Rollup merge of #81050 - yoshuawuyts:stabilize-task-ready, r=m-ou-se

Stabilize core::task::ready!

_Tracking issue: https://github.com/rust-lang/rust/issues/70922_

This PR stabilizes the `task::ready!` macro. Similar to #80886, this PR was waiting on #74355 to be fixed.

The `task::ready!` API has existed in the futures ecosystem for several years, and was added on nightly last year in #70817. The motivation for this macro is the same as it was back then: virtually every single manual future implementation makes use of this; so much so that it's one of the few things included in the [futures-core](https://docs.rs/futures-core/0.3.12/futures_core) library.

r? ``@tmandry``

cc/ ``@rust-lang/wg-async-foundations`` ``@rust-lang/libs``

## Example
```rust
use core::task::{Context, Poll};
use core::future::Future;
use core::pin::Pin;

async fn get_num() -> usize {
    42
}

pub fn do_poll(cx: &mut Context<'_>) -> Poll<()> {
    let mut f = get_num();
    let f = unsafe { Pin::new_unchecked(&mut f) };

    let num = ready!(f.poll(cx));
    // ... use num

    Poll::Ready(())
}
```

library/core/src/task/mod.rs

@@ -11,5 +11,5 @@ mod wake;
 pub use self::wake::{Context, RawWaker, RawWakerVTable, Waker};
 
 mod ready;
-#[unstable(feature = "ready_macro", issue = "70922")]
+#[stable(feature = "ready_macro", since = "1.56.0")]
 pub use ready::ready;

library/core/src/task/ready.rs

@@ -8,8 +8,6 @@
 /// # Examples
 ///
 /// ```
-/// #![feature(ready_macro)]
-///
 /// use std::task::{ready, Context, Poll};
 /// use std::future::{self, Future};
 /// use std::pin::Pin;
@@ -29,8 +27,6 @@
 /// The `ready!` call expands to:
 ///
 /// ```
-/// # #![feature(ready_macro)]
-/// #
 /// # use std::task::{Context, Poll};
 /// # use std::future::{self, Future};
 /// # use std::pin::Pin;
@@ -49,7 +45,7 @@
 ///     # Poll::Ready(())
 /// # }
 /// ```
-#[unstable(feature = "ready_macro", issue = "70922")]
+#[stable(feature = "ready_macro", since = "1.56.0")]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro ready($e:expr) {
     match $e {

library/std/src/lib.rs

@@ -304,7 +304,6 @@
 #![feature(pin_static_ref)]
 #![feature(prelude_import)]
 #![feature(ptr_internals)]
-#![feature(ready_macro)]
 #![feature(rustc_attrs)]
 #![feature(rustc_private)]
 #![feature(shrink_to)]