Make IndexRange accessible from alloc

Specifically make it public + unstable under `std_internals` + `doc(hidden)`.
This is honestly Not Great, but I do not know a better solution :(

Author: WaffleLapkin

library/core/src/ops/index_range.rs

@@ -1,3 +1,5 @@
+#![unstable(feature = "std_internals", issue = "none")]
+#![doc(hidden)]
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::num::NonZero;
 use crate::ub_checks;
@@ -9,7 +11,7 @@ use crate::ub_checks;
 /// (Normal `Range` code needs to handle degenerate ranges like `10..0`,
 ///  which takes extra checks compared to only handling the canonical form.)
 #[derive(Clone, Debug, PartialEq, Eq)]
-pub(crate) struct IndexRange {
+pub struct IndexRange {
     start: usize,
     end: usize,
 }

library/core/src/ops/mod.rs

@@ -181,7 +181,9 @@ pub(crate) use self::drop::fallback_surface_drop;
 pub use self::function::{Fn, FnMut, FnOnce};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::index::{Index, IndexMut};
-pub(crate) use self::index_range::IndexRange;
+#[doc(hidden)]
+#[unstable(feature = "std_internals", issue = "none")]
+pub use self::index_range::IndexRange;
 #[unstable(feature = "one_sided_range", issue = "69780")]
 pub use self::range::OneSidedRange;
 #[stable(feature = "inclusive_range", since = "1.26.0")]

library/core/src/slice/index.rs

@@ -149,6 +149,8 @@ mod private_slice_index {
     #[unstable(feature = "new_range_api", issue = "125687")]
     impl Sealed for range::RangeFrom<usize> {}
 
+    #[doc(hidden)]
+    #[unstable(feature = "std_internals", issue = "none")]
     impl Sealed for ops::IndexRange {}
 }
 
@@ -283,6 +285,8 @@ unsafe impl<T> SliceIndex<[T]> for usize {
 
 /// Because `IndexRange` guarantees `start <= end`, fewer checks are needed here
 /// than there are for a general `Range<usize>` (which might be `100..3`).
+#[doc(hidden)]
+#[unstable(feature = "std_internals", issue = "none")]
 unsafe impl<T> SliceIndex<[T]> for ops::IndexRange {
     type Output = [T];