Add note

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,