fix refining_impl_trait suggestion with return_type_notation

Author: enthropy7

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -6,6 +6,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint_defs::builtin::{REFINING_IMPL_TRAIT_INTERNAL, REFINING_IMPL_TRAIT_REACHABLE};
 use rustc_middle::span_bug;
 use rustc_middle::traits::ObligationCause;
+use rustc_middle::ty::print::with_types_for_signature;
 use rustc_middle::ty::{
     self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable,
     TypeVisitableExt, TypeVisitor, TypingMode,
@@ -332,6 +333,10 @@ fn report_mismatched_rpitit_signature<'tcx>(
             hir::FnRetTy::Return(ty) => ty.span,
         });
 
+    // Use ForSignature mode to ensure RPITITs are printed as `impl Trait` rather than
+    // `impl Trait { T::method(..) }` when RTN is enabled.
+    let return_ty_suggestion = with_types_for_signature!(format!("{return_ty}"));
+
     let span = unmatched_bound.unwrap_or(span);
     tcx.emit_node_span_lint(
         if is_internal { REFINING_IMPL_TRAIT_INTERNAL } else { REFINING_IMPL_TRAIT_REACHABLE },
@@ -342,7 +347,7 @@ fn report_mismatched_rpitit_signature<'tcx>(
             trait_return_span,
             pre,
             post,
-            return_ty,
+            return_ty: return_ty_suggestion,
             unmatched_bound,
         },
     );

compiler/rustc_hir_analysis/src/errors.rs

@@ -1023,7 +1023,7 @@ pub(crate) struct UnusedAssociatedTypeBounds {
 #[diag(hir_analysis_rpitit_refined)]
 #[note]
 #[note(hir_analysis_feedback_note)]
-pub(crate) struct ReturnPositionImplTraitInTraitRefined<'tcx> {
+pub(crate) struct ReturnPositionImplTraitInTraitRefined {
     #[suggestion(applicability = "maybe-incorrect", code = "{pre}{return_ty}{post}")]
     pub impl_return_span: Span,
     #[label]
@@ -1033,7 +1033,7 @@ pub(crate) struct ReturnPositionImplTraitInTraitRefined<'tcx> {
 
     pub pre: &'static str,
     pub post: &'static str,
-    pub return_ty: Ty<'tcx>,
+    pub return_ty: String,
 }
 
 #[derive(LintDiagnostic)]

tests/ui/impl-trait/in-trait/refine-return-type-notation.rs

@@ -0,0 +1,13 @@
+#![feature(return_type_notation)]
+#![deny(refining_impl_trait)]
+
+trait Trait {
+    fn f() -> impl Sized;
+}
+
+impl Trait for () {
+    fn f() {}
+    //~^ ERROR impl trait in impl method signature does not match trait method signature
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/refine-return-type-notation.stderr

@@ -0,0 +1,24 @@
+error: impl trait in impl method signature does not match trait method signature
+  --> $DIR/refine-return-type-notation.rs:9:5
+   |
+LL |     fn f() -> impl Sized;
+   |               ---------- return type from trait method defined here
+...
+LL |     fn f() {}
+   |     ^^^^^^
+   |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
+   = note: we are soliciting feedback, see issue #121718 <https://github.com/rust-lang/rust/issues/121718> for more information
+note: the lint level is defined here
+  --> $DIR/refine-return-type-notation.rs:2:9
+   |
+LL | #![deny(refining_impl_trait)]
+   |         ^^^^^^^^^^^^^^^^^^^
+   = note: `#[deny(refining_impl_trait_internal)]` implied by `#[deny(refining_impl_trait)]`
+help: replace the return type so that it matches the trait
+   |
+LL |     fn f()-> impl Sized  {}
+   |           +++++++++++++
+
+error: aborting due to 1 previous error
+