Add Iterator::join to combine Iterator and Join

Iterator::join will be added as extention trait IteratorExt inside
alloc crate as it requries allocation while joining which is the
reason why Join avaliable in alloc but not core. It is an easier
alternative to `.collect::<Vec<_>>().join(sep)`.

Tracking issue: rust-lang#75638

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;