cargo build needed for panics in constant expressions

[Jarcho]

Currently cargo check will not evaluate associated constants or const blocks. This results in cargo check succeeding while cargo build fails to compile when it evaluates the constant.

Example

trait A { const A: (); }
impl A for () { const A: () = panic!(); }

fn main() {
    <() as A>::A;
}

This s probably best done with a force_eval_constants flag which would switch to using cargo build for any rust version (currently all of them) for which cargo check will miss constant panics.

[dtolnay]

This is a duplicate of #225.