trait_impl_difference.rs

//! Error Reporting for `impl` items that do not match the obligations from their `trait`.

use crate::infer::error_reporting::nice_region_error::NiceRegionError;
use crate::infer::lexical_region_resolve::RegionResolutionError;
use crate::infer::{Subtype, TyCtxtInferExt, ValuePairs};
use crate::traits::ObligationCauseCode::CompareImplMethodObligation;
use rustc_errors::ErrorReported;
use rustc_hir as hir;
use rustc_hir::def::Res;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::Visitor;
use rustc_middle::ty::error::ExpectedFound;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_span::{MultiSpan, Span};

impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
    /// Print the error message for lifetime errors when the `impl` doesn't conform to the `trait`.
    pub(super) fn try_report_impl_not_conforming_to_trait(&self) -> Option<ErrorReported> {
        if let Some(ref error) = self.error {
            debug!("try_report_impl_not_conforming_to_trait {:?}", error);
            if let RegionResolutionError::SubSupConflict(
                _,
                var_origin,
                sub_origin,
                _sub,
                sup_origin,
                _sup,
            ) = error.clone()
            {
                if let (&Subtype(ref sup_trace), &Subtype(ref sub_trace)) =
                    (&sup_origin, &sub_origin)
                {
                    if let (
                        ValuePairs::Types(sub_expected_found),
                        ValuePairs::Types(sup_expected_found),
                        CompareImplMethodObligation { trait_item_def_id, .. },
                    ) = (&sub_trace.values, &sup_trace.values, &sub_trace.cause.code)
                    {
                        if sup_expected_found == sub_expected_found {
                            self.emit_err(
                                var_origin.span(),
                                sub_expected_found.expected,
                                sub_expected_found.found,
                                *trait_item_def_id,
                            );
                            return Some(ErrorReported);
                        }
                    }
                }
            }
        }
        None
    }

    fn emit_err(&self, sp: Span, expected: Ty<'tcx>, found: Ty<'tcx>, trait_def_id: DefId) {
        let tcx = self.tcx();
        let trait_sp = self.tcx().def_span(trait_def_id);
        let mut err = self
            .tcx()
            .sess
            .struct_span_err(sp, "`impl` item signature doesn't match `trait` item signature");
        err.span_label(sp, &format!("found `{:?}`", found));
        err.span_label(trait_sp, &format!("expected `{:?}`", expected));

        // Get the span of all the used type parameters in the method.
        let assoc_item = self.tcx().associated_item(trait_def_id);
        let mut visitor = TypeParamSpanVisitor { tcx: self.tcx(), types: vec![] };
        match assoc_item.kind {
            ty::AssocKind::Fn => {
                let hir = self.tcx().hir();
                if let Some(hir_id) = assoc_item.def_id.as_local().map(|id| hir.as_local_hir_id(id))
                {
                    if let Some(decl) = hir.fn_decl_by_hir_id(hir_id) {
                        visitor.visit_fn_decl(decl);
                    }
                }
            }
            _ => {}
        }
        let mut type_param_span: MultiSpan = visitor.types.to_vec().into();
        for &span in &visitor.types {
            type_param_span.push_span_label(
                span,
                "consider borrowing this type parameter in the trait".to_string(),
            );
        }

        if let Some((expected, found)) = tcx
            .infer_ctxt()
            .enter(|infcx| infcx.expected_found_str_ty(&ExpectedFound { expected, found }))
        {
            // Highlighted the differences when showing the "expected/found" note.
            err.note_expected_found(&"", expected, &"", found);
        } else {
            // This fallback shouldn't be necessary, but let's keep it in just in case.
            err.note(&format!("expected `{:?}`\n   found `{:?}`", expected, found));
        }
        err.span_help(
            type_param_span,
            "the lifetime requirements from the `impl` do not correspond to the requirements in \
             the `trait`",
        );
        if visitor.types.is_empty() {
            err.help(
                "verify the lifetime relationships in the `trait` and `impl` between the `self` \
                 argument, the other inputs and its output",
            );
        }
        err.emit();
    }
}

struct TypeParamSpanVisitor<'tcx> {
    tcx: TyCtxt<'tcx>,
    types: Vec<Span>,
}

impl Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
    type Map = rustc_middle::hir::map::Map<'tcx>;

    fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
        hir::intravisit::NestedVisitorMap::OnlyBodies(self.tcx.hir())
    }

    fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
        match arg.kind {
            hir::TyKind::Rptr(_, ref mut_ty) => {
                // We don't want to suggest looking into borrowing `&T` or `&Self`.
                hir::intravisit::walk_ty(self, mut_ty.ty);
                return;
            }
            hir::TyKind::Path(hir::QPath::Resolved(None, path)) => match &path.segments {
                [segment]
                    if segment
                        .res
                        .map(|res| match res {
                            Res::SelfTy(_, _) | Res::Def(hir::def::DefKind::TyParam, _) => true,
                            _ => false,
                        })
                        .unwrap_or(false) =>
                {
                    self.types.push(path.span);
                }
                _ => {}
            },
            _ => {}
        }
        hir::intravisit::walk_ty(self, arg);
    }
}