Only reachable traits

compiler/rustc_lint/src/async_fn_in_trait.rs

@@ -93,10 +93,16 @@ impl<'tcx> LateLintPass<'tcx> for AsyncFnInTrait {
  if let hir::TraitItemKind::Fn(sig, body) = item.kind
  && let hir::IsAsync::Async(async_span) = sig.header.asyncness
  {
+ // RTN can be used to bound `async fn` in traits in a better way than "always"
  if cx.tcx.features().return_type_notation {
  return;
  }
 
+ // Only need to think about library implications of reachable traits
+ if !cx.tcx.effective_visibilities(()).is_reachable(item.owner_id.def_id) {
+ return;
+ }
+
  let hir::FnRetTy::Return(hir::Ty { kind: hir::TyKind::OpaqueDef(def, ..), .. }) =
  sig.decl.output
  else {

tests/ui/async-await/in-trait/warn.rs

@@ -3,9 +3,21 @@
 #![feature(async_fn_in_trait)]
 #![deny(async_fn_in_trait)]
 
-trait Foo {
+pub trait Foo {
  async fn not_send();
- //~^ ERROR
+ //~^ ERROR use of `async fn` in public traits is discouraged
+}
+
+mod private {
+ pub trait FooUnreachable {
+ async fn not_send();
+ // No warning
+ }
+}
+
+pub(crate) trait FooCrate {
+ async fn not_send();
+ // No warning
 }
 
 fn main() {}