diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs index 9743dc7c69fbc..61a7d2405e5ed 100644 --- a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs +++ b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs @@ -874,9 +874,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> { } else { // Only point to return type if the expected type is the return type, as if they // are not, the expectation must have been caused by something else. - debug!("return type {:?}", hir_ty); + debug!("return type (hir::Ty) {:?}", hir_ty); let ty = self.lowerer().lower_ty(hir_ty); - debug!("return type {:?}", ty); + debug!("return type (ty) {:?}", ty); debug!("expected type {:?}", expected); let bound_vars = self.tcx.late_bound_vars(hir_ty.hir_id.owner.into()); let ty = Binder::bind_with_vars(ty, bound_vars); @@ -888,7 +888,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> { errors::ExpectedReturnTypeLabel::Other { span: hir_ty.span, expected }, ); self.try_suggest_return_impl_trait(err, expected, found, fn_id); - self.note_caller_chooses_ty_for_ty_param(err, expected, found); + self.try_note_caller_chooses_ty_for_ty_param(err, expected, found); return true; } } @@ -898,21 +898,34 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> { false } - fn note_caller_chooses_ty_for_ty_param( + fn try_note_caller_chooses_ty_for_ty_param( &self, diag: &mut Diag<'_>, expected: Ty<'tcx>, found: Ty<'tcx>, ) { - if let ty::Param(expected_ty_as_param) = expected.kind() { - diag.subdiagnostic( - self.dcx(), - errors::NoteCallerChoosesTyForTyParam { - ty_param_name: expected_ty_as_param.name, - found_ty: found, - }, - ); + // Only show the note if: + // 1. `expected` ty is a type parameter; + // 2. The `expected` type parameter does *not* occur in the return expression type. This can + // happen for e.g. `fn foo(t: &T) -> T { t }`, where `expected` is `T` but `found` is + // `&T`. Saying "the caller chooses a type for `T` which can be different from `&T`" is + // "well duh" and is only confusing and not helpful. + + let ty::Param(expected_ty_as_param) = expected.kind() else { + return; + }; + + if found.contains(expected) { + return; } + + diag.subdiagnostic( + self.dcx(), + errors::NoteCallerChoosesTyForTyParam { + ty_param_name: expected_ty_as_param.name, + found_ty: found, + }, + ); } /// check whether the return type is a generic type with a trait bound