Add a "peekable" iterator adaptor, with a peek() method that returns the next element.

[SimonSapin]

No description provided.

[SimonSapin]

The name is unfortunately close to the existing peek_ method, but the behavior is different.

[bluss]

This is very useful, basically an identical adaptor is going in with the TreeMap/union/intersection iterator changes, and can be refactored to use peekable as soon as this merges.

[bluss]

We don't strictly need .peek_clone(); it can be removed if it's contentious? extra::tremap would look great using .peek(): https://github.com/blake2-ppc/rust/commit/d33a2e658536160759992e6a32c4259fe01225ff

[thestinger]

@blake2-ppc: I think I would rather just have peek as long as there's a way to get the same behaviour as peek_clone without hitting a borrow checking error - can you open a PR with the treemap changes included?

[bluss]

It's possible. .peek_clone is Something like .peek().map(|x| (**x).clone()) (just deref enough to clone the right thing)

[bluss]

Rebased & combined with treemap changes in the new PR. peek_clone was removed, Hope this is ok simon!

[SimonSapin]

Sure @blake2-ppc , that’s fine. Closing this in favor of #8428.