Auto merge of #12982 - jridgewell:into_future, r=Veykril

Implement IntoFuture type inference

One of my projects is using [IntoFuture](https://doc.rust-lang.org/std/future/trait.IntoFuture.html) to make our async code a little less verbose. However, rust-analyzer can't infer the output type of an await expression if the value uses `IntoFuture` to convert into another type. So we're getting `{unknown}` types everywhere since switching.

`foo.await` itself [desugars](e4417cf020/compiler/rustc_ast_lowering/src/expr.rs (L644-L658)) into a `match into_future(foo) {}`, with every `Future` impl getting a [default](e4417cf020/library/core/src/future/into_future.rs (L131-L139)) `IntoFuture` implementation. I'm not sure if we want to disable the old `future_trait` paths, since this only recently [stabilize](https://github.com/rust-lang/rust/pull/98718).

crates/ide/src/goto_definition.rs

@@ -1664,6 +1664,40 @@ fn f() {
         );
     }
 
+    #[test]
+    fn goto_await_into_future_poll() {
+        check(
+            r#"
+//- minicore: future
+
+struct Futurable;
+
+impl core::future::IntoFuture for Futurable {
+    type IntoFuture = MyFut;
+}
+
+struct MyFut;
+
+impl core::future::Future for MyFut {
+    type Output = ();
+
+    fn poll(
+     //^^^^
+        self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>
+    ) -> std::task::Poll<Self::Output>
+    {
+        ()
+    }
+}
+
+fn f() {
+    Futurable.await$0;
+}
+"#,
+        );
+    }
+
     #[test]
     fn goto_try_op() {
         check(