Rollup merge of rust-lang#51511 - Centril:feature/stabilize_iterator_flatten, r=SimonSapin

Stabilize Iterator::flatten in 1.29, fixes rust-lang#48213.

This PR stabilizes [`Iterator::flatten`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.flatten) in *version 1.29* (1.28 goes to beta in 10 days, I don't think there's enough time to land it in that time, but let's see...).

Tracking issue is:  rust-lang#48213.

cc @bluss re. itertools.
r? @SimonSapin
ping @pietroalbini -- let's do a crater run when this passes CI :)

Author: pietroalbini

src/libcore/iter/iterator.rs

@@ -1036,8 +1036,6 @@ pub trait Iterator {
     /// Basic usage:
     ///
     /// ```
-    /// #![feature(iterator_flatten)]
-    ///
     /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
     /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
     /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
@@ -1046,8 +1044,6 @@ pub trait Iterator {
     /// Mapping and then flattening:
     ///
     /// ```
-    /// #![feature(iterator_flatten)]
-    ///
     /// let words = ["alpha", "beta", "gamma"];
     ///
     /// // chars() returns an iterator
@@ -1074,8 +1070,6 @@ pub trait Iterator {
     /// Flattening once only removes one level of nesting:
     ///
     /// ```
-    /// #![feature(iterator_flatten)]
-    ///
     /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
     ///
     /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
@@ -1093,7 +1087,7 @@ pub trait Iterator {
     ///
     /// [`flat_map()`]: #method.flat_map
     #[inline]
-    #[unstable(feature = "iterator_flatten", issue = "48213")]
+    #[stable(feature = "iterator_flatten", since = "1.29")]
     fn flatten(self) -> Flatten<Self>
     where Self: Sized, Self::Item: IntoIterator {
         Flatten { inner: flatten_compat(self) }

src/libcore/iter/mod.rs

@@ -2575,13 +2575,13 @@ impl<I, U, F> FusedIterator for FlatMap<I, U, F>
 /// [`flatten`]: trait.Iterator.html#method.flatten
 /// [`Iterator`]: trait.Iterator.html
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
-#[unstable(feature = "iterator_flatten", issue = "48213")]
+#[stable(feature = "iterator_flatten", since = "1.29")]
 pub struct Flatten<I: Iterator>
 where I::Item: IntoIterator {
     inner: FlattenCompat<I, <I::Item as IntoIterator>::IntoIter>,
 }
 
-#[unstable(feature = "iterator_flatten", issue = "48213")]
+#[stable(feature = "iterator_flatten", since = "1.29")]
 impl<I, U> fmt::Debug for Flatten<I>
     where I: Iterator + fmt::Debug, U: Iterator + fmt::Debug,
           I::Item: IntoIterator<IntoIter = U, Item = U::Item>,
@@ -2591,15 +2591,15 @@ impl<I, U> fmt::Debug for Flatten<I>
     }
 }
 
-#[unstable(feature = "iterator_flatten", issue = "48213")]
+#[stable(feature = "iterator_flatten", since = "1.29")]
 impl<I, U> Clone for Flatten<I>
     where I: Iterator + Clone, U: Iterator + Clone,
           I::Item: IntoIterator<IntoIter = U, Item = U::Item>,
 {
     fn clone(&self) -> Self { Flatten { inner: self.inner.clone() } }
 }
 
-#[unstable(feature = "iterator_flatten", issue = "48213")]
+#[stable(feature = "iterator_flatten", since = "1.29")]
 impl<I, U> Iterator for Flatten<I>
     where I: Iterator, U: Iterator,
           I::Item: IntoIterator<IntoIter = U, Item = U::Item>
@@ -2627,7 +2627,7 @@ impl<I, U> Iterator for Flatten<I>
     }
 }
 
-#[unstable(feature = "iterator_flatten", issue = "48213")]
+#[stable(feature = "iterator_flatten", since = "1.29")]
 impl<I, U> DoubleEndedIterator for Flatten<I>
     where I: DoubleEndedIterator, U: DoubleEndedIterator,
           I::Item: IntoIterator<IntoIter = U, Item = U::Item>
@@ -2650,7 +2650,7 @@ impl<I, U> DoubleEndedIterator for Flatten<I>
     }
 }
 
-#[unstable(feature = "iterator_flatten", issue = "48213")]
+#[stable(feature = "iterator_flatten", since = "1.29")]
 impl<I, U> FusedIterator for Flatten<I>
     where I: FusedIterator, U: Iterator,
           I::Item: IntoIterator<IntoIter = U, Item = U::Item> {}

src/libcore/lib.rs

@@ -89,7 +89,6 @@
 #![feature(extern_types)]
 #![feature(fundamental)]
 #![feature(intrinsics)]
-#![feature(iterator_flatten)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
 #![feature(never_type)]

src/libcore/tests/lib.rs

@@ -23,7 +23,6 @@
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
 #![feature(hashmap_internals)]
-#![feature(iterator_flatten)]
 #![feature(pattern)]
 #![feature(range_is_empty)]
 #![feature(raw)]