Add `Default` impls for iterators and adapters (original) (raw)

Proposal

Problem statement

Currently many iterators don't implement Default which means one can't derive(Default) structs containing them, mem::take() or Cell::take them.

Motivation, use-cases

https://github.com/rust-lang/rust/blob/dc2d232c7485c60dd856f8b9aee83426492d4661/library/alloc/src/vec/drain.rs#L129

    let iter = mem::replace(&mut self.iter, (&mut []).iter());

could be replaced with

    let iter = mem::take(&mut self.iter);

Solution sketches

Iterator sources should implement Default when it is unambiguous that the default should be an empty iterator. Examples:

slice::Iter{Mut}
vec::IntoIter
iter::Empty (already implements it)
btree_map::Iter
etc.

Adapters should implement it when the inner iterator does and when they don't have any closure that would have to be materialized

Skip<I> where I: Default
Flatten<I> where I: Default
Chain<A, B> where A: Default, B: Default

Examples where it should not be implemented

array::IntoIter since one could expect [T; N]::default().into_iter() where T: Default
iter::Once since one could expect iter::once(Default::default())
adapters::Map since it would require summoning an FnMut ex nihilo

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.