1 files changed, 86 insertions, 0 deletions
diff --git a/library/core/src/iter/adapters/by_ref_sized.rs b/library/core/src/iter/adapters/by_ref_sized.rs
new file mode 100644
index 000000000..cc1e8e8a2
--- /dev/null
+++ b/library/core/src/iter/adapters/by_ref_sized.rs
@@ -0,0 +1,86 @@
+use crate::ops::Try;
+
+/// Like `Iterator::by_ref`, but requiring `Sized` so it can forward generics.
+///
+/// Ideally this will no longer be required, eventually, but as can be seen in
+/// the benchmarks (as of Feb 2022 at least) `by_ref` can have performance cost.
+#[unstable(feature = "std_internals", issue = "none")]
+#[derive(Debug)]
+pub struct ByRefSized<'a, I>(pub &'a mut I);
+
+#[unstable(feature = "std_internals", issue = "none")]
+impl<I: Iterator> Iterator for ByRefSized<'_, I> {
+    type Item = I::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
+
+    #[inline]
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        self.0.advance_by(n)
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.0.nth(n)
+    }
+
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.0.fold(init, f)
+    }
+
+    #[inline]
+    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        self.0.try_fold(init, f)
+    }
+}
+
+#[unstable(feature = "std_internals", issue = "none")]
+impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.0.next_back()
+    }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        self.0.advance_back_by(n)
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        self.0.nth_back(n)
+    }
+
+    #[inline]
+    fn rfold<B, F>(self, init: B, f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.0.rfold(init, f)
+    }
+
+    #[inline]
+    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
+    where
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        self.0.try_rfold(init, f)
+    }
+}