Fix FnOnce impl for AsyncFnOnce closures in new solver

Author: compiler-errors

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);
                 }
 

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);
+}