Migrate a trait selection error to use diagnostic translation

compiler/rustc_middle/src/ty/closure.rs

@@ -7,6 +7,7 @@ use std::fmt::Write;
 
 use crate::query::Providers;
 use rustc_data_structures::fx::FxIndexMap;
+use rustc_errors::{DiagnosticArgValue, IntoDiagnosticArg};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{self as hir, LangItem};
 use rustc_span::def_id::LocalDefIdMap;
@@ -89,10 +90,18 @@ pub enum ClosureKind {
     FnOnce,
 }
 
-impl<'tcx> ClosureKind {
+impl ClosureKind {
     /// This is the initial value used when doing upvar inference.
     pub const LATTICE_BOTTOM: ClosureKind = ClosureKind::Fn;
 
+    pub const fn as_str(self) -> &'static str {
+        match self {
+            ClosureKind::Fn => "Fn",
+            ClosureKind::FnMut => "FnMut",
+            ClosureKind::FnOnce => "FnOnce",
+        }
+    }
+
     /// Returns `true` if a type that impls this closure kind
     /// must also implement `other`.
     pub fn extends(self, other: ty::ClosureKind) -> bool {
@@ -115,7 +124,7 @@ impl<'tcx> ClosureKind {
 
     /// Returns the representative scalar type for this closure kind.
     /// See `Ty::to_opt_closure_kind` for more details.
-    pub fn to_ty(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+    pub fn to_ty<'tcx>(self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
         match self {
             ClosureKind::Fn => tcx.types.i8,
             ClosureKind::FnMut => tcx.types.i16,
@@ -124,6 +133,12 @@ impl<'tcx> ClosureKind {
     }
 }
 
+impl IntoDiagnosticArg for ClosureKind {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        DiagnosticArgValue::Str(self.as_str().into())
+    }
+}
+
 /// A composite describing a `Place` that is captured by a closure.
 #[derive(PartialEq, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
 #[derive(TypeFoldable, TypeVisitable)]

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2870,11 +2870,7 @@ define_print_and_forward_display! {
     }
 
     ty::ClosureKind {
-        match *self {
-            ty::ClosureKind::Fn => p!("Fn"),
-            ty::ClosureKind::FnMut => p!("FnMut"),
-            ty::ClosureKind::FnOnce => p!("FnOnce"),
-        }
+        p!(write("{}", self.as_str()))
     }
 
     ty::Predicate<'tcx> {

compiler/rustc_trait_selection/messages.ftl

@@ -8,6 +8,15 @@ trait_selection_adjust_signature_remove_borrow = consider adjusting the signatur
         *[other] arguments
     }
 
+trait_selection_closure_fn_mut_label = closure is `FnMut` because it mutates the variable `{$place}` here
+
+trait_selection_closure_fn_once_label = closure is `FnOnce` because it moves the variable `{$place}` out of its environment
+
+trait_selection_closure_kind_mismatch = expected a closure that implements the `{$expected}` trait, but this closure only implements `{$found}`
+    .label = this closure implements `{$found}`, not `{$expected}`
+
+trait_selection_closure_kind_requirement = the requirement to implement `{$expected}` derives from here
+
 trait_selection_dump_vtable_entries = vtable entries for `{$trait_ref}`: {$entries}
 
 trait_selection_empty_on_clause_in_rustc_on_unimplemented = empty `on`-clause in `#[rustc_on_unimplemented]`

compiler/rustc_trait_selection/src/errors.rs

@@ -4,7 +4,7 @@ use rustc_errors::{
     SubdiagnosticMessage,
 };
 use rustc_macros::Diagnostic;
-use rustc_middle::ty::{self, PolyTraitRef, Ty};
+use rustc_middle::ty::{self, ClosureKind, PolyTraitRef, Ty};
 use rustc_span::{Span, Symbol};
 
 #[derive(Diagnostic)]
@@ -131,3 +131,37 @@ impl AddToDiagnostic for AdjustSignatureBorrow {
         }
     }
 }
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_closure_kind_mismatch, code = "E0525")]
+pub struct ClosureKindMismatch {
+    #[primary_span]
+    #[label]
+    pub closure_span: Span,
+    pub expected: ClosureKind,
+    pub found: ClosureKind,
+    #[label(trait_selection_closure_kind_requirement)]
+    pub cause_span: Span,
+
+    #[subdiagnostic]
+    pub fn_once_label: Option<ClosureFnOnceLabel>,
+
+    #[subdiagnostic]
+    pub fn_mut_label: Option<ClosureFnMutLabel>,
+}
+
+#[derive(Subdiagnostic)]
+#[label(trait_selection_closure_fn_once_label)]
+pub struct ClosureFnOnceLabel {
+    #[primary_span]
+    pub span: Span,
+    pub place: String,
+}
+
+#[derive(Subdiagnostic)]
+#[label(trait_selection_closure_fn_mut_label)]
+pub struct ClosureFnMutLabel {
+    #[primary_span]
+    pub span: Span,
+    pub place: String,
+}

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -7,6 +7,7 @@ use super::{
     ObligationCauseCode, ObligationCtxt, OutputTypeParameterMismatch, Overflow,
     PredicateObligation, SelectionError, TraitNotObjectSafe,
 };
+use crate::errors::{ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch};
 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{self, InferCtxt};
@@ -3142,55 +3143,38 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         kind: ty::ClosureKind,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let closure_span = self.tcx.def_span(closure_def_id);
-        let mut err = struct_span_err!(
-            self.tcx.sess,
-            closure_span,
-            E0525,
-            "expected a closure that implements the `{}` trait, \
-                but this closure only implements `{}`",
-            kind,
-            found_kind
-        );
 
-        err.span_label(
+        let mut err = ClosureKindMismatch {
             closure_span,
-            format!("this closure implements `{found_kind}`, not `{kind}`"),
-        );
-        err.span_label(
-            obligation.cause.span,
-            format!("the requirement to implement `{kind}` derives from here"),
-        );
+            expected: kind,
+            found: found_kind,
+            cause_span: obligation.cause.span,
+            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.hir().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.span_label(
-                        *span,
-                        format!(
-                            "closure is `FnOnce` because it moves the \
-                            variable `{}` out of its environment",
-                            ty::place_to_string_for_capture(self.tcx, 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.span_label(
-                        *span,
-                        format!(
-                            "closure is `FnMut` because it mutates the \
-                            variable `{}` here",
-                            ty::place_to_string_for_capture(self.tcx, place)
-                        ),
-                    );
+                    err.fn_mut_label = Some(ClosureFnMutLabel {
+                        span: *span,
+                        place: ty::place_to_string_for_capture(self.tcx, &place),
+                    })
                 }
                 _ => {}
             }
         }
 
-        err
+        self.tcx.sess.create_err(err)
     }
 
     fn report_type_parameter_mismatch_cyclic_type_error(