fix: Fix panicking while resolving callable sigs for AsyncFnXs

Author: ShoyuVanilla

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

@@ -9,10 +9,11 @@ use rustc_hash::FxHashSet;
 use rustc_type_ir::{
     DebruijnIndex, InferConst, InferTy, RegionVid, TyVid, TypeFoldable, TypeFolder,
     TypeSuperFoldable, TypeVisitableExt, UpcastFrom,
-    inherent::{Const as _, IntoKind, Ty as _},
+    inherent::{Const as _, GenericArgs as _, IntoKind, Ty as _},
     solve::{Certainty, GoalSource},
 };
 use smallvec::SmallVec;
+use stdx::never;
 use triomphe::Arc;
 
 use crate::{
@@ -674,10 +675,21 @@ impl<'db> InferenceTable<'db> {
             let args = [ty, arg_ty];
             let trait_ref = TraitRef::new(self.interner(), fn_trait.into(), args);
 
+            let mut proj_args: SmallVec<[_; 3]> =
+                GenericArgs::identity_for_item(self.interner(), output_assoc_type.into())
+                    .into_iter()
+                    .collect();
+            if proj_args.len() < 2 {
+                never!();
+                return None;
+            }
+            proj_args[0] = ty.into();
+            proj_args[1] = arg_ty.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<'a>
+            215..219 'self': &'? F
+            215..221 'self()': <F as AsyncFnMut<()>>::CallRefFuture<'a>
+        "#]],
+    );
+}

crates/hir-ty/src/tests/traits.rs

@@ -4880,20 +4880,20 @@ async fn baz<T: AsyncFnOnce(u32) -> i32>(c: T) {
             37..38 'a': T
             43..83 '{     ...ait; }': ()
             43..83 '{     ...ait; }': impl Future<Output = ()>
-            53..57 'fut1': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'?>
+            53..57 'fut1': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'a>
             60..61 'a': T
-            60..64 'a(0)': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'?>
+            60..64 'a(0)': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'a>
             62..63 '0': u32
-            70..74 'fut1': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'?>
+            70..74 'fut1': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'a>
             70..80 'fut1.await': i32
             124..129 'mut b': T
             134..174 '{     ...ait; }': ()
             134..174 '{     ...ait; }': impl Future<Output = ()>
-            144..148 'fut2': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'?>
+            144..148 'fut2': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'a>
             151..152 'b': T
-            151..155 'b(0)': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'?>
+            151..155 'b(0)': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'a>
             153..154 '0': u32
-            161..165 'fut2': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'?>
+            161..165 'fut2': <T as AsyncFnMut<(u32,)>>::CallRefFuture<'a>
             161..171 'fut2.await': i32
             216..217 'c': T
             222..262 '{     ...ait; }': ()