From 353683e4e9590d80e82902e5d538dff15e0f4663 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sat, 30 Sep 2023 19:28:40 +0000
Subject: [PATCH] Only reachable traits

---
 compiler/rustc_lint/src/async_fn_in_trait.rs |  6 ++++++
 tests/ui/async-await/in-trait/warn.rs        | 16 ++++++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_lint/src/async_fn_in_trait.rs b/compiler/rustc_lint/src/async_fn_in_trait.rs
index 2ef9627eac54a..ba1a3b5eb16d5 100644
--- a/compiler/rustc_lint/src/async_fn_in_trait.rs
+++ b/compiler/rustc_lint/src/async_fn_in_trait.rs
@@ -92,10 +92,16 @@ 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 {
diff --git a/tests/ui/async-await/in-trait/warn.rs b/tests/ui/async-await/in-trait/warn.rs
index defbdcffccd7f..4f981c31f5c06 100644
--- a/tests/ui/async-await/in-trait/warn.rs
+++ b/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() {}