Repository: rust
Issue: Tracking Issue for `PeekableIterator`
Labels: T-libs-api,C-tracking-issue

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Feature gate: `#![feature(peekable_iterator)]`

This is a tracking issue for the `PeekableIterator` trait, which extends `Iterator` with `peek` and related methods that inspect the next element without consuming it.

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### Public API

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```rust
// core::iter

pub trait PeekableIterator: Iterator {
    type PeekedItem<'a>: Borrow<Self::Item> + 'a
        where
            Self: 'a;
    fn peek(&self) -> Option<PeekedItem<'_>>;
    fn next_if(&mut self, func: impl FnOnce(&Self::Item) -> bool) -> Option<Self::Item>;
    fn next_if_eq(&mut self, expected: &T) -> Option<Self::Item> where Self::Item: PartialEq<T>, T: ?Sized;
}
```

### Steps / History

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- [x] ACP: rust-lang/libs-team#176
- [ ] Implementation: #132976
- [ ] Final comment period (FCP)[^1]
- [ ] Stabilization PR

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### Unresolved Questions

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- Should `peek` take `&mut self` or `&self`? `&self` makes sense for iterators such as `core::slice::iter` but precludes implementing the trait on `Peekable`.
- What about the return type of `peek`? We could always make it return `Self::Item` like itertools’s [`PeekingNext`](https://docs.rs/itertools/latest/itertools/trait.PeekingNext.html), but that would prevent this trait from being implemented for consuming iterators such as `vec::IntoIter`, as well as `Peekable` itself.
  - If we use an associated type, then what should be the bound for it: `Borrow`, `AsRef`, or something else?

[^1]: https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html