ICE when const-evaling a block that does not end with an expression

[scurest]

Unreachable code is entered when trying to const-eval a block that doesn't have a terminating expression.

const fn f(_: ()) -> usize { 1 }
fn main() {
    [0; f({})];
}

The relevant line is

rust/src/librustc/middle/const_eval.rs

Line 1124 in 8c77ffb

None => unreachable!(),

[oli-obk]

That was me! I found no way to trigger this so I made it unreachable, but I forgot about const fn arguments. We don't have a way of creating a unit constant. Technically it would be an empty tuple, but our constant tuples are just an expression id, we can't create new ones in the const evaluator.

[oli-obk]

does not ice anymore, but reports a normal error