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

Improve design of `assert_len`

It was discussed in the [tracking issue](https://github.com/rust-lang/rust/issues/76393#issuecomment-761765448) 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 #81157

1 files changed, 3 insertions, 3 deletions

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index b6166617789..dbb7708b600 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/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);
         }