Actually use the right closure kind when checking async Fn goals

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -934,7 +934,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))
             }
             ty::Closure(_, args) => {
-                let sig = args.as_closure().sig();
+                let args = args.as_closure();
+                let sig = args.sig();
                 let trait_ref = sig.map_bound(|sig| {
                     ty::TraitRef::new(
                         self.tcx(),
@@ -950,7 +951,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         ty::TraitRef::new(tcx, future_trait_def_id, [sig.output()])
                     }),
                 ));
-                (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))
+                (trait_ref, args.kind_ty())
             }
             _ => bug!("expected callable type for AsyncFn candidate"),
         };

tests/ui/async-await/async-closures/wrong-fn-kind.rs

@@ -1,7 +1,5 @@
 //@ edition:2021
 
-// FIXME(async_closures): This needs a better error message!
-
 #![feature(async_closure)]
 
 fn main() {
@@ -12,4 +10,10 @@ fn main() {
         //~^ ERROR expected a closure that implements the `async Fn` trait, but this closure only implements `async FnMut`
         x += 1;
     });
+
+    let x = String::new();
+    needs_async_fn(move || async move {
+        //~^ ERROR expected a closure that implements the `async Fn` trait, but this closure only implements `async FnOnce`
+        println!("{x}");
+    });
 }

tests/ui/async-await/async-closures/wrong-fn-kind.stderr

@@ -1,5 +1,5 @@
 error[E0525]: expected a closure that implements the `async Fn` trait, but this closure only implements `async FnMut`
-  --> $DIR/wrong-fn-kind.rs:11:20
+  --> $DIR/wrong-fn-kind.rs:9:20
    |
 LL |       needs_async_fn(async || {
    |       -------------- -^^^^^^^
@@ -14,11 +14,32 @@ LL | |     });
    | |_____- the requirement to implement `async Fn` derives from here
    |
 note: required by a bound in `needs_async_fn`
-  --> $DIR/wrong-fn-kind.rs:8:31
+  --> $DIR/wrong-fn-kind.rs:6:31
    |
 LL |     fn needs_async_fn(_: impl async Fn()) {}
    |                               ^^^^^^^^^^ required by this bound in `needs_async_fn`
 
-error: aborting due to 1 previous error
+error[E0525]: expected a closure that implements the `async Fn` trait, but this closure only implements `async FnOnce`
+  --> $DIR/wrong-fn-kind.rs:15:20
+   |
+LL |       needs_async_fn(move || async move {
+   |       -------------- -^^^^^^
+   |       |              |
+   |  _____|______________this closure implements `async FnOnce`, not `async Fn`
+   | |     |
+   | |     required by a bound introduced by this call
+LL | |
+LL | |         println!("{x}");
+   | |                    - closure is `async FnOnce` because it moves the variable `x` out of its environment
+LL | |     });
+   | |_____- the requirement to implement `async Fn` derives from here
+   |
+note: required by a bound in `needs_async_fn`
+  --> $DIR/wrong-fn-kind.rs:6:31
+   |
+LL |     fn needs_async_fn(_: impl async Fn()) {}
+   |                               ^^^^^^^^^^ required by this bound in `needs_async_fn`
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0525`.