        if let Some(local_def_id) = data.trait_ref.def_id.as_local()
                            && let Some(hir::Node::Item(hir::Item {
                                ident: trait_name,
                                kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs),
                                ..
                            })) = self.tcx.opt_hir_node_by_def_id(local_def_id)
                            && let Some(method_ref) = trait_item_refs
                                .iter()
                                .find(|item_ref| item_ref.ident == *assoc_item_name)
                        {
                            err.span_label(
                                method_ref.span,
                                format!("`{trait_name}::{assoc_item_name}` defined here"),
                            );
                        }

                        err.span_label(span, format!("cannot {verb} associated {noun} of trait"));

                        let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);

                        if let Some(impl_def_id) =
                            trait_impls.non_blanket_impls().values().flatten().next()
                        {
                            let non_blanket_impl_count =
                                trait_impls.non_blanket_impls().values().flatten().count();
                            // If there is only one implementation of the trait, suggest using it.
                            // Otherwise, use a placeholder comment for the implementation.
                            let (message, self_type) = if non_blanket_impl_count == 1 {
                                (
                                    "use the fully-qualified path to the only available \
                                     implementation",
                                    format!(
                                        "{}",
                                        self.tcx.type_of(impl_def_id).instantiate_identity()
                                    ),
                                )
                            } else {
                                (
                                    "use a fully-qualified path to a specific available \
                                     implementation",
                                    "/* self type */".to_string(),
                                )
                            };
                            let mut suggestions =
                                vec![(path.span.shrink_to_lo(), format!("<{self_type} as "))];
                            if let Some(generic_arg) = trait_path_segment.args {
                                let between_span =
                                    trait_path_segment.ident.span.between(generic_arg.span_ext);
                                // get rid of :: between Trait and <type>
                                // must be '::' between them, otherwise the parser won't accept the code
                                suggestions.push((between_span, "".to_string()));
                                suggestions
                                    .push((generic_arg.span_ext.shrink_to_hi(), ">".to_string()));
                            } else {
                                suggestions.push((
                                    trait_path_segment.ident.span.shrink_to_hi(),
                                    ">".to_string(),
                                ));
                            }
                            err.multipart_suggestion(
                                message,
                                suggestions,
                                Applicability::MaybeIncorrect,
                            );
                        }
                    }
                };

                err
            }

            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
                // Same hacky approach as above to avoid deluging user
                // with error messages.

                if let Err(e) = arg.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }

                self.emit_inference_failure_err(
                    obligation.cause.body_id,
                    span,
                    arg,
                    ErrorCode::E0282,
                    false,
                )
            }

            ty::PredicateKind::Subtype(data) => {
                if let Err(e) = data.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                let SubtypePredicate { a_is_expected: _, a, b } = data;
                // both must be type variables, or the other would've been instantiated
                assert!(a.is_ty_var() && b.is_ty_var());
                self.emit_inference_failure_err(
                    obligation.cause.body_id,
                    span,
                    a.into(),
                    ErrorCode::E0282,
                    true,
                )
            }
            ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => {
                if let Err(e) = predicate.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }

                if let Err(guar) =
                    self.tcx.ensure().coherent_trait(self.tcx.parent(data.projection_ty.def_id))
                {
                    // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for Foo` in case
                    // other `Foo` impls are incoherent.
                    return guar;
                }
                let arg = data
                    .projection_ty
                    .args
                    .iter()
                    .chain(Some(data.term.into_arg()))
                    .find(|g| g.has_non_region_infer());
                if let Some(arg) = arg {
                    self.emit_inference_failure_err(
                        obligation.cause.body_id,
                        span,
                        arg,
                        ErrorCode::E0284,
                        true,
                    )
                    .with_note(format!("cannot satisfy `{predicate}`"))
                } else {
                    // If we can't find a generic parameter, just print a generic error
                    struct_span_code_err!(
                        self.dcx(),
                        span,
                        E0284,
                        "type annotations needed: cannot satisfy `{}`",
                        predicate,
                    )
                    .with_span_label(span, format!("cannot satisfy `{predicate}`"))
                }
            }

            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(data)) => {
                if let Err(e) = predicate.error_reported() {
                    return e;
                }
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                let arg = data.walk().find(|g| g.is_non_region_infer());
                if let Some(arg) = arg {
                    let err = self.emit_inference_failure_err(
                        obligation.cause.body_id,
                        span,
                        arg,
                        ErrorCode::E0284,
                        true,
                    );
                    err
                } else {
                    // If we can't find a generic parameter, just print a generic error
                    struct_span_code_err!(
                        self.dcx(),
                        span,
                        E0284,
                        "type annotations needed: cannot satisfy `{}`",
                        predicate,
                    )
                    .with_span_label(span, format!("cannot satisfy `{predicate}`"))
                }
            }
            _ => {
                if let Some(e) = self.tainted_by_errors() {
                    return e;
                }
                struct_span_code_err!(
                    self.dcx(),
                    span,
                    E0284,
                    "type annotations needed: cannot satisfy `{}`",
                    predicate,
                )
                .with_span_label(span, format!("cannot satisfy `{predicate}`"))
            }
        };
        self.note_obligation_cause(&mut err, obligation);
        err.emit()
    }

    fn annotate_source_of_ambiguity(
        &self,
        err: &mut Diag<'_>,
        ambiguities: &[ambiguity::Ambiguity],
        predicate: ty::Predicate<'tcx>,
    ) {
        let mut spans = vec![];
        let mut crates = vec![];
        let mut post = vec![];
        let mut has_param_env = false;
        for ambiguity in ambiguities {
            match ambiguity {
                ambiguity::Ambiguity::DefId(impl_def_id) => {
                    match self.tcx.span_of_impl(*impl_def_id) {
                        Ok(span) => spans.push(span),
                        Err(name) => {
                            crates.push(name);
                            if let Some(header) = to_pretty_impl_header(self.tcx, *impl_def_id) {
                                post.push(header);
                            }
                        }
                    }
                }
                ambiguity::Ambiguity::ParamEnv(span) => {
                    has_param_env = true;
                    spans.push(*span);
                }
            }
        }
        let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{n}`")).collect();
        crate_names.sort();
        crate_names.dedup();
        post.sort();
        post.dedup();

        if self.tainted_by_errors().is_some()
            && (crate_names.len() == 1
                && spans.len() == 0
                && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
                || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
        {
            // Avoid complaining about other inference issues for expressions like
            // `42 >> 1`, where the types are still `{integer}`, but we want to
            // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
            // NOTE(eddyb) this was `.cancel()`, but `err`
            // is borrowed, so we can't fully defuse it.
            err.downgrade_to_delayed_bug();
            return;
        }

        let msg = format!(
            "multiple `impl`s{} satisfying `{}` found",
            if has_param_env { " or `where` clauses" } else { "" },
            predicate
        );
        let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
            format!(":\n{}", post.iter().map(|p| format!("- {p}")).collect::<Vec<_>>().join("\n"),)
        } else if post.len() == 1 {
            format!(": `{}`", post[0])
        } else {
            String::new()
        };

        match (spans.len(), crates.len(), crate_names.len()) {
            (0, 0, 0) => {
                err.note(format!("cannot satisfy `{predicate}`"));
            }
            (0, _, 1) => {
                err.note(format!("{} in the `{}` crate{}", msg, crates[0], post,));
            }
            (0, _, _) => {
                err.note(format!(
                    "{} in the following crates: {}{}",
                    msg,
                    crate_names.join(", "),
                    post,
                ));
            }
            (_, 0, 0) => {
                let span: MultiSpan = spans.into();
                err.span_note(span, msg);
            }
            (_, 1, 1) => {
                let span: MultiSpan = spans.into();
                err.span_note(span, msg);
                err.note(format!("and another `impl` found in the `{}` crate{}", crates[0], post,));
            }
            _ => {
                let span: MultiSpan = spans.into();
                err.span_note(span, msg);
                err.note(format!(
                    "and more `impl`s found in the following crates: {}{}",
                    crate_names.join(", "),
                    post,
                ));
            }
        }
    }

    /// Returns `true` if the trait predicate may apply for *some* assignment
    /// to the type parameters.
    fn predicate_can_apply(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        pred: ty::PolyTraitPredicate<'tcx>,
    ) -> bool {
        struct ParamToVarFolder<'a, 'tcx> {
            infcx: &'a InferCtxt<'tcx>,
            var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
        }

        impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for ParamToVarFolder<'a, 'tcx> {
            fn interner(&self) -> TyCtxt<'tcx> {
                self.infcx.tcx
            }

            fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
                if let ty::Param(_) = *ty.kind() {
                    let infcx = self.infcx;
                    *self.var_map.entry(ty).or_insert_with(|| {
                        infcx.next_ty_var(TypeVariableOrigin {
                            kind: TypeVariableOriginKind::MiscVariable,
                            span: DUMMY_SP,
                        })
                    })
                } else {
                    ty.super_fold_with(self)
                }
            }
        }

        self.probe(|_| {
            let cleaned_pred =
                pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });

            let InferOk { value: cleaned_pred, .. } =
                self.infcx.at(&ObligationCause::dummy(), param_env).normalize(cleaned_pred);

            let obligation =
                Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);

            self.predicate_may_hold(&obligation)
        })
    }

    fn note_obligation_cause(&self, err: &mut Diag<'_>, obligation: &PredicateObligation<'tcx>) {
        // First, attempt to add note to this error with an async-await-specific
        // message, and fall back to regular note otherwise.
        if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
            self.note_obligation_cause_code(
                obligation.cause.body_id,
                err,
                obligation.predicate,
                obligation.param_env,
                obligation.cause.code(),
                &mut vec![],
                &mut Default::default(),
            );
            self.suggest_unsized_bound_if_applicable(err, obligation);
            if let Some(span) = err.span.primary_span()
                && let Some(mut diag) =
                    self.tcx.dcx().steal_non_err(span, StashKey::AssociatedTypeSuggestion)
                && let Ok(ref mut s1) = err.suggestions
                && let Ok(ref mut s2) = diag.suggestions
            {
                s1.append(s2);
                diag.cancel()
            }
        }
    }

    #[instrument(level = "debug", skip_all)]
    fn suggest_unsized_bound_if_applicable(
        &self,
        err: &mut Diag<'_>,
        obligation: &PredicateObligation<'tcx>,
    ) {
        let ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) =
            obligation.predicate.kind().skip_binder()
        else {
            return;
        };
        let (ObligationCauseCode::BindingObligation(item_def_id, span)
        | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..)) =
            *obligation.cause.code().peel_derives()
        else {
            return;
        };
        debug!(?pred, ?item_def_id, ?span);

        let (Some(node), true) = (
            self.tcx.hir().get_if_local(item_def_id),
            Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
        ) else {
            return;
        };
        self.maybe_suggest_unsized_generics(err, span, node);
    }

    #[instrument(level = "debug", skip_all)]
    fn maybe_suggest_unsized_generics(&self, err: &mut Diag<'_>, span: Span, node: Node<'tcx>) {
        let Some(generics) = node.generics() else {
            return;
        };
        let sized_trait = self.tcx.lang_items().sized_trait();
        debug!(?generics.params);
        debug!(?generics.predicates);
        let Some(param) = generics.params.iter().find(|param| param.span == span) else {
            return;
        };
        // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
        // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
        let explicitly_sized = generics
            .bounds_for_param(param.def_id)
            .flat_map(|bp| bp.bounds)
            .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
        if explicitly_sized {
            return;
        }
        debug!(?param);
        match node {
            hir::Node::Item(
                item @ hir::Item {
                    // Only suggest indirection for uses of type parameters in ADTs.
                    kind:
                        hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
                    ..
                },
            ) => {
                if self.maybe_indirection_for_unsized(err, item, param) {
                    return;
                }
            }
            _ => {}
        };
        // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
        let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
        {
            (s, " +")
        } else {
            (param.name.ident().span.shrink_to_hi(), ":")
        };
        err.span_suggestion_verbose(
            span,
            "consider relaxing the implicit `Sized` restriction",
            format!("{separator} ?Sized"),
            Applicability::MachineApplicable,
        );
    }

    fn maybe_indirection_for_unsized(
        &self,
        err: &mut Diag<'_>,
        item: &Item<'tcx>,
        param: &GenericParam<'tcx>,
    ) -> bool {
        // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
        // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
        // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
        let mut visitor =
            FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
        visitor.visit_item(item);
        if visitor.invalid_spans.is_empty() {
            return false;
        }
        let mut multispan: MultiSpan = param.span.into();
        multispan.push_span_label(
            param.span,
            format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
        );
        for sp in visitor.invalid_spans {
            multispan.push_span_label(
                sp,
                format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
            );
        }
        err.span_help(
            multispan,
            format!(
                "you could relax the implicit `Sized` bound on `{T}` if it were \
                used through indirection like `&{T}` or `Box<{T}>`",
                T = param.name.ident(),
            ),
        );
        true
    }

    fn is_recursive_obligation(
        &self,
        obligated_types: &mut Vec<Ty<'tcx>>,
        cause_code: &ObligationCauseCode<'tcx>,
    ) -> bool {
        if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
            let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
            let self_ty = parent_trait_ref.skip_binder().self_ty();
            if obligated_types.iter().any(|ot| ot == &self_ty) {
                return true;
            }
            if let ty::Adt(def, args) = self_ty.kind()
                && let [arg] = &args[..]
                && let ty::GenericArgKind::Type(ty) = arg.unpack()
                && let ty::Adt(inner_def, _) = ty.kind()
                && inner_def == def
            {
                return true;
            }
        }
        false
    }

    fn get_standard_error_message(
        &self,
        trait_predicate: &ty::PolyTraitPredicate<'tcx>,
        message: Option<String>,
        predicate_is_const: bool,
        append_const_msg: Option<AppendConstMessage>,
        post_message: String,
    ) -> String {
        message
            .and_then(|cannot_do_this| {
                match (predicate_is_const, append_const_msg) {
                    // do nothing if predicate is not const
                    (false, _) => Some(cannot_do_this),
                    // suggested using default post message
                    (true, Some(AppendConstMessage::Default)) => {
                        Some(format!("{cannot_do_this} in const contexts"))
                    }
                    // overridden post message
                    (true, Some(AppendConstMessage::Custom(custom_msg, _))) => {
                        Some(format!("{cannot_do_this}{custom_msg}"))
                    }
                    // fallback to generic message
                    (true, None) => None,
                }
            })
            .unwrap_or_else(|| {
                format!("the trait bound `{trait_predicate}` is not satisfied{post_message}")
            })
    }

    fn get_safe_transmute_error_and_reason(
        &self,
        obligation: PredicateObligation<'tcx>,
        trait_ref: ty::PolyTraitRef<'tcx>,
        span: Span,
    ) -> GetSafeTransmuteErrorAndReason {
        use rustc_transmute::Answer;

        // Erase regions because layout code doesn't particularly care about regions.
        let trait_ref =
            self.tcx.erase_regions(self.tcx.instantiate_bound_regions_with_erased(trait_ref));

        let src_and_dst = rustc_transmute::Types {
            dst: trait_ref.args.type_at(0),
            src: trait_ref.args.type_at(1),
        };
        let Some(assume) = rustc_transmute::Assume::from_const(
            self.infcx.tcx,
            obligation.param_env,
            trait_ref.args.const_at(2),
        ) else {
            self.dcx().span_delayed_bug(
                span,
                "Unable to construct rustc_transmute::Assume where it was previously possible",
            );
            return GetSafeTransmuteErrorAndReason::Silent;
        };

        match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
            obligation.cause,
            src_and_dst,
            assume,
        ) {
            Answer::No(reason) => {
                let dst = trait_ref.args.type_at(0);
                let src = trait_ref.args.type_at(1);
                let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
                let safe_transmute_explanation = match reason {
                    rustc_transmute::Reason::SrcIsUnspecified => {
                        format!("`{src}` does not have a well-specified layout")
                    }

                    rustc_transmute::Reason::DstIsUnspecified => {
                        format!("`{dst}` does not have a well-specified layout")
                    }

                    rustc_transmute::Reason::DstIsBitIncompatible => {
                        format!("At least one value of `{src}` isn't a bit-valid value of `{dst}`")
                    }

                    rustc_transmute::Reason::DstMayHaveSafetyInvariants => {
                        format!("`{dst}` may carry safety invariants")
                    }
                    rustc_transmute::Reason::DstIsTooBig => {
                        format!("The size of `{src}` is smaller than the size of `{dst}`")
                    }
                    rustc_transmute::Reason::DstRefIsTooBig { src, dst } => {
                        let src_size = src.size;
                        let dst_size = dst.size;
                        format!(
                            "The referent size of `{src}` ({src_size} bytes) is smaller than that of `{dst}` ({dst_size} bytes)"
                        )
                    }
                    rustc_transmute::Reason::SrcSizeOverflow => {
                        format!(
                            "values of the type `{src}` are too big for the current architecture"
                        )
                    }
                    rustc_transmute::Reason::DstSizeOverflow => {
                        format!(
                            "values of the type `{dst}` are too big for the current architecture"
                        )
                    }
                    rustc_transmute::Reason::DstHasStricterAlignment {
                        src_min_align,
                        dst_min_align,
                    } => {
                        format!(
                            "The minimum alignment of `{src}` ({src_min_align}) should be greater than that of `{dst}` ({dst_min_align})"
                        )
                    }
                    rustc_transmute::Reason::DstIsMoreUnique => {
                        format!("`{src}` is a shared reference, but `{dst}` is a unique reference")
                    }
                    // Already reported by rustc
                    rustc_transmute::Reason::TypeError => {
                        return GetSafeTransmuteErrorAndReason::Silent;
                    }
                    rustc_transmute::Reason::SrcLayoutUnknown => {
                        format!("`{src}` has an unknown layout")
                    }
                    rustc_transmute::Reason::DstLayoutUnknown => {
                        format!("`{dst}` has an unknown layout")
                    }
                };
                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation }
            }
            // Should never get a Yes at this point! We already ran it before, and did not get a Yes.
            Answer::Yes => span_bug!(
                span,
                "Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
            ),
            other => span_bug!(span, "Unsupported rustc_transmute::Answer variant: `{other:?}`"),
        }
    }

    fn add_tuple_trait_message(
        &self,
        obligation_cause_code: &ObligationCauseCode<'tcx>,
        err: &mut Diag<'_>,
    ) {
        match obligation_cause_code {
            ObligationCauseCode::RustCall => {
                err.primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
            }
            ObligationCauseCode::BindingObligation(def_id, _)
            | ObligationCauseCode::ItemObligation(def_id)
                if self.tcx.is_fn_trait(*def_id) =>
            {
                err.code(E0059);
                err.primary_message(format!(
                    "type parameter to bare `{}` trait must be a tuple",
                    self.tcx.def_path_str(*def_id)
                ));
            }
            _ => {}
        }
    }

    fn try_to_add_help_message(
        &self,
        obligation: &PredicateObligation<'tcx>,
        trait_ref: ty::PolyTraitRef<'tcx>,
        trait_predicate: &ty::PolyTraitPredicate<'tcx>,
        err: &mut Diag<'_>,
        span: Span,
        is_fn_trait: bool,
        suggested: bool,
        unsatisfied_const: bool,
    ) {
        let body_def_id = obligation.cause.body_id;
        let span = if let ObligationCauseCode::BinOp { rhs_span: Some(rhs_span), .. } =
            obligation.cause.code()
        {
            *rhs_span
        } else {
            span
        };

        // Try to report a help message
        if is_fn_trait
            && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
                obligation.param_env,
                trait_ref.self_ty(),
                trait_predicate.skip_binder().polarity,
            )
        {
            self.add_help_message_for_fn_trait(trait_ref, err, implemented_kind, params);
        } else if !trait_ref.has_non_region_infer()
            && self.predicate_can_apply(obligation.param_env, *trait_predicate)
        {
            // If a where-clause may be useful, remind the
            // user that they can add it.
            //
            // don't display an on-unimplemented note, as
            // these notes will often be of the form
            //     "the type `T` can't be frobnicated"
            // which is somewhat confusing.
            self.suggest_restricting_param_bound(
                err,
                *trait_predicate,
                None,
                obligation.cause.body_id,
            );
        } else if trait_ref.def_id().is_local()
            && self.tcx.trait_impls_of(trait_ref.def_id()).is_empty()
            && !self.tcx.trait_is_auto(trait_ref.def_id())
            && !self.tcx.trait_is_alias(trait_ref.def_id())
        {
            err.span_help(
                self.tcx.def_span(trait_ref.def_id()),
                crate::fluent_generated::trait_selection_trait_has_no_impls,
            );
        } else if !suggested && !unsatisfied_const {
            // Can't show anything else useful, try to find similar impls.
            let impl_candidates = self.find_similar_impl_candidates(*trait_predicate);
            if !self.report_similar_impl_candidates(
                &impl_candidates,
                trait_ref,
                body_def_id,
                err,
                true,
                obligation.param_env,
            ) {
                self.report_similar_impl_candidates_for_root_obligation(
                    obligation,
                    *trait_predicate,
                    body_def_id,
                    err,
                );
            }

            self.suggest_convert_to_slice(
                err,
                obligation,
                trait_ref,
                impl_candidates.as_slice(),
                span,
            );
        }
    }

    fn add_help_message_for_fn_trait(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        err: &mut Diag<'_>,
        implemented_kind: ty::ClosureKind,
        params: ty::Binder<'tcx, Ty<'tcx>>,
    ) {
        // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
        // suggestion to add trait bounds for the type, since we only typically implement
        // these traits once.

        // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
        // to implement.
        let selected_kind = self
            .tcx
            .fn_trait_kind_from_def_id(trait_ref.def_id())
            .expect("expected to map DefId to ClosureKind");
        if !implemented_kind.extends(selected_kind) {
            err.note(format!(
                "`{}` implements `{}`, but it must implement `{}`, which is more general",
                trait_ref.skip_binder().self_ty(),
                implemented_kind,
                selected_kind
            ));
        }

        // Note any argument mismatches
        let given_ty = params.skip_binder();
        let expected_ty = trait_ref.skip_binder().args.type_at(1);
        if let ty::Tuple(given) = given_ty.kind()
            && let ty::Tuple(expected) = expected_ty.kind()
        {
            if expected.len() != given.len() {
                // Note number of types that were expected and given
                err.note(
                    format!(
                        "expected a closure taking {} argument{}, but one taking {} argument{} was given",
                        given.len(),
                        pluralize!(given.len()),
                        expected.len(),
                        pluralize!(expected.len()),
                    )
                );
            } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
                // Print type mismatch
                let (expected_args, given_args) = self.cmp(given_ty, expected_ty);
                err.note_expected_found(
                    &"a closure with arguments",
                    expected_args,
                    &"a closure with arguments",
                    given_args,
                );
            }
        }
    }

    fn maybe_add_note_for_unsatisfied_const(
        &self,
        _trait_predicate: &ty::PolyTraitPredicate<'tcx>,
        _err: &mut Diag<'_>,
        _span: Span,
    ) -> UnsatisfiedConst {
        let unsatisfied_const = UnsatisfiedConst(false);
        // FIXME(effects)
        unsatisfied_const
    }

    fn report_closure_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        closure_def_id: DefId,
        found_kind: ty::ClosureKind,
        kind: ty::ClosureKind,
        trait_prefix: &'static str,
    ) -> Diag<'tcx> {
        let closure_span = self.tcx.def_span(closure_def_id);

        let mut err = ClosureKindMismatch {
            closure_span,
            expected: kind,
            found: found_kind,
            cause_span: obligation.cause.span,
            trait_prefix,
            fn_once_label: None,
            fn_mut_label: None,
        };

        // Additional context information explaining why the closure only implements
        // a particular trait.
        if let Some(typeck_results) = &self.typeck_results {
            let hir_id = self.tcx.local_def_id_to_hir_id(closure_def_id.expect_local());
            match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
                (ty::ClosureKind::FnOnce, Some((span, place))) => {
                    err.fn_once_label = Some(ClosureFnOnceLabel {
                        span: *span,
                        place: ty::place_to_string_for_capture(self.tcx, place),
                    })
                }
                (ty::ClosureKind::FnMut, Some((span, place))) => {
                    err.fn_mut_label = Some(ClosureFnMutLabel {
                        span: *span,
                        place: ty::place_to_string_for_capture(self.tcx, place),
                    })
                }
                _ => {}
            }
        }

        self.dcx().create_err(err)
    }

    fn report_cyclic_signature_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        found_trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
        expected_trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
        terr: TypeError<'tcx>,
    ) -> Diag<'tcx> {
        let self_ty = found_trait_ref.self_ty().skip_binder();
        let (cause, terr) = if let ty::Closure(def_id, _) = self_ty.kind() {
            (
                ObligationCause::dummy_with_span(self.tcx.def_span(def_id)),
                TypeError::CyclicTy(self_ty),
            )
        } else {
            (obligation.cause.clone(), terr)
        };
        self.report_and_explain_type_error(
            TypeTrace::poly_trait_refs(&cause, true, expected_trait_ref, found_trait_ref),
            terr,
        )
    }

    fn report_opaque_type_auto_trait_leakage(
        &self,
        obligation: &PredicateObligation<'tcx>,
        def_id: DefId,
    ) -> ErrorGuaranteed {
        let name = match self.tcx.opaque_type_origin(def_id.expect_local()) {
            hir::OpaqueTyOrigin::FnReturn(_) | hir::OpaqueTyOrigin::AsyncFn(_) => {
                "opaque type".to_string()
            }
            hir::OpaqueTyOrigin::TyAlias { .. } => {
                format!("`{}`", self.tcx.def_path_debug_str(def_id))
            }
        };
        let mut err = self.dcx().struct_span_err(
            obligation.cause.span,
            format!("cannot check whether the hidden type of {name} satisfies auto traits"),
        );

        err.note(
            "fetching the hidden types of an opaque inside of the defining scope is not supported. \
            You can try moving the opaque type and the item that actually registers a hidden type into a new submodule",
        );
        err.span_note(self.tcx.def_span(def_id), "opaque type is declared here");

        self.note_obligation_cause(&mut err, &obligation);
        self.point_at_returns_when_relevant(&mut err, &obligation);
        self.dcx().try_steal_replace_and_emit_err(self.tcx.def_span(def_id), StashKey::Cycle, err)
    }

    // FIXME(@lcnr): This function could be changed to trait `TraitRef` directly
    // instead of using a `Binder`.
    fn report_signature_mismatch_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        span: Span,
        found_trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
        expected_trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
    ) -> Result<Diag<'tcx>, ErrorGuaranteed> {
        let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
        let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);

        expected_trait_ref.self_ty().error_reported()?;

        let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
            self.dcx().bug("bound vars outside binder");
        };

        let found_did = match *found_trait_ty.kind() {
            ty::Closure(did, _) | ty::FnDef(did, _) | ty::Coroutine(did, ..) => Some(did),
            _ => None,
        };

        let found_node = found_did.and_then(|did| self.tcx.hir().get_if_local(did));
        let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));

        if !self.reported_signature_mismatch.borrow_mut().insert((span, found_span)) {
            // We check closures twice, with obligations flowing in different directions,
            // but we want to complain about them only once.
            return Err(self.dcx().span_delayed_bug(span, "already_reported"));
        }

        let mut not_tupled = false;

        let found = match found_trait_ref.skip_binder().args.type_at(1).kind() {
            ty::Tuple(tys) => vec![ArgKind::empty(); tys.len()],
            _ => {
                not_tupled = true;
                vec![ArgKind::empty()]
            }
        };

        let expected_ty = expected_trait_ref.skip_binder().args.type_at(1);
        let expected = match expected_ty.kind() {
            ty::Tuple(tys) => {
                tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
            }
            _ => {
                not_tupled = true;
                vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
            }
        };

        // If this is a `Fn` family trait and either the expected or found
        // is not tupled, then fall back to just a regular mismatch error.
        // This shouldn't be common unless manually implementing one of the
        // traits manually, but don't make it more confusing when it does
        // happen.
        Ok(
            if Some(expected_trait_ref.def_id()) != self.tcx.lang_items().coroutine_trait()
                && not_tupled
            {
                self.report_and_explain_type_error(
                    TypeTrace::poly_trait_refs(
                        &obligation.cause,
                        true,
                        expected_trait_ref,
                        found_trait_ref,
                    ),
                    ty::error::TypeError::Mismatch,
                )
            } else if found.len() == expected.len() {
                self.report_closure_arg_mismatch(
                    span,
                    found_span,
                    found_trait_ref,
                    expected_trait_ref,
                    obligation.cause.code(),
                    found_node,
                    obligation.param_env,
                )
            } else {
                let (closure_span, closure_arg_span, found) = found_did
                    .and_then(|did| {
                        let node = self.tcx.hir().get_if_local(did)?;
                        let (found_span, closure_arg_span, found) =
                            self.get_fn_like_arguments(node)?;
                        Some((Some(found_span), closure_arg_span, found))
                    })
                    .unwrap_or((found_span, None, found));

                self.report_arg_count_mismatch(
                    span,
                    closure_span,
                    expected,
                    found,
                    found_trait_ty.is_closure(),
                    closure_arg_span,
                )
            },
        )
    }

    fn report_not_const_evaluatable_error(
        &self,
        obligation: &PredicateObligation<'tcx>,
        span: Span,
    ) -> Result<Diag<'tcx>, ErrorGuaranteed> {
        if !self.tcx.features().generic_const_exprs {
            let guar = self
                .dcx()
                .struct_span_err(span, "constant expression depends on a generic parameter")
                // FIXME(const_generics): we should suggest to the user how they can resolve this
                // issue. However, this is currently not actually possible
                // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
                //
                // Note that with `feature(generic_const_exprs)` this case should not
                // be reachable.
                .with_note("this may fail depending on what value the parameter takes")
                .emit();
            return Err(guar);
        }

        match obligation.predicate.kind().skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(ct)) => match ct.kind() {
                ty::ConstKind::Unevaluated(uv) => {
                    let mut err =
                        self.dcx().struct_span_err(span, "unconstrained generic constant");
                    let const_span = self.tcx.def_span(uv.def);
                    match self.tcx.sess.source_map().span_to_snippet(const_span) {
                            Ok(snippet) => err.help(format!(
                                "try adding a `where` bound using this expression: `where [(); {snippet}]:`"
                            )),
                            _ => err.help("consider adding a `where` bound using this expression"),
                        };
                    Ok(err)
                }
                ty::ConstKind::Expr(_) => {
                    let err = self
                        .dcx()
                        .struct_span_err(span, format!("unconstrained generic constant `{ct}`"));
                    Ok(err)
                }
                _ => {
                    bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
                }
            },
            _ => {
                span_bug!(
                    span,
                    "unexpected non-ConstEvaluatable predicate, this should not be reachable"
                )
            }
        }
    }
}