core::slice::Items is lacking the proper lifetime annotations

[nikomatsakis]

std::slice::Items should require that T:'a, but it does not. It is able to do this due to its unsafe implementation. The following test case fails to compile as a result:

use std::slice::Items;

pub trait MyVec<T> : Slice<T> {
  fn iter<'a>(&'a self) -> Items<'a, T> {
    self.as_slice().iter()
  }
}

fn main() {

}

[nikomatsakis]

This is not quite as trivial as adding T:'a to Items -- the problem is that MoveItems abuses Items by embedding a Items<'static, T>, which then requires that T:'static to be valid (which is no good). The proper fix is probably to have MoveItems (and Items) build on an UnsafeItems, or else just both embed some unsafe pointers.

[brson]

Nominating because this is an important type.

[pnkfelix]

We can fix this backwards compatibly, so this is P-high, not a 1.0 blocker.

[dschatzberg]

As the 2nd comment indicates, this cannot be fixed backwards compatibly. As soon as we add the proper lifetime specifications to Items and MoveItems, any code which depends on them may break. vec's MoveItems is just an example.

[pnkfelix]

Renominating, I am not sure how we came to the prior conclusion that this could be fixed backwards compatibly (unless the implication was that we would adopt some solution other than adding the lifetime bound, which is not clear to me from my earlier note).

I suspect this should actually be P-backcompat-lang, 1.0.

[aturon]

@pnkfelix It's a breaking change, but it's a library issue not a language issue. I'm fine with 1.0 on this.

There's a PR already, in any case.