Add note

Author: compiler-errors

compiler/rustc_hir_analysis/messages.ftl

@@ -226,6 +226,7 @@ hir_analysis_rpitit_refined = impl trait in impl method signature does not match
     .suggestion = replace the return type so that it matches the trait
     .label = return type from trait method defined here
     .unmatched_bound_label = this bound is stronger than that defined on the trait
+    .note = add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
 
 hir_analysis_self_in_impl_self =
     `Self` is not valid in the self type of an impl block

compiler/rustc_hir_analysis/src/errors.rs

@@ -921,6 +921,7 @@ pub struct UnusedAssociatedTypeBounds {
 
 #[derive(LintDiagnostic)]
 #[diag(hir_analysis_rpitit_refined)]
+#[note]
 pub(crate) struct ReturnPositionImplTraitInTraitRefined<'tcx> {
     #[suggestion(applicability = "maybe-incorrect", code = "{pre}{return_ty}{post}")]
     pub impl_return_span: Span,

tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.stderr

@@ -21,6 +21,7 @@ LL |     fn iter(&self) -> impl '_ + Iterator<Item = Self::Item<'_>>;
 LL |     fn iter(&self) -> impl 'a + Iterator<Item = I::Item<'a>> {
    |                            ^^ this bound is stronger than that defined on the trait
    |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
    = note: `#[warn(refining_impl_trait)]` on by default
 help: replace the return type so that it matches the trait
    |

tests/ui/impl-trait/in-trait/refine.stderr

@@ -7,6 +7,7 @@ LL |     fn bar() -> impl Sized;
 LL |     fn bar() -> impl Copy {}
    |                      ^^^^ this bound is stronger than that defined on the trait
    |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
 note: the lint level is defined here
   --> $DIR/refine.rs:2:9
    |
@@ -26,6 +27,7 @@ LL |     fn bar() -> impl Sized;
 LL |     fn bar() {}
    |     ^^^^^^^^
    |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
 help: replace the return type so that it matches the trait
    |
 LL |     fn bar() -> impl Sized {}
@@ -40,6 +42,7 @@ LL |     fn bar() -> impl Sized;
 LL |     fn bar() -> () {}
    |                 ^^
    |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
 help: replace the return type so that it matches the trait
    |
 LL |     fn bar() -> impl Sized {}
@@ -54,6 +57,7 @@ LL |     fn bar<'a>(&'a self) -> impl Sized + 'a;
 LL |     fn bar(&self) -> impl Copy + '_ {}
    |                           ^^^^ this bound is stronger than that defined on the trait
    |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
 help: replace the return type so that it matches the trait
    |
 LL |     fn bar(&self) -> impl Sized + '_ {}