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 {