Add a helpful suggestion

Author: compiler-errors

compiler/rustc_ast_lowering/messages.ftl

@@ -10,6 +10,7 @@ ast_lowering_argument = argument
 
 ast_lowering_assoc_ty_binding_in_dyn =
     associated type bounds are not allowed in `dyn` types
+    .suggestion = use `impl` to introduce a type instead
 
 ast_lowering_assoc_ty_parentheses =
     parenthesized generic arguments cannot be used in associated type constraints

compiler/rustc_ast_lowering/src/errors.rs

@@ -102,6 +102,8 @@ pub struct MisplacedImplTrait<'a> {
 pub struct MisplacedAssocTyBinding {
     #[primary_span]
     pub span: Span,
+    #[suggestion(code = " = impl", applicability = "maybe-incorrect", style = "verbose")]
+    pub suggestion: Option<Span>,
 }
 
 #[derive(Diagnostic, Clone, Copy)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -1081,13 +1081,29 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             AssocConstraintKind::Bound { bounds } => {
                 enum DesugarKind {
                     ImplTrait,
-                    Error,
+                    Error(Option<Span>),
                     Bound,
                 }
 
                 // Piggy-back on the `impl Trait` context to figure out the correct behavior.
                 let desugar_kind = match itctx {
-                    _ if self.is_in_dyn_type => DesugarKind::Error,
+                    ImplTraitContext::ReturnPositionOpaqueTy { .. }
+                    | ImplTraitContext::TypeAliasesOpaqueTy { .. }
+                    | ImplTraitContext::Universal
+                        if self.is_in_dyn_type =>
+                    {
+                        let bound_end_span = constraint
+                            .gen_args
+                            .as_ref()
+                            .map_or(constraint.ident.span, |args| args.span());
+                        let colon_span = if bound_end_span.eq_ctxt(constraint.span) {
+                            Some(self.tcx.sess.source_map().next_point(bound_end_span))
+                        } else {
+                            None
+                        };
+                        DesugarKind::Error(colon_span)
+                    }
+                    _ if self.is_in_dyn_type => DesugarKind::Error(None),
 
                     // We are in the return position:
                     //
@@ -1143,10 +1159,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                         hir::TypeBindingKind::Constraint { bounds }
                     }
-                    DesugarKind::Error => {
-                        let guar = self
-                            .dcx()
-                            .emit_err(errors::MisplacedAssocTyBinding { span: constraint.span });
+                    DesugarKind::Error(suggestion) => {
+                        let guar = self.dcx().emit_err(errors::MisplacedAssocTyBinding {
+                            span: constraint.span,
+                            suggestion,
+                        });
                         let err_ty =
                             &*self.arena.alloc(self.ty(constraint.span, hir::TyKind::Err(guar)));
                         hir::TypeBindingKind::Equality { term: err_ty.into() }

tests/ui/associated-type-bounds/assoc-type-eq-with-dyn-atb-fail.stderr

@@ -3,6 +3,11 @@ error: associated type bounds are not allowed in `dyn` types
    |
 LL |     type Out = Box<dyn Bar<Assoc: Copy>>;
    |                            ^^^^^^^^^^^
+   |
+help: use `impl` to introduce a type instead
+   |
+LL |     type Out = Box<dyn Bar<Assoc = impl Copy>>;
+   |                                  ~~~~~~
 
 error: aborting due to 1 previous error
 

tests/ui/associated-type-bounds/elision.stderr

@@ -15,6 +15,11 @@ error: associated type bounds are not allowed in `dyn` types
    |
 LL | fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: use `impl` to introduce a type instead
+   |
+LL | fn f(x: &mut dyn Iterator<Item = impl Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
+   |                                ~~~~~~
 
 error: aborting due to 2 previous errors