Rejecting higher-kinded lifetime bounds is a breaking change

[comex]

fn foo<'s, F>(s: &'s i32, f: F) where for<'a: 's> F: FnOnce(&'a i32) {
    f(s)
}

fn main() {
    foo(&42, |x| println!("{}", x));
}

Playground Link

This compiles on stable; on beta and above, it produces:

error: lifetime bounds cannot be used in this context

Apparently this is on purpose:

commit 49abd8748357012e5db10bf11077384f727e2177
Author: Ralf Jung <post@ralfj.de>
Date:   Tue Mar 6 11:22:24 2018 +0100

    make bounds on higher-kinded lifetimes a hard error in ast_validation
    
    Also move the check for not having type parameters into ast_validation.
    
    I was not sure what to do with compile-fail/issue-23046.rs: The issue looks like
    maybe the bounds actually played a role in triggering the ICE, but that seems
    unlikely given that the compiler seems to entirely ignore them.  However, I
    couldn't find a testcase without the bounds, so I figured the best I could do is
    to just remove the bounds and make sure at least that keeps working.

But it's still a gratuitous breaking change, which seems unfortunate… even if the lifetimes weren't being handled correctly before.

cc @RalfJung

[ExpHP]

This is in the 1.26 that's about to ship, right? Maybe it should be mentioned in the release notes?

I don't think it's too far of a stretch to say that there is live code out there with a for<'a: 'b> somewhere in it. Barely weeks ago, I myself was playing around with a for<'a, 'b: 'a> fn on the playground, wondering if I'd ever need to impl a trait for one.

[comex]

For the record, I reported this after running into it with real code I was developing on stable. I needed to debug a macro, so I switched to nightly in order to use --pretty=expanded; then my code stopped compiling.

[RalfJung]

Yes it is a breaking change indeed, introduced in #48326. That's why we did a crater run, which came back with a single failure.

However, @comex your code is equivalent to

fn foo<'s, F>(s: &'s i32, f: F) where for<'a> F: FnOnce(&'a i32) {
...
}

That is, the bound you wrote is entirely ignored. So, this only breaks code that already did not do what the author thought it would. Given that so far, not many people seem to have made this mistake (writing lifetime bounds that are ignored), it seemed worth the breaking change to make sure no more people will do so in the future.

[comex]

Welp, I didn't realize we were so close to a stable release containing the change in question. I guess this is now moot. But I wish the change had gone through a warning cycle.

[RSSchermer]

Apologies for resurrecting an old issue; I find myself in need of for<'a, 'b: 'a> ... and my google-fu turned up this thread, which makes it seem like it might have been possible at some point.

Is this a false impression, was it just that the syntax didn't trigger an error, but no semantics were attached to it? Or was this indeed a working feature that was removed (presumably for soundness reasons)?

[RalfJung]

was it just that the syntax didn't trigger an error, but no semantics were attached to it?

That was indeed the case. The bound got parsed, and then ignored.