From ca912d794d42336ddd54389480b56bfc2bc6685f Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 21 May 2025 14:55:47 +0000
Subject: [PATCH 1/2] Make captures state error more precise

---
 .../src/error_reporting/traits/fulfillment_errors.rs | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
index 970160ba212af..7d8c4df6341c4 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -841,16 +841,17 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 return None;
             };
 
-        let (closure_def_id, found_args, by_ref_captures) = match *self_ty.kind() {
+        let (closure_def_id, found_args, has_self_borrows) = match *self_ty.kind() {
             ty::Closure(def_id, args) => {
-                (def_id, args.as_closure().sig().map_bound(|sig| sig.inputs()[0]), None)
+                (def_id, args.as_closure().sig().map_bound(|sig| sig.inputs()[0]), false)
             }
             ty::CoroutineClosure(def_id, args) => (
                 def_id,
                 args.as_coroutine_closure()
                     .coroutine_closure_sig()
                     .map_bound(|sig| sig.tupled_inputs_ty),
-                Some(args.as_coroutine_closure().coroutine_captures_by_ref_ty()),
+                !args.as_coroutine_closure().tupled_upvars_ty().is_ty_var()
+                    && args.as_coroutine_closure().has_self_borrows(),
             ),
             _ => return None,
         };
@@ -884,10 +885,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         // If the closure has captures, then perhaps the reason that the trait
         // is unimplemented is because async closures don't implement `Fn`/`FnMut`
         // if they have captures.
-        if let Some(by_ref_captures) = by_ref_captures
-            && let ty::FnPtr(sig_tys, _) = by_ref_captures.kind()
-            && !sig_tys.skip_binder().output().is_unit()
-        {
+        if has_self_borrows && expected_kind != ty::ClosureKind::FnOnce {
             let mut err = self.dcx().create_err(AsyncClosureNotFn {
                 span: self.tcx.def_span(closure_def_id),
                 kind: expected_kind.as_str(),

From e0f80558716372902b2eda1e43ab2e169bfb3669 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 21 May 2025 18:07:29 +0000
Subject: [PATCH 2/2] Fix FnOnce impl for AsyncFn/AsyncFnMut closures in new
 solver

---
 .../src/solve/assembly/structural_traits.rs       |  2 +-
 .../async-closures/async-fn-mut-impl-fn-once.rs   | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/async-await/async-closures/async-fn-mut-impl-fn-once.rs

diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
index 2a2b462a36cbd..26ad72bea806c 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
@@ -327,7 +327,7 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_callable<I: Intern
                 // always be called once. It additionally implements `Fn`/`FnMut`
                 // only if it has no upvars referencing the closure-env lifetime,
                 // and if the closure kind permits it.
-                if closure_kind != ty::ClosureKind::FnOnce && args.has_self_borrows() {
+                if goal_kind != ty::ClosureKind::FnOnce && args.has_self_borrows() {
                     return Err(NoSolution);
                 }
 
diff --git a/tests/ui/async-await/async-closures/async-fn-mut-impl-fn-once.rs b/tests/ui/async-await/async-closures/async-fn-mut-impl-fn-once.rs
new file mode 100644
index 0000000000000..59d034953d700
--- /dev/null
+++ b/tests/ui/async-await/async-closures/async-fn-mut-impl-fn-once.rs
@@ -0,0 +1,15 @@
+//@ edition:2021
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+fn call_once<F>(_: impl FnOnce() -> F) {}
+
+fn main() {
+    let mut i = 0;
+    let c = async || {
+        i += 1;
+    };
+    call_once(c);
+}