Static assert idiom compiles very slowly for certain values

[thomcc]

I wrote the following code in a small project to help assert constant integers are equivalent. There are other ways to write this test but this tells me the values on failure.

macro_rules! static_assert_ints_eq {
    ($a:expr, $b:expr) => {
        // todo: enforce that args are reasonable integers
        const _: [(); $a as usize] = [(); $b as usize];
    };
}

Later, I called it in a manner where the values were equal but negative. They were the result of a const function, but something like this is equivalent:

static_assert_ints_eq!(-1i32, -1i32);

I expected this to be relatively cheap for rustc — I was hoping that the const _: indicates that I'm just looking for a type check.

Unfortunately, I had to kill the compiler because it was taking too long. I imagine there's a bug somewhere for this but could not find it, but if not, this can be that bug.

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (38d911dfc 2020-10-09)
binary: rustc
commit-hash: 38d911dfc55a7a1eea1c80139113ed2ff0151087
commit-date: 2020-10-09
host: x86_64-apple-darwin
release: 1.49.0-nightly
LLVM version: 11.0

[thomcc]

Note: I'm aware of several other ways to write this that are better along various metrics — that's not my point. My point is that this way (which is somewhat common) does poorly.

[jonas-schievink]

Duplicate of #68010