Nightly regression around HRTBs, likely connected to wf changes

[aturon]

The following code:

struct Parser<'i: 't, 't>(&'i u8, &'t u8);

impl<'i, 't> Parser<'i, 't> {
    fn parse_nested_block<F, T>(&mut self, parse: F) -> Result<T, ()>
        where for<'tt> F: FnOnce(&mut Parser<'i, 'tt>) -> T { panic!() }

    fn expect_exhausted(&mut self) -> Result<(), ()> { Ok(()) }
}

fn main() {
    let x = 0u8;
    Parser(&x, &x).parse_nested_block(|input| input.expect_exhausted()).unwrap();
}

compiles on 1.3 stable and 1.4 beta, but not on nightly. This seems likely related to wf changes.

[aturon]

cc @SimonSapin @nikomatsakis

[aturon]

cc @arielb1

[SimonSapin]

The above is a reduction of servo/rust-cssparser@29f757d

[SimonSapin]

I can reproduce in rustc 1.5.0-nightly (d0cae14 2015-10-08) but not in rustc 1.5.0-dev (168a23e 2015-10-01). (The latter is what Servo uses at the moment.)

[aturon]

A quick scan of the intervening commits suggests 603a75c may be related.

[arielb1]

The eta-reduced version fails on all versions of rustc:

struct Parser<'i: 't, 't>(&'i u8, &'t u8);

impl<'i, 't> Parser<'i, 't> {
    fn parse_nested_block<F, T>(&mut self, parse: F) -> Result<T, ()>
        where for<'tt> F: FnOnce(&mut Parser<'i, 'tt>) -> T { panic!() }

    fn expect_exhausted(&mut self) -> Result<(), ()> { Ok(()) }
}

fn main() {
    let x = 0u8;
    Parser(&x, &x).parse_nested_block(Parser::expect_exhausted).unwrap();
}

<anon>:12:20: 12:64 error: type mismatch: the type `fn(&mut Parser<'_, '_>) -> core::result::Result<(), ()> {Parser<'i, 't>::expect_exhausted}` implements the trait `for<'r> core::ops::FnOnce<(&'r mut Parser<'_, '_>,)>`, but the trait `for<'r, 'tt> core::ops::FnOnce<(&'r mut Parser<'_, 'tt>,)>` is required (expected concrete lifetime, found bound lifetime parameter 'tt) [E0281]
<anon>:12     Parser(&x, &x).parse_nested_block(Parser::expect_exhausted).unwrap();
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:12:20: 12:64 error: type mismatch resolving `for<'r, 'tt> <fn(&mut Parser<'_, '_>) -> core::result::Result<(), ()> {Parser<'i, 't>::expect_exhausted} as core::ops::FnOnce<(&'r mut Parser<'_, 'tt>,)>>::Output == _`:
 expected bound lifetime parameter 'tt,
    found concrete lifetime [E0271]
<anon>:12     Parser(&x, &x).parse_nested_block(Parser::expect_exhausted).unwrap();
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

<anon>:12:20: 12:64 help: see the detailed explanation for E0271
error: aborting due to 2 previous errors

The wf fixes just bought it in line.

[arielb1]

The UFCS version fails on 1.4+, but succeeds if you annotate the parameter type - this looks like a closure inference issue.

[arielb1]

This was not fixed by #29006

[nikomatsakis]

triage: P-high

[nikomatsakis]

/me investigating THIS now

[nikomatsakis]

This seems to boil down to a bug in the givens list for region inference. It is fixed by #29188 (at least, it is with the latest commit).