Document expected relationships between FromIterator, Default, and Extend

[ExpHP]

Somebody who creates a type that implements both Default and Extend should be aware that their type can be created through Iterator::unzip. In fact, any such type should probably also implement FromIterator, with the semantics that:

let mut a = TheType::default()
a.extend(iter);

is identical to iter.collect().

(I wonder why unzip didn't decide to use FromIterator + Extend instead?)

[hkBst]

A blanket impl of FromIterator for types that impl Default + Extend would accomplish what you want I think. The question is whether that is a good idea and what the fallout would be. A good way to get that feedback is to make a PR, which also enables a crater run to check for fallout.

Writing this blanket impl should not be hard and it has a natural home in: https://github.com/rust-lang/rust/blob/master/library/core/src/iter/traits/collect.rs. Don't expect your PR to get merged!

@rustbot label E-easy

[yoges]

@rustbot claim

[yoges]

@rustbot release-assignment

[HernandoR]

@rustbot claim

[hkBst]

Adding this impl causes problems with specialization, so
@rustbot label -E-easy E-medium