Nightly regression: Unable to create array temporary in static

[Aaron1011]

This code snippet:

struct Foo<'a>(&'a [&'a bool]);

static TEST: bool = true;
static FOO: Foo = Foo(&[&TEST]);

fn main() {}

compiles on stable rust rustc 1.20.0 (f3d6973f4 2017-08-27), but not on rustc 1.22.0-nightly (2f1ef9ef1 2017-09-05).

Playground link

[Aaron1011]

I'd like to work on this issue.

[Mark-Simulacrum]

cc @eddyb -- this seems related to the work you've been recently doing

[eddyb]

Hmm, this looks like an rvalue promotion failure. @Aaron1011 can you try, on this line:

rust/src/librustc_mir/transform/qualify_consts.rs

Line 69 in a209539

const NEVER_PROMOTE = 0b111_1111,

adding & !Qualif::STATIC_REF.bits?

That should allow &[&TEST] to promote and thus let the code compile again.
The limitations around static are completely artificial given a correct implementation, and will hopefully be removed when miri starts being used for evaluating these constants.

[nikomatsakis]

triage: P-high

Assigning to @eddyb to mentor @Aaron1011 towards a fix!

[eddyb]

I just realized I may have never seen the actual error emitted, I thought it was MIR constant checking that was banning a reference to a static. But static lvalue is not an rvalue so the error on nightly should be impossible to reach, given the mem_categorization result for it.

EDIT: nevermind, I just realized [&TEST] is what doesn't get promoted.

[Aaron1011]

My fix has been merged, and is included in the latest nightly.