Slow compile times for promotion of [(); usize::MAX]

[carbotaniuman]

I tried this code:

fn main() {
    let _ = &[(); usize::MAX];
}

I expected to see this happen: code compiles in a reasonable amount of times or the slice gets elided entirely.

Instead, this happened: code takes multiple minutes to compile/gets killed by the playground.

Meta

rustc --version --verbose:

rustc 1.48.0-nightly (9b4154193 2020-09-14)
binary: rustc
commit-hash: 9b4154193e8471f36b1a9e781f1ef7d492fc6a6c
commit-date: 2020-09-14
host: x86_64-pc-windows-msvc
release: 1.48.0-nightly
LLVM version: 11.0

Also tested on playground stable and nightly.

[tesuji]

The code compiled in about 6 seconds in Rust 1.29 and 1.30 : https://rust.godbolt.org/z/hbdora .
Before that it had capacity error. After that it exceeded 15 seconds limit of godbolt.

This is a regression-from-stable-to-stable but I cannot tag it.

[mstallmo]

It looks like this also works correctly on 1.31 stable as well: https://rust.godbolt.org/z/v5vEcE

[mstallmo]

Decided on P-medium as discussed in the prioritization working group procedure.

[RalfJung]

Cc @rust-lang/wg-const-eval

[wesleywiser]

Fixed in #78061.