Lifetime extension in consts/statics sometimes makes *less* code compile

@traviscross found this fun example:

const C1: &[u8] = { let x: &'static mut [u8] = &mut []; x }; //~ OK
const C2: &[u8] = { &mut [] }; //~ ERROR

C1 compiles because empty arrays get promoted even behind mutable references.

C2 does not compile because that borrow is in lifetime extension position, and then the const-checks reject it due to being a non-transient mutable borrow. If we didn't do lifetime extension here, then the code would work, since then const-checks would consider the borrow to be transient, and finally promotion would make the rest work.

Not sure if this is worth fixing, but it's definitely odd. If we did want to fix it, we would have to either

• ensure that const-checks accept non-transient borrows that will later get promoted (duplicating the same logic... ugh)
• or run promotion before const-checks (which seems to be the opposite of what @oli-obk has planned)

@rust-lang/wg-const-eval

Reproduce the C1 and C2 examples and read the linked Rust Reference lifetime-extension section first. Then trace how lifetime extension, const-checks, and promotion process these borrows; a complete resolution would establish whether this behavior should change and make the intended result consistent.