[beta] back out #53793 - stabilize outlives requirements

[arielb1]

Fixes #54467 for beta, looks like a less risky fix than #54701

[rust-highfive]

r? @eddyb

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[rust-highfive]

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[Mark-Simulacrum]

cc @Centril @aturon -- outlives requirements are a part of 2018 I believe which makes this worrisome. Do we need to prioritize resources to finding the underlying cause? (Not exactly sure what the status is here)

[Centril]

@Mark-Simulacrum They are; but we could also drop it from 2018 if it can't be done. However, that would be quite unfortunate and make 2018 less exciting. As for finding the underlying cause I can't help out myself, but I do think it's worth prioritizing this. I'd wait for @aturon's input tho.

[arielb1]

@Centril

We have a fix for #54467 on nightly. I just don't want to push it to beta. If you think that getting more experience for stable outlives requirements is a priority, there's no reason not to backport #54467.

[Centril]

@arielb1 I'm quite unsure what to do here so I'll defer to someone else (e.g. @aturon).

[nikomatsakis]

Perhaps I am missing something. This PR merely removes the stabilization from the current beta — this change will still be considered stable on nightly and in RC2 when that branch breaks, and hence it will still be part of Rust 2018. It's just arriving a bit later to avoid a more complex backport.

The only real reason I can see to land this change earlier would be to help us flush out more bugs, but finding those bugs on stable is usually not the sort of thing we try to do (rather, we tend to prefer to give a patch more time for testing on nightly and beta).

[nikomatsakis]

r? @nikomatsakis -- stealing this from @eddyb

[nikomatsakis]

@bors r+

[bors]

📌 Commit e954510 has been approved by nikomatsakis

[nikomatsakis]

After some brief conversation on Discord, it seems clear we can move forward with this.

[emilyalbini]

@bors p=1 (beta backport)

[bors]

⌛ Testing commit e954510 with merge 1320d21...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 1320d21 to beta...