From cfbe6af4f25d12f7eade4106f978363d11c4d125 Mon Sep 17 00:00:00 2001
From: cijiugechu <cijiugechu@gmail.com>
Date: Thu, 28 May 2026 10:15:29 +0800
Subject: [PATCH] Fix tupled closure signature in AsyncFn arg mismatch
 diagnostic

---
 .../rustc_middle/src/middle/lang_items.rs     | 19 +++++++++++++++
 .../src/error_reporting/traits/suggestions.rs |  2 +-
 .../async-fn/closure-arg-type-mismatch.rs     |  9 +++++++
 .../async-fn/closure-arg-type-mismatch.stderr | 24 +++++++++++++++++++
 4 files changed, 53 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/async-await/async-fn/closure-arg-type-mismatch.rs
 create mode 100644 tests/ui/async-await/async-fn/closure-arg-type-mismatch.stderr

diff --git a/compiler/rustc_middle/src/middle/lang_items.rs b/compiler/rustc_middle/src/middle/lang_items.rs
index a0e4c288c4a85..8a2595e2938b8 100644
--- a/compiler/rustc_middle/src/middle/lang_items.rs
+++ b/compiler/rustc_middle/src/middle/lang_items.rs
@@ -55,6 +55,25 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// Given a [`DefId`], returns whether it is one of the built-in callable
+    /// traits: `Fn`/`FnMut`/`FnOnce` or `AsyncFn`/`AsyncFnMut`/`AsyncFnOnce`.
+    ///
+    /// These built-in callable traits all model their inputs using the
+    /// `rust-call` ABI, which is tupled at the type level.
+    pub fn is_callable_trait(self, id: DefId) -> bool {
+        matches!(
+            self.as_lang_item(id),
+            Some(
+                LangItem::Fn
+                    | LangItem::FnMut
+                    | LangItem::FnOnce
+                    | LangItem::AsyncFn
+                    | LangItem::AsyncFnMut
+                    | LangItem::AsyncFnOnce
+            )
+        )
+    }
+
     /// Given a [`ty::ClosureKind`], get the [`DefId`] of its corresponding `Fn`-family
     /// trait, if it is defined.
     pub fn fn_trait_kind_to_def_id(self, kind: ty::ClosureKind) -> Option<DefId> {
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 66eaa49cbd5d4..beea76434f1b3 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -2346,7 +2346,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         ) -> Ty<'tcx> {
             let inputs = trait_ref.args.type_at(1);
             let sig = match inputs.kind() {
-                ty::Tuple(inputs) if infcx.tcx.is_fn_trait(trait_ref.def_id) => {
+                ty::Tuple(inputs) if infcx.tcx.is_callable_trait(trait_ref.def_id) => {
                     infcx.tcx.mk_fn_sig_safe_rust_abi(*inputs, infcx.next_ty_var(DUMMY_SP))
                 }
                 _ => infcx.tcx.mk_fn_sig_safe_rust_abi([inputs], infcx.next_ty_var(DUMMY_SP)),
diff --git a/tests/ui/async-await/async-fn/closure-arg-type-mismatch.rs b/tests/ui/async-await/async-fn/closure-arg-type-mismatch.rs
new file mode 100644
index 0000000000000..be1ad20e4c221
--- /dev/null
+++ b/tests/ui/async-await/async-fn/closure-arg-type-mismatch.rs
@@ -0,0 +1,9 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/155636>
+//@ edition:2021
+
+fn foo(_: impl AsyncFn(&mut i32)) {}
+
+fn main() {
+    foo(|_: i32| async {});
+    //~^ ERROR type mismatch in closure arguments
+}
diff --git a/tests/ui/async-await/async-fn/closure-arg-type-mismatch.stderr b/tests/ui/async-await/async-fn/closure-arg-type-mismatch.stderr
new file mode 100644
index 0000000000000..a3b47bb448d17
--- /dev/null
+++ b/tests/ui/async-await/async-fn/closure-arg-type-mismatch.stderr
@@ -0,0 +1,24 @@
+error[E0631]: type mismatch in closure arguments
+  --> $DIR/closure-arg-type-mismatch.rs:7:5
+   |
+LL |     foo(|_: i32| async {});
+   |     ^^^^--------^^^^^^^^^^
+   |     |   |
+   |     |   found signature defined here
+   |     expected due to this
+   |
+   = note: expected closure signature `for<'a> fn(&'a mut _) -> _`
+              found closure signature `fn(_) -> _`
+note: required by a bound in `foo`
+  --> $DIR/closure-arg-type-mismatch.rs:4:16
+   |
+LL | fn foo(_: impl AsyncFn(&mut i32)) {}
+   |                ^^^^^^^^^^^^^^^^^ required by this bound in `foo`
+help: consider adjusting the signature so it borrows its argument
+   |
+LL |     foo(|_: &mut i32| async {});
+   |             ++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0631`.