Make sure that async closures (and fns) only capture their parent callable's parameters by move, and nothing else

1 files changed, 5 insertions, 4 deletions

diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 6b772c1295f..a122b4c5113 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -1278,10 +1278,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         };
         let closure_id = coroutine_kind.closure_id();
         let coroutine_expr = self.make_desugared_coroutine_expr(
-            // FIXME(async_closures): This should only move locals,
-            // and not upvars. Capturing closure upvars by ref doesn't
-            // work right now anyways, so whatever.
-            CaptureBy::Value { move_kw: rustc_span::DUMMY_SP },
+            // The default capture mode here is by-ref. Later on during upvar analysis,
+            // we will force the captured arguments to by-move, but for async closures,
+            // we want to make sure that we avoid unnecessarily moving captures, or else
+            // all async closures would default to `FnOnce` as their calling mode.
+            CaptureBy::Ref,
             closure_id,
             return_type_hint,
             body_span,