iter::Zip is now invariant

[Stebalien]

The following compiles on stable but not beta:

use std::iter::Zip;

fn test<'a, A, B>(iter: Zip<&'static A, &'static B>) -> Zip<&'a A, &'a B> { iter }

fn main() {}

[Stebalien]

(also a regression from stable to beta)

[brson]

If this is a beta regression that means it is going to hit stable thursday.

[brson]

If somebody fixed and landed this right now and we started a stable build tonight we might be able to manage getting this into a Thursday release.

[brson]

... or we could just end up flubbing and missing our date.

[alexcrichton]

I don't think this is a serious enough regression to halt the release. @Stebalien this was just manufactured, right? There's no code that actually hit this?

[Stebalien]

@brson. The fix is to either revert the zip specializations (simplest) or put the extra fields needed by specialization into the main Zip struct. We can keep the enumerate specialization as-is (it doesn't need extra specialized fields).

@alexcrichton Not my code.

[bluss]

Ah this is troublesome, indeed it's the associated types/struct field specialization that causes this, not just the regular "behavioral" specialization isn't it. I can't think of a workaround, we could back out the whole optimization, or just the associated type part?

[alexcrichton]

Discussed at @rust-lang/libs triage today, we concluded that we're likely to revert but we're not sure. @aturon wasn't present unfortunately and he'll likely also have opinions on this as well. We'll discuss again soon.

Our other conclusion was that this wasn't super urgent because it's already hit stable.