Meeting Proposal: How does match interact with UnsafeCell

[chorman0773]

Summary

Currently, both Stacked Borrows and Treed Borrows considers the following code to have defined behaviour:

use std::cell::Cell;

pub fn evil(x: &Result<bool, Cell<u8>>) -> bool {
    unsafe { *(x as *const _ as *const bool as *mut bool).add(1) = false }
    false
}

pub fn main() {
    let x = Ok(true);
    match &x {
        Ok(true) if evil(&x) => unreachable!(),
        Ok(false) => println!("Ok(false) = {x:?}"),
        Ok(true) => println!("Ok(true) = {x:?}"),
        Err(_) => unreachable!(),
    }
}

Under the current match rules, this should evaluate and print Ok(false) = Ok(false). However, on the current lowering by rustc, it prints Ok(true) = Ok(false). If both lowerings (linear, and rustc's current lowering) are correct, this code must have undefined behaviour (or some other rule must allow it). The meeting would be to help address how this code has UB, or if it does/should.

Reading

UCG Issue: unsafe-code-guidelines#399.
Zulip Thread: https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/How.20does.20match.20interact.20with.20.60.26UnsafeCell.60

There will likely be a summary document prepared in time for the meeting, based on the asynchronous discussion that has occured by that time.

Comment policy

These issues are meant to be used as an "announcements channel" regarding the proposal, and not as a
place to discuss the technical details. Feel free to subscribe to updates. We'll post comments when
reviewing the proposal in meetings or making a scheduling decision. In the meantime, if you have
questions or ideas, ping the proposers on Zulip (or elsewhere).

[chorman0773]

This meeting has been held as of 2023-05-09. The question has not been resovled, but the underlying questions can be refined asynchronously prior to holding another Opsem Meeting on this topic. I am therefore closing this issue.