4 files changed, 202 insertions, 1 deletions
diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index 2adc968bd46..48b6177434b 100644
--- a/library/core/src/fmt/mod.rs
+++ b/library/core/src/fmt/mod.rs
@@ -510,7 +510,7 @@ impl<'a> Arguments<'a> {
     /// assert_eq!(format_args!("{}", 1).as_str(), None);
     /// ```
     #[stable(feature = "fmt_as_str", since = "1.52.0")]
-    #[rustc_const_unstable(feature = "const_arguments_as_str", issue = "none")]
+    #[rustc_const_unstable(feature = "const_arguments_as_str", issue = "103900")]
     #[must_use]
     #[inline]
     pub const fn as_str(&self) -> Option<&'static str> {
diff --git a/library/core/src/iter/mod.rs b/library/core/src/iter/mod.rs
index ef0f397825b..bb35d50b4bf 100644
--- a/library/core/src/iter/mod.rs
+++ b/library/core/src/iter/mod.rs
@@ -401,6 +401,8 @@ pub use self::sources::{once, Once};
 pub use self::sources::{once_with, OnceWith};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::sources::{repeat, Repeat};
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+pub use self::sources::{repeat_n, RepeatN};
 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
 pub use self::sources::{repeat_with, RepeatWith};
 #[stable(feature = "iter_successors", since = "1.34.0")]
diff --git a/library/core/src/iter/sources.rs b/library/core/src/iter/sources.rs
index d34772cd304..3ec426a3ad9 100644
--- a/library/core/src/iter/sources.rs
+++ b/library/core/src/iter/sources.rs
@@ -4,6 +4,7 @@ mod from_generator;
 mod once;
 mod once_with;
 mod repeat;
+mod repeat_n;
 mod repeat_with;
 mod successors;
 
@@ -16,6 +17,9 @@ pub use self::empty::{empty, Empty};
 #[stable(feature = "iter_once", since = "1.2.0")]
 pub use self::once::{once, Once};
 
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+pub use self::repeat_n::{repeat_n, RepeatN};
+
 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
 pub use self::repeat_with::{repeat_with, RepeatWith};
 
diff --git a/library/core/src/iter/sources/repeat_n.rs b/library/core/src/iter/sources/repeat_n.rs
new file mode 100644
index 00000000000..dc69bf4df59
--- /dev/null
+++ b/library/core/src/iter/sources/repeat_n.rs
@@ -0,0 +1,195 @@
+use crate::iter::{FusedIterator, TrustedLen};
+use crate::mem::ManuallyDrop;
+
+/// Creates a new iterator that repeats a single element a given number of times.
+///
+/// The `repeat_n()` function repeats a single value exactly `n` times.
+///
+/// This is very similar to using [`repeat()`] with [`Iterator::take()`],
+/// but there are two differences:
+/// - `repeat_n()` can return the original value, rather than always cloning.
+/// - `repeat_n()` produces an [`ExactSizeIterator`].
+///
+/// [`repeat()`]: crate::iter::repeat
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// #![feature(iter_repeat_n)]
+/// use std::iter;
+///
+/// // four of the the number four:
+/// let mut four_fours = iter::repeat_n(4, 4);
+///
+/// assert_eq!(Some(4), four_fours.next());
+/// assert_eq!(Some(4), four_fours.next());
+/// assert_eq!(Some(4), four_fours.next());
+/// assert_eq!(Some(4), four_fours.next());
+///
+/// // no more fours
+/// assert_eq!(None, four_fours.next());
+/// ```
+///
+/// For non-`Copy` types,
+///
+/// ```
+/// #![feature(iter_repeat_n)]
+/// use std::iter;
+///
+/// let v: Vec<i32> = Vec::with_capacity(123);
+/// let mut it = iter::repeat_n(v, 5);
+///
+/// for i in 0..4 {
+///     // It starts by cloning things
+///     let cloned = it.next().unwrap();
+///     assert_eq!(cloned.len(), 0);
+///     assert_eq!(cloned.capacity(), 0);
+/// }
+///
+/// // ... but the last item is the original one
+/// let last = it.next().unwrap();
+/// assert_eq!(last.len(), 0);
+/// assert_eq!(last.capacity(), 123);
+///
+/// // ... and now we're done
+/// assert_eq!(None, it.next());
+/// ```
+#[inline]
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+#[doc(hidden)] // waiting on ACP#120 to decide whether to expose publicly
+pub fn repeat_n<T: Clone>(element: T, count: usize) -> RepeatN<T> {
+    let mut element = ManuallyDrop::new(element);
+
+    if count == 0 {
+        // SAFETY: we definitely haven't dropped it yet, since we only just got
+        // passed it in, and because the count is zero the instance we're about
+        // to create won't drop it, so to avoid leaking we need to now.
+        unsafe { ManuallyDrop::drop(&mut element) };
+    }
+
+    RepeatN { element, count }
+}
+
+/// An iterator that repeats an element an exact number of times.
+///
+/// This `struct` is created by the [`repeat_n()`] function.
+/// See its documentation for more.
+#[derive(Clone, Debug)]
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+#[doc(hidden)] // waiting on ACP#120 to decide whether to expose publicly
+pub struct RepeatN<A> {
+    count: usize,
+    // Invariant: has been dropped iff count == 0.
+    element: ManuallyDrop<A>,
+}
+
+impl<A> RepeatN<A> {
+    /// If we haven't already dropped the element, return it in an option.
+    ///
+    /// Clears the count so it won't be dropped again later.
+    #[inline]
+    fn take_element(&mut self) -> Option<A> {
+        if self.count > 0 {
+            self.count = 0;
+            // SAFETY: We just set count to zero so it won't be dropped again,
+            // and it used to be non-zero so it hasn't already been dropped.
+            unsafe { Some(ManuallyDrop::take(&mut self.element)) }
+        } else {
+            None
+        }
+    }
+}
+
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+impl<A> Drop for RepeatN<A> {
+    fn drop(&mut self) {
+        self.take_element();
+    }
+}
+
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+impl<A: Clone> Iterator for RepeatN<A> {
+    type Item = A;
+
+    #[inline]
+    fn next(&mut self) -> Option<A> {
+        if self.count == 0 {
+            return None;
+        }
+
+        self.count -= 1;
+        Some(if self.count == 0 {
+            // SAFETY: the check above ensured that the count used to be non-zero,
+            // so element hasn't been dropped yet, and we just lowered the count to
+            // zero so it won't be dropped later, and thus it's okay to take it here.
+            unsafe { ManuallyDrop::take(&mut self.element) }
+        } else {
+            A::clone(&mut self.element)
+        })
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.len();
+        (len, Some(len))
+    }
+
+    #[inline]
+    fn advance_by(&mut self, skip: usize) -> Result<(), usize> {
+        let len = self.count;
+
+        if skip >= len {
+            self.take_element();
+        }
+
+        if skip > len {
+            Err(len)
+        } else {
+            self.count = len - skip;
+            Ok(())
+        }
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<A> {
+        self.take_element()
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.len()
+    }
+}
+
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+impl<A: Clone> ExactSizeIterator for RepeatN<A> {
+    fn len(&self) -> usize {
+        self.count
+    }
+}
+
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+impl<A: Clone> DoubleEndedIterator for RepeatN<A> {
+    #[inline]
+    fn next_back(&mut self) -> Option<A> {
+        self.next()
+    }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        self.advance_by(n)
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<A> {
+        self.nth(n)
+    }
+}
+
+#[unstable(feature = "iter_repeat_n", issue = "104434")]
+impl<A: Clone> FusedIterator for RepeatN<A> {}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<A: Clone> TrustedLen for RepeatN<A> {}