lub: don't bail out due to empty binders

[lcnr]

allows for the following to compile. The equivalent code using struct Wrapper<'upper>(fn(&'upper ()); already compiles on stable.

let _: fn(&'upper ()) = match v {
    true => lt_in_fn::<'a>(),
    false => lt_in_fn::<'b>(),
};

see https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7034a677190110941223cafac6632f70 for a complete example

r? @rust-lang/types

[rust-highfive]

r? @estebank

(rust-highfive has picked a reviewer for you, use r? to override)

[jackh726]

r? @jackh726

[jackh726]

This does allow more behavior, but I don't think there's anything that we don't want from a more "complete" system. Right?

[lcnr]

I think the only way this change could be a future compat concern is if we infer the
LUB of fn(fn(&'a ())) and fn(fn(&'b ())) to fn(for<'c> fn(&'c ())) in the future, at which point something like the following would break.

// if we infer `x` to be higher ranked in the future,
// this would cause a type error.
let x = match v {
    true => lt_in_fn_fn::<'a>(),
    false => lt_in_fn_fn::<'b>(),
};

let y: fn(fn(&'lower ())) = x;

Adding that as a test.

[compiler-errors]

rust-lang/team#778

[compiler-errors]

Let's try this again:

@rfcbot merge

Do we want to commit to not inferring the LUB of fn(fn(&'a ())) and fn(fn(&'b ())) to fn(for<'c> fn(&'c ())) in the future?

[rfcbot]

Team member @compiler-errors has proposed to merge this. The next step is review by the rest of the tagged team members:

• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[lcnr]

(as a side note, I don't expect this to be frequent enough that it would ever actually block us from inferring the lub to be higher ranked. People don't tend to have nested function pointers)

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[nikomatsakis]

@rfcbot reviewed

For future references, we had a detailed discussion in the types team meeting, where we went over what was being proposed here, how it fit historically, and how it relates to the new subtyping algorithm that we are developing in a-mir-formality.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[oli-obk]

@bors r+

[bors]

📌 Commit 0667b00 has been approved by oli-obk