Rollup merge of rust-lang#119222 - eholk:into-async-iterator, r=compiler-errors,dtolnay

Add `IntoAsyncIterator`

This introduces the `IntoAsyncIterator` trait and uses it in the desugaring of the unstable `for await` loop syntax. This is mostly added for symmetry with `Iterator` and `IntoIterator`.

r? `@compiler-errors`

cc `@rust-lang/libs-api,` `@rust-lang/wg-async`

Author: compiler-errors

core/src/async_iter/async_iter.rs

@@ -135,3 +135,26 @@ impl<T> Poll<Option<T>> {
     #[lang = "AsyncGenFinished"]
     pub const FINISHED: Self = Poll::Ready(None);
 }
+
+/// Convert something into an async iterator
+#[unstable(feature = "async_iterator", issue = "79024")]
+pub trait IntoAsyncIterator {
+    /// The type of the item yielded by the iterator
+    type Item;
+    /// The type of the resulting iterator
+    type IntoAsyncIter: AsyncIterator<Item = Self::Item>;
+
+    /// Converts `self` into an async iterator
+    #[cfg_attr(not(bootstrap), lang = "into_async_iter_into_iter")]
+    fn into_async_iter(self) -> Self::IntoAsyncIter;
+}
+
+#[unstable(feature = "async_iterator", issue = "79024")]
+impl<I: AsyncIterator> IntoAsyncIterator for I {
+    type Item = I::Item;
+    type IntoAsyncIter = I;
+
+    fn into_async_iter(self) -> Self::IntoAsyncIter {
+        self
+    }
+}

core/src/async_iter/mod.rs

@@ -124,5 +124,5 @@
 mod async_iter;
 mod from_iter;
 
-pub use async_iter::AsyncIterator;
+pub use async_iter::{AsyncIterator, IntoAsyncIterator};
 pub use from_iter::{from_iter, FromIter};

core/tests/async_iter/mod.rs

@@ -0,0 +1,17 @@
+use core::async_iter::{self, AsyncIterator, IntoAsyncIterator};
+use core::pin::pin;
+use core::task::Poll;
+
+#[test]
+fn into_async_iter() {
+    let async_iter = async_iter::from_iter(0..3);
+    let mut async_iter = pin!(async_iter.into_async_iter());
+
+    let waker = core::task::Waker::noop();
+    let mut cx = &mut core::task::Context::from_waker(&waker);
+
+    assert_eq!(async_iter.as_mut().poll_next(&mut cx), Poll::Ready(Some(0)));
+    assert_eq!(async_iter.as_mut().poll_next(&mut cx), Poll::Ready(Some(1)));
+    assert_eq!(async_iter.as_mut().poll_next(&mut cx), Poll::Ready(Some(2)));
+    assert_eq!(async_iter.as_mut().poll_next(&mut cx), Poll::Ready(None));
+}

core/tests/lib.rs

@@ -4,6 +4,8 @@
 #![feature(array_windows)]
 #![feature(ascii_char)]
 #![feature(ascii_char_variants)]
+#![feature(async_iter_from_iter)]
+#![feature(async_iterator)]
 #![feature(bigint_helper_methods)]
 #![feature(cell_update)]
 #![feature(const_align_offset)]
@@ -55,6 +57,7 @@
 #![feature(maybe_uninit_write_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(min_specialization)]
+#![feature(noop_waker)]
 #![feature(numfmt)]
 #![feature(num_midpoint)]
 #![feature(isqrt)]
@@ -125,6 +128,7 @@ mod any;
 mod array;
 mod ascii;
 mod asserting;
+mod async_iter;
 mod atomic;
 mod bool;
 mod cell;