Rollup merge of #72584 - CAD97:stabilize-58957, r=dtolnay

Stabilize vec::Drain::as_slice

and add `AsRef<[T]> for Drain<'_, T>`.

Tracking issue: #58957. Does not stabilize `slice::IterMut::as_slice` yet. cc @cuviper
This PR proposes stabilizing just the `vec::Drain::as_slice` part of that tracking issue.

My ultimate goal here: being able to use `for<T, I: Iterator<Item=T> + AsRef<[T]>> I` to refer to `vec::IntoIter`, `vec::Drain`, and eventually `array::IntoIter`, as an approximation of the set of by-value iterators that can be "previewed" as by-ref iterators. (Actually expressing that as a trait requires GAT.)

Author: RalfJung

src/liballoc/vec.rs

@@ -2779,19 +2779,25 @@ impl<'a, T> Drain<'a, T> {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(vec_drain_as_slice)]
     /// let mut vec = vec!['a', 'b', 'c'];
     /// let mut drain = vec.drain(..);
     /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
     /// let _ = drain.next().unwrap();
     /// assert_eq!(drain.as_slice(), &['b', 'c']);
     /// ```
-    #[unstable(feature = "vec_drain_as_slice", reason = "recently added", issue = "58957")]
+    #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
     pub fn as_slice(&self) -> &[T] {
         self.iter.as_slice()
     }
 }
 
+#[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
+impl<'a, T> AsRef<[T]> for Drain<'a, T> {
+    fn as_ref(&self) -> &[T] {
+        self.as_slice()
+    }
+}
+
 #[stable(feature = "drain", since = "1.6.0")]
 unsafe impl<T: Sync> Sync for Drain<'_, T> {}
 #[stable(feature = "drain", since = "1.6.0")]