evaluate required_consts when pushing stack frame in Miri engine

[RalfJung]

Just like codegen, Miri needs to make sure all required_consts evaluate successfully, to catch post-monomorphization errors.

While at it I also moved the const_eval error reporting logic into rustc_mir::const_eval::error; there is no reason it should be in rustc_middle. I kept this in a separate commit for easier reviewing.

Helps with rust-lang/miri#1382. I will add a test on the Miri side (done now: rust-lang/miri#1504).
r? @oli-obk

[RalfJung]

This is the key new loop.

[oli-obk]

#![warn(const_err, unconditional_panic)]

struct PrintName<T>(T);
impl<T> PrintName<T> {
    const VOID: () = [()][2]; //~WARN any use of this value will cause an error
}

const fn no_codegen<T>() {
    if false {
        let _ = PrintName::<T>::VOID; //~ERROR referenced constant has errors
    }
}

pub static FOO: () = no_codegen::<i32>();

fn main() {
    FOO
}

works on beta, so we should probably beta backport this (and add the above as a test).

The beta backport should not contain the first commit though

[RalfJung]

I don't see why this should be beta-backported -- it's an ooold bug for that to work.

[oli-obk]

it's an ooold bug for that to work.

not really, before this beta there was no conditional execution. So the constant was always evaluated unless optimized out... ok... true, we used to optimize them out. So nevermind on the beta nomination, but the test should still be in this repo, not just in miri

[oli-obk]

So this is a breaking change, but only if your code was UB anyway

[RalfJung]

So this is a breaking change, but only if your code was UB anyway

It is, just like the corresponding change in codegen was a breaking change.
I am not sure if the old code was UB (dead code cannot cause UB), but it is a bug that we fixed for codegen and should likewise fix for CTFE.

the test should still be in this repo, not just in miri

Fair, I'll add it.

[RalfJung]

@oli-obk I added a test.

[oli-obk]

@bors r+

[bors]

📌 Commit fd3851a has been approved by oli-obk