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

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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
This commit is contained in:
Dylan DPC 2021-02-23 02:51:43 +01:00 • committed by GitHub
commit 72e6d51583
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9 changed files with 117 additions and 113 deletions
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6
library/alloc/src/vec/mod.rs
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@ -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);
}