Remove identity_future indirection

This was previously needed because the indirection used to hide some unexplained lifetime errors, which it turned out were related to the `min_choice` algorithm.

Removing the indirection also solves a couple of cycle errors, large moves and makes async blocks support the `#[track_caller]` annotation.

library/core/src/future/mod.rs

@@ -67,14 +67,10 @@ pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
     unsafe { &mut *cx.0.as_ptr().cast() }
 }
 
-// FIXME(swatinem): This fn is currently needed to work around shortcomings
-// in type and lifetime inference.
-// See the comment at the bottom of `LoweringContext::make_async_expr` and
-// <https://github.com/rust-lang/rust/issues/104826>.
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[inline]
-#[lang = "identity_future"]
+#[cfg_attr(bootstrap, lang = "identity_future")]
 pub const fn identity_future<O, Fut: Future<Output = O>>(f: Fut) -> Fut {
     f
 }