ReentrantLockGuard's Sync impl is unsound

[programmerjake]

I tried running this code in miri:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=4b45c8a022255be4404228c01922b632

#![feature(reentrant_lock)]
use std::cell::Cell;
use std::sync::ReentrantLock;
use std::thread::scope;

fn main() {
    let l = ReentrantLock::new(Cell::new(0u8));
    let lg = l.lock();
    scope(|s| {
        s.spawn(|| dbg!(lg.get()));
        lg.set(1);
    });
}

I expected to see this happen: compile error

Instead, this happened: compiled successfully and miri reported a data race

this is because the automatic impl<T: Send> Sync for ReentrantLockGuard<T> is incorrect:
https://doc.rust-lang.org/1.78.0/std/sync/struct.ReentrantLockGuard.html#impl-Sync-for-ReentrantLockGuard%3C'a,+T%3E

ReentrantLock's tracking issue: #121440

[programmerjake]

@rustbot label I-unsound

[programmerjake]

@rustbot label requires-nightly