fix: Fix panicking while resolving callable sigs for AsyncFnMut

Author: ShoyuVanilla

crates/hir-ty/src/infer/unify.rs

@@ -674,10 +674,13 @@ impl<'db> InferenceTable<'db> {
             let args = [ty, arg_ty];
             let trait_ref = TraitRef::new(self.interner(), fn_trait.into(), args);
 
+            let proj_args = self
+                .infer_ctxt
+                .fill_rest_fresh_args(output_assoc_type.into(), args.into_iter().map(Into::into));
             let projection = Ty::new_alias(
                 self.interner(),
                 rustc_type_ir::AliasTyKind::Projection,
-                AliasTy::new(self.interner(), output_assoc_type.into(), args),
+                AliasTy::new(self.interner(), output_assoc_type.into(), proj_args),
             );
 
             let pred = Predicate::upcast_from(trait_ref, self.interner());

crates/hir-ty/src/tests/regression/new_solver.rs

@@ -524,3 +524,31 @@ fn g(it: *const (dyn Trait)) {
 "#,
     );
 }
+
+#[test]
+fn regression_20951() {
+    check_infer(
+        r#"
+//- minicore: async_fn
+trait DoesSomething {
+    fn do_something(&self) -> impl Future<Output = usize>;
+}
+
+impl<F> DoesSomething for F
+where
+    F: AsyncFn() -> usize,
+{
+    fn do_something(&self) -> impl Future<Output = usize> {
+        self()
+    }
+}
+"#,
+        expect![[r#"
+            43..47 'self': &'? Self
+            168..172 'self': &'? F
+            205..227 '{     ...     }': <F as AsyncFnMut<()>>::CallRefFuture<'<erased>>
+            215..219 'self': &'? F
+            215..221 'self()': <F as AsyncFnMut<()>>::CallRefFuture<'<erased>>
+        "#]],
+    );
+}