Avoid GenFuture shim when compiling async constructs

Previously, async constructs would be lowered to "normal" generators,
with an additional `from_generator` / `GenFuture` shim in between to
convert from `Generator` to `Future`.

The compiler will now special-case these generators internally so that
async constructs will *directly* implement `Future` without the need
to go through the `from_generator` / `GenFuture` shim.

The primary motivation for this change was hiding this implementation
detail in stack traces and debuginfo, but it can in theory also help
the optimizer as there is less abstractions to see through.

Author: Swatinem

core/src/future/mod.rs

@@ -9,12 +9,8 @@
 //! [`await`]: ../../std/keyword.await.html
 //! [async book]: https://rust-lang.github.io/async-book/
 
-use crate::{
-    ops::{Generator, GeneratorState},
-    pin::Pin,
-    ptr::NonNull,
-    task::{Context, Poll},
-};
+use crate::ptr::NonNull;
+use crate::task::Context;
 
 mod future;
 mod into_future;
@@ -48,6 +44,7 @@ pub use poll_fn::{poll_fn, PollFn};
 ///    non-Send/Sync as well, and we don't want that.
 ///
 /// It also simplifies the HIR lowering of `.await`.
+#[cfg_attr(not(bootstrap), lang = "ResumeTy")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[derive(Debug, Copy, Clone)]
@@ -64,15 +61,21 @@ unsafe impl Sync for ResumeTy {}
 /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
 /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
 // This is `const` to avoid extra errors after we recover from `const async fn`
-#[lang = "from_generator"]
+#[cfg_attr(bootstrap, lang = "from_generator")]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[rustc_const_unstable(feature = "gen_future", issue = "50547")]
 #[inline]
 pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
 where
-    T: Generator<ResumeTy, Yield = ()>,
+    T: crate::ops::Generator<ResumeTy, Yield = ()>,
 {
+    use crate::{
+        ops::{Generator, GeneratorState},
+        pin::Pin,
+        task::Poll,
+    };
+
     #[rustc_diagnostic_item = "gen_future"]
     struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
 
@@ -109,3 +112,11 @@ pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
     // that fulfills all the requirements for a mutable reference.
     unsafe { &mut *cx.0.as_ptr().cast() }
 }
+
+#[cfg_attr(not(bootstrap), lang = "identity_future")]
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[inline]
+pub const fn identity_future<O, Fut: Future<Output = O>>(f: Fut) -> Fut {
+    f
+}

core/src/task/poll.rs

@@ -9,6 +9,7 @@ use crate::task::Ready;
 /// scheduled to receive a wakeup instead.
 #[must_use = "this `Poll` may be a `Pending` variant, which should be handled"]
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[cfg_attr(not(bootstrap), lang = "Poll")]
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub enum Poll<T> {
     /// Represents that a value is immediately ready.