Expose iter::ByRefSized as unstable feature and use it

library/alloc/src/collections/vec_deque/spec_extend.rs

@@ -1,6 +1,6 @@
 use crate::alloc::Allocator;
 use crate::vec;
-use core::iter::TrustedLen;
+use core::iter::{ByRefSized, TrustedLen};
 use core::slice;
 
 use super::VecDeque;

library/alloc/src/lib.rs

@@ -143,6 +143,7 @@
 #![feature(unchecked_math)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
+#![feature(std_internals)]
 //
 // Language features:
 #![feature(allocator_internals)]

library/core/src/iter/adapters/by_ref_sized.rs

@@ -4,8 +4,11 @@ use crate::ops::Try;
 ///
 /// 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.
-pub(crate) struct ByRefSized<'a, I>(pub &'a mut I);
+#[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;
 
@@ -47,6 +50,7 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     }
 }
 
+#[unstable(feature = "std_internals", issue = "none")]
 impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     #[inline]
     fn next_back(&mut self) -> Option<Self::Item> {

library/core/src/iter/adapters/mod.rs

@@ -32,7 +32,8 @@ pub use self::{
     scan::Scan, skip::Skip, skip_while::SkipWhile, take::Take, take_while::TakeWhile, zip::Zip,
 };
 
-pub(crate) use self::by_ref_sized::ByRefSized;
+#[unstable(feature = "std_internals", issue = "none")]
+pub use self::by_ref_sized::ByRefSized;
 
 #[stable(feature = "iter_cloned", since = "1.1.0")]
 pub use self::cloned::Cloned;

library/core/src/iter/mod.rs

@@ -398,6 +398,8 @@ pub use self::traits::{
 
 #[stable(feature = "iter_zip", since = "1.59.0")]
 pub use self::adapters::zip;
+#[unstable(feature = "std_internals", issue = "none")]
+pub use self::adapters::ByRefSized;
 #[stable(feature = "iter_cloned", since = "1.1.0")]
 pub use self::adapters::Cloned;
 #[stable(feature = "iter_copied", since = "1.36.0")]
@@ -422,7 +424,7 @@ pub use self::adapters::{
 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
 pub use self::adapters::{Intersperse, IntersperseWith};
 
-pub(crate) use self::adapters::{try_process, ByRefSized};
+pub(crate) use self::adapters::try_process;
 
 mod adapters;
 mod range;