Add Iterator::join to combine Iterator and Join

library/alloc/src/iter.rs

@@ -0,0 +1,40 @@
+//! Allocation extensions for [`Iterator`].
+//!
+//! *[See also the Iterator trait][Iterator].*
+#![unstable(feature = "iterator_join", issue = "75638")]
+
+use crate::slice::Join;
+use crate::vec::Vec;
+
+/// Iterator extension traits that requires allocation.
+#[unstable(feature = "iterator_join", issue = "75638")]
+pub trait IteratorExt: Iterator {
+    /// Flattens an iterator into a single value with the given separator in
+    /// between.
+    ///
+    /// Combines `collect` with `join` to convert a sequence into a value
+    /// separated with the specified separator.
+    ///
+    /// Allows `.join(sep)` instead of `.collect::<Vec<_>>().join(sep)`.
+    ///
+    /// ```
+    /// #![feature(iterator_join)]
+    /// use alloc::iter::IteratorExt;
+    ///
+    /// assert_eq!(["hello", "world"].iter().copied().join(" "), "hello world");
+    /// assert_eq!([[1, 2], [3, 4]].iter().copied().join(&0), [1, 2, 0, 3, 4]);
+    /// assert_eq!([[1, 2], [3, 4]].iter().copied().join(&[0, 0][..]), [1, 2, 0, 0, 3, 4]);
+    /// ```
+    #[inline]
+    #[unstable(feature = "iterator_join", issue = "75638")]
+    #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
+    fn join<Separator>(self, sep: Separator) -> <[Self::Item] as Join<Separator>>::Output
+    where
+        [Self::Item]: Join<Separator>,
+        Self: Sized,
+    {
+        Join::join(self.collect::<Vec<Self::Item>>().as_slice(), sep)
+    }
+}
+
+impl<T: Iterator + ?Sized> IteratorExt for T {}

library/alloc/src/lib.rs

@@ -167,6 +167,7 @@ mod boxed {
 pub mod borrow;
 pub mod collections;
 pub mod fmt;
+pub mod iter;
 pub mod prelude;
 pub mod raw_vec;
 pub mod rc;