Const-time immutability does not carry over to runtime

[BennoLossin]

I tried this code:

#![feature(sync_unsafe_cell)]
use std::cell::SyncUnsafeCell;

static PARAM: SyncUnsafeCell<i64> = SyncUnsafeCell::new(0);

static BAD: &'static i64 = unsafe { &*PARAM.get() };

fn main() {
    println!("{BAD}");
    unsafe { *PARAM.get() = 42 };
    println!("{BAD}");
}

And expected miri to complain about UB. But it doesn't (I also tried tree-borrows & strict-provenance).

Zulip discussion.

(feel free to improve the issue title)

[RalfJung]

Thanks for filing an issue!

This is at the moment more of a spec question, since it is unclear whether we want this to be UB. The fact that a pointer gets created during const-eval and then used at runtime makes this a very special case. That question is tracked at rust-lang/unsafe-code-guidelines#424.

Even if we say this should be UB, it'll be hard to actually represent this in the general case... though the simple case you gave here should be fairly easy to implement with Tree Borrows.

[BennoLossin]

thanks for the pointer, didn't expect to find this much complexity behind my question :) I will just treat it as UB until a decision is made.