Tracking issue for future-incompatibility lint `const_eval_mutable_ptr_in_final_value`

[pnkfelix]

This is a tracking issue for the CONST_EVAL_MUTABLE_PTR_IN_FINAL_VALUE lint, which is being added in #122204. The lint detects cases where a const expression evaluates to a value that holds a pointer to mutable state, which is undefined behavior and may be rejected by a future version of the compiler.

Example

This lint will trigger for code like this:

use std::cell::Cell;

pub enum JsValue {
    Undefined,
    Object(Cell<bool>),
}

impl ::std::ops::Drop for JsValue {
    fn drop(&mut self) {}
}

const UNDEFINED: &JsValue = &JsValue::Undefined;

fn main() {
}

Cc #121610