rustc: lifetime elision should not apply to covariant uses of lifetimes

[aturon]

As was noted in the RFC discussion, covariant lifetime positions (which I was mistakenly referring to as "contravariant") should require explicit lifetime annotations.

cc #15552

[aturon]

Nominating, as this is a backwards-incompatible change.

[pcwalton]

I'm going to bring this up at the meeting, because I don't think it's worth doing. Long story short: This would require a whole new pass in typeck that would involve duplicating all the variance rules just to enforce this one rule that seems like it rarely ever comes up.

[pcwalton]

Based on some quick analysis I believe that, in our existing codebase (rustc+libs), covariant lifetimes never come up in any situations where lifetime elision could possibly apply. So I have extra reason to believe this is not worth doing.

[aturon]

Thanks for collecting that information -- could you post a few example fn names? I'm just curious.

[zwarich]

@pcwalton Covariant lifetimes being used in a meaningful way doesn't really come up that much in general in the existing codebase. When I removed covariance altogether, I only found one instance up to libsyntax (and even that was potentially just a bug in the inference algorithm, since it wasn't obvious to me what the issue was). That said, if we are going to support them then lifetime elision shouldn't do some obviously incorrect thing for them, especially after specifically stating in the RFC discussion that it would be handled correctly.

[pcwalton]

@aturon Not sure what you mean. What function names?

@zwarich I don't want to add an entire extra pass to the typechecker, one that duplicates all the variance information, to guard against some potentially confusing behavior that never comes up in practice. That just strikes me as gratuitous language complexity.

[aturon]

@pcwalton I'm curious about the places where covariant lifetimes do come up.

I was assuming they did show up in function arguments, and you were saying that the elision rules wouldn't apply anyway; I'd just be curious to know what the functions are.

But maybe you're saying they never show up in argument position?

[pcwalton]

They show up on every closure that has a lifetime in it. However, according to my analysis, none of those appear in argument position when there was a by-reference return value present. Examples of functions that take closures that take arguments by reference are filter and skip_while.

[aturon]

@pcwalton Those closures are higher-ranked.

For embedded closures like that, it's critical that the lifetime elision rules apply in a nested fashion, so that

fn filter<A>(predicate: |&A|: 'r -> bool) -> Filter<'r, A, Self>

becomes

fn filter<'r, A>(predicate: <'a>|&'a A|: 'r -> bool) -> Filter<'r, A, Self>

The rule in question was about a hypothetical situation like:

fn foo(callback: Callback) -> V

where Callback<'a> contains a closure |&'a T| -> U.

[pcwalton]

Ah, right. I'll rerun the analysis.

[pnkfelix]

Assigning P-backcompat-lang, 1.0 milestone, with expectation that we will discuss further in the team mtg next week.