Borrowing static array that contains another static array represented by literal doesn't work when expressed inline

[pepyakin]

Why does this work:

use std::borrow::Cow;

pub const X: [u8; 3] = *b"ABC";
pub const Y: Cow<'static, [ [u8; 3] ]> = Cow::Borrowed(&[X]);

but this doesn't?

use std::borrow::Cow;

pub const Z: Cow<'static, [ [u8; 3] ]> = Cow::Borrowed(&[*b"ABC"]);

[Havvy]

error[E0597]: borrowed value does not live long enough
 --> src/lib.rs:3:57
  |
3 | pub const Z: Cow<'static, [ [u8; 3] ]> = Cow::Borrowed(&[*b"ABC"]);
  |                                                         ^^^^^^^^^- temporary value only lives until here
  |                                                         |
  |                                                         temporary value does not live long enough
  |
  = note: borrowed value must be valid for the static lifetime...

[memoryruins]

If we add #![feature(nll)], it compiles. (playground)

It appears related to the issue described in #43058, which was fixed by nll.
cc @rust-lang/wg-compiler-nll

[nikomatsakis]

This is a bit curious. I'm digging in a bit more.

[eddyb]

It's a bug that NLL "fixes" this - see #51990 (comment).

@pepyakin This is not a bug (ignoring NLL), but we need to document it better.

*b"ABC" is not considered a "promotable-to-'static constant expression". We rely on evaluating it later to ensure that it is valid where written, but it's not treated as promotable because we don't track references/pointers in the promotion analysis, and promotion could change behavior.

We could maybe relax the rules a bit here, since we're in a constant anyway, and promotion rules require not involving named variables, but we'd need to be careful around Drop and unsafe code.