Spurious "does not live long enough" for closure coerced to fn pointer in static item (AST borrowck only)

[SimonSapin]

The test case below is reduced from a phf map.

rustc 1.26.0-nightly (28a1e4f 2018-02-24) with -Z borrowck=mir: OK

Without any -Z flag: error below, even though I believe the program to be valid. But maybe it’s not worth fixing if MIR-based borrow-checking will become the default soon? CC @nikomatsakis

A work-around is to have the closure in an intermediate const item.

fn main() {}

struct Static<T: 'static>(&'static [T]);

static FOO: Static<fn(&u32) -> &u32> = Static(&[|x| x]);

error[E0597]: borrowed value does not live long enough
 --> a.rs:5:52
  |
5 | pub static FOO: Static<fn(&u32) -> &u32> = Static(&[|x| x]);
  |                                                    ^^^^^^^- temporary value only lives until here
  |                                                    |
  |                                                    temporary value does not live long enough
  |
  = note: borrowed value must be valid for the static lifetime...

error: aborting due to previous error

fn main() {}

struct Static<T: 'static>(&'static [T]);

static FOO: Static<fn(&u32) -> &u32> = Static(&[
    {
        const F: fn(&u32) -> &u32 = |x| x;  // work around
        F
    },
]);

Hey I learned something new, like:
adding #![feature(nll)] is, apparently, equivalent to -Z borrowck=mir and to -Z nll

NLL tracking issue

how to pass -Z flags with cargo

Thanks to durka42_ (on IRC) for telling me to how to pass those pesky -Z flags :D
eg. cargo rustc -- -Z borrowck=mir

rustc -Z help

[nikomatsakis]

It's actually equivalent to -Zborrowck=mir -Znll -Ztwo-phase-borrows, I think

[nikomatsakis]

@SimonSapin is this a regression, do you know?

[SimonSapin]

@nikomatsakis Not as far as I know. 1.24.0 stable gives the same error. Note that this is fixed by MIR-borrowck.

[nikomatsakis]

OK. In that case I am inclined to wait, yes.

[SimonSapin]

Adding the "fixed by NLL" label as an approximation of fixed by MIR borrowck