Rollup merge of rust-lang#81154 - dylni:improve-design-of-assert-len, r=KodrAus

Improve design of `assert_len`

It was discussed in the [tracking issue](rust-lang#76393 (comment)) that `assert_len`'s name and usage are confusing. This PR improves them based on a suggestion by ``@scottmcm`` in that issue.

I also improved the documentation to make it clearer when you might want to use this method.

Old example:

```rust
let range = range.assert_len(slice.len());
```

New example:

```rust
let range = range.ensure_subset_of(..slice.len());
```

Fixes rust-lang#81157

Author: Dylan-DPC

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

@@ -1063,7 +1063,7 @@ impl<T> VecDeque<T> {
     where
         R: RangeBounds<usize>,
     {
-        let Range { start, end } = range.assert_len(self.len());
+        let Range { start, end } = slice::range(range, ..self.len());
         let tail = self.wrap_add(self.tail, start);
         let head = self.wrap_add(self.tail, end);
         (tail, head)

library/alloc/src/lib.rs

@@ -115,14 +115,14 @@
 #![feature(or_patterns)]
 #![feature(pattern)]
 #![feature(ptr_internals)]
-#![feature(range_bounds_assert_len)]
 #![feature(rustc_attrs)]
 #![feature(receiver_trait)]
 #![cfg_attr(bootstrap, feature(min_const_generics))]
 #![feature(min_specialization)]
 #![feature(set_ptr_value)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]
+#![feature(slice_range)]
 #![feature(staged_api)]
 #![feature(str_internals)]
 #![feature(trusted_len)]

library/alloc/src/slice.rs

@@ -92,6 +92,8 @@ use crate::borrow::ToOwned;
 use crate::boxed::Box;
 use crate::vec::Vec;
 
+#[unstable(feature = "slice_range", issue = "76393")]
+pub use core::slice::range;
 #[unstable(feature = "array_chunks", issue = "74985")]
 pub use core::slice::ArrayChunks;
 #[unstable(feature = "array_chunks", issue = "74985")]

library/alloc/src/string.rs

@@ -49,6 +49,7 @@ use core::iter::{FromIterator, FusedIterator};
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{self, Add, AddAssign, Index, IndexMut, Range, RangeBounds};
 use core::ptr;
+use core::slice;
 use core::str::{lossy, pattern::Pattern};
 
 use crate::borrow::{Cow, ToOwned};
@@ -1510,14 +1511,14 @@ impl String {
         // of the vector version. The data is just plain bytes.
         // Because the range removal happens in Drop, if the Drain iterator is leaked,
         // the removal will not happen.
-        let Range { start, end } = range.assert_len(self.len());
+        let Range { start, end } = slice::range(range, ..self.len());
         assert!(self.is_char_boundary(start));
         assert!(self.is_char_boundary(end));
 
         // Take out two simultaneous borrows. The &mut String won't be accessed
         // until iteration is over, in Drop.
         let self_ptr = self as *mut _;
-        // SAFETY: `assert_len` and `is_char_boundary` do the appropriate bounds checks.
+        // SAFETY: `slice::range` and `is_char_boundary` do the appropriate bounds checks.
         let chars_iter = unsafe { self.get_unchecked(start..end) }.chars();
 
         Drain { start, end, iter: chars_iter, string: self_ptr }

library/alloc/src/vec/mod.rs

@@ -1651,7 +1651,7 @@ impl<T, A: Allocator> Vec<T, A> {
         // the hole, and the vector length is restored to the new length.
         //
         let len = self.len();
-        let Range { start, end } = range.assert_len(len);
+        let Range { start, end } = slice::range(range, ..len);
 
         unsafe {
             // set self.vec length's to start, to be safe in case Drain is leaked
@@ -2037,11 +2037,11 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
     where
         R: RangeBounds<usize>,
     {
-        let range = src.assert_len(self.len());
+        let range = slice::range(src, ..self.len());
         self.reserve(range.len());
 
         // SAFETY:
-        // - `assert_len` guarantees  that the given range is valid for indexing self
+        // - `slice::range` guarantees  that the given range is valid for indexing self
         unsafe {
             self.spec_extend_from_within(range);
         }

library/core/src/ops/range.rs

@@ -1,9 +1,5 @@
 use crate::fmt;
 use crate::hash::Hash;
-use crate::slice::index::{
-    slice_end_index_len_fail, slice_end_index_overflow_fail, slice_index_order_fail,
-    slice_start_index_overflow_fail,
-};
 
 /// An unbounded range (`..`).
 ///
@@ -764,92 +760,6 @@ pub trait RangeBounds<T: ?Sized> {
     #[stable(feature = "collections_range", since = "1.28.0")]
     fn end_bound(&self) -> Bound<&T>;
 
-    /// Performs bounds-checking of this range.
-    ///
-    /// The returned [`Range`] is safe to pass to [`slice::get_unchecked`] and
-    /// [`slice::get_unchecked_mut`] for slices of the given length.
-    ///
-    /// [`slice::get_unchecked`]: ../../std/primitive.slice.html#method.get_unchecked
-    /// [`slice::get_unchecked_mut`]: ../../std/primitive.slice.html#method.get_unchecked_mut
-    ///
-    /// # Panics
-    ///
-    /// Panics if the range would be out of bounds.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(range_bounds_assert_len)]
-    ///
-    /// use std::ops::RangeBounds;
-    ///
-    /// let v = [10, 40, 30];
-    /// assert_eq!(1..2, (1..2).assert_len(v.len()));
-    /// assert_eq!(0..2, (..2).assert_len(v.len()));
-    /// assert_eq!(1..3, (1..).assert_len(v.len()));
-    /// ```
-    ///
-    /// Panics when [`Index::index`] would panic:
-    ///
-    /// ```should_panic
-    /// #![feature(range_bounds_assert_len)]
-    ///
-    /// use std::ops::RangeBounds;
-    ///
-    /// (2..1).assert_len(3);
-    /// ```
-    ///
-    /// ```should_panic
-    /// #![feature(range_bounds_assert_len)]
-    ///
-    /// use std::ops::RangeBounds;
-    ///
-    /// (1..4).assert_len(3);
-    /// ```
-    ///
-    /// ```should_panic
-    /// #![feature(range_bounds_assert_len)]
-    ///
-    /// use std::ops::RangeBounds;
-    ///
-    /// (1..=usize::MAX).assert_len(3);
-    /// ```
-    ///
-    /// [`Index::index`]: crate::ops::Index::index
-    #[track_caller]
-    #[unstable(feature = "range_bounds_assert_len", issue = "76393")]
-    fn assert_len(self, len: usize) -> Range<usize>
-    where
-        Self: RangeBounds<usize>,
-    {
-        let start: Bound<&usize> = self.start_bound();
-        let start = match start {
-            Bound::Included(&start) => start,
-            Bound::Excluded(start) => {
-                start.checked_add(1).unwrap_or_else(|| slice_start_index_overflow_fail())
-            }
-            Bound::Unbounded => 0,
-        };
-
-        let end: Bound<&usize> = self.end_bound();
-        let end = match end {
-            Bound::Included(end) => {
-                end.checked_add(1).unwrap_or_else(|| slice_end_index_overflow_fail())
-            }
-            Bound::Excluded(&end) => end,
-            Bound::Unbounded => len,
-        };
-
-        if start > end {
-            slice_index_order_fail(start, end);
-        }
-        if end > len {
-            slice_end_index_len_fail(end, len);
-        }
-
-        Range { start, end }
-    }
-
     /// Returns `true` if `item` is contained in the range.
     ///
     /// # Examples

library/core/src/slice/index.rs

@@ -37,28 +37,28 @@ fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
 #[inline(never)]
 #[cold]
 #[track_caller]
-pub(crate) fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
+fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
     panic!("range end index {} out of range for slice of length {}", index, len);
 }
 
 #[inline(never)]
 #[cold]
 #[track_caller]
-pub(crate) fn slice_index_order_fail(index: usize, end: usize) -> ! {
+fn slice_index_order_fail(index: usize, end: usize) -> ! {
     panic!("slice index starts at {} but ends at {}", index, end);
 }
 
 #[inline(never)]
 #[cold]
 #[track_caller]
-pub(crate) fn slice_start_index_overflow_fail() -> ! {
+fn slice_start_index_overflow_fail() -> ! {
     panic!("attempted to index slice from after maximum usize");
 }
 
 #[inline(never)]
 #[cold]
 #[track_caller]
-pub(crate) fn slice_end_index_overflow_fail() -> ! {
+fn slice_end_index_overflow_fail() -> ! {
     panic!("attempted to index slice up to maximum usize");
 }
 
@@ -449,3 +449,100 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeToInclusive<usize> {
         (0..=self.end).index_mut(slice)
     }
 }
+
+/// Performs bounds-checking of a range.
+///
+/// This method is similar to [`Index::index`] for slices, but it returns a
+/// [`Range`] equivalent to `range`. You can use this method to turn any range
+/// into `start` and `end` values.
+///
+/// `bounds` is the range of the slice to use for bounds-checking. It should
+/// be a [`RangeTo`] range that ends at the length of the slice.
+///
+/// The returned [`Range`] is safe to pass to [`slice::get_unchecked`] and
+/// [`slice::get_unchecked_mut`] for slices with the given range.
+///
+/// [`Range`]: ops::Range
+/// [`RangeTo`]: ops::RangeTo
+/// [`slice::get_unchecked`]: ../../std/primitive.slice.html#method.get_unchecked
+/// [`slice::get_unchecked_mut`]: ../../std/primitive.slice.html#method.get_unchecked_mut
+///
+/// # Panics
+///
+/// Panics if `range` would be out of bounds.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(slice_range)]
+///
+/// use std::slice;
+///
+/// let v = [10, 40, 30];
+/// assert_eq!(1..2, slice::range(1..2, ..v.len()));
+/// assert_eq!(0..2, slice::range(..2, ..v.len()));
+/// assert_eq!(1..3, slice::range(1.., ..v.len()));
+/// ```
+///
+/// Panics when [`Index::index`] would panic:
+///
+/// ```should_panic
+/// #![feature(slice_range)]
+///
+/// use std::slice;
+///
+/// slice::range(2..1, ..3);
+/// ```
+///
+/// ```should_panic
+/// #![feature(slice_range)]
+///
+/// use std::slice;
+///
+/// slice::range(1..4, ..3);
+/// ```
+///
+/// ```should_panic
+/// #![feature(slice_range)]
+///
+/// use std::slice;
+///
+/// slice::range(1..=usize::MAX, ..3);
+/// ```
+///
+/// [`Index::index`]: ops::Index::index
+#[track_caller]
+#[unstable(feature = "slice_range", issue = "76393")]
+pub fn range<R>(range: R, bounds: ops::RangeTo<usize>) -> ops::Range<usize>
+where
+    R: ops::RangeBounds<usize>,
+{
+    let len = bounds.end;
+
+    let start: ops::Bound<&usize> = range.start_bound();
+    let start = match start {
+        ops::Bound::Included(&start) => start,
+        ops::Bound::Excluded(start) => {
+            start.checked_add(1).unwrap_or_else(|| slice_start_index_overflow_fail())
+        }
+        ops::Bound::Unbounded => 0,
+    };
+
+    let end: ops::Bound<&usize> = range.end_bound();
+    let end = match end {
+        ops::Bound::Included(end) => {
+            end.checked_add(1).unwrap_or_else(|| slice_end_index_overflow_fail())
+        }
+        ops::Bound::Excluded(&end) => end,
+        ops::Bound::Unbounded => len,
+    };
+
+    if start > end {
+        slice_index_order_fail(start, end);
+    }
+    if end > len {
+        slice_end_index_len_fail(end, len);
+    }
+
+    ops::Range { start, end }
+}

library/core/src/slice/mod.rs

@@ -18,6 +18,7 @@ use crate::option::Option::{None, Some};
 use crate::ptr;
 use crate::result::Result;
 use crate::result::Result::{Err, Ok};
+use crate::slice;
 
 #[unstable(
     feature = "slice_internals",
@@ -29,7 +30,7 @@ pub mod memchr;
 
 mod ascii;
 mod cmp;
-pub(crate) mod index;
+mod index;
 mod iter;
 mod raw;
 mod rotate;
@@ -76,6 +77,9 @@ pub use sort::heapsort;
 #[stable(feature = "slice_get_slice", since = "1.28.0")]
 pub use index::SliceIndex;
 
+#[unstable(feature = "slice_range", issue = "76393")]
+pub use index::range;
+
 #[lang = "slice"]
 #[cfg(not(test))]
 impl<T> [T] {
@@ -3075,7 +3079,7 @@ impl<T> [T] {
     where
         T: Copy,
     {
-        let Range { start: src_start, end: src_end } = src.assert_len(self.len());
+        let Range { start: src_start, end: src_end } = slice::range(src, ..self.len());
         let count = src_end - src_start;
         assert!(dest <= self.len() - count, "dest is out of bounds");
         // SAFETY: the conditions for `ptr::copy` have all been checked above,