The code is sound if it uses UnsafeCell::get, and all the fields inside that type is covered by UnsafeCell. It should use UnsafeCell::get or UnsafeCell::raw_get instead of pointer-casting.

The code is sound if it uses UnsafeCell::get, and all the fields inside that type is covered by UnsafeCell. It should use UnsafeCell::get or UnsafeCell::raw_get instead of pointer-casting.

Edit: I think what I want to ask about is the following example, which is almost identical to the original question and passes miri's check:

use std::cell::Cell;

struct S(Cell<i32>);
struct U(S);

fn main() {
    let u = U(S(Cell::new(10)));
    let p = &u as *const _ as *mut U;
    let s = unsafe { &mut (*p).0 };
    println!("{}", s.0.take());
}

@NichtsHsu No, you should not do that.

So this is what is happening in the standard library:

There is a structure like Key { inner: LazyKeyInner(UnsafeCell), ... };

In Key::try_register_dtor, &self is converted to *mut u8 and passed to register_dtor;

In destroy_value, *mut u8 is converted to ptr: *mut Key, then (*ptr).inner.take() is called.

LazyKeyInner::take requires &mut self, so it is actually LazyKeyInner::take(&mut (*ptr).inner), which means that a &mut LazyKeyInner is born from a &Key.

In the entire process, we haven’t touched on anything related to UnsafeCell. And, I also believe that containing an UnsafeCell is not a reason for LazyKeyInner to be treated differently.

Yes, LazyKeyInner::take should be fn(&self) -> Option<T> and not fn(&mut self) -> Option<T>.

@NichtsHsu No, you should not do that.

I should clarify that my "should not" here is that it can potentially pass miri, but my understanding is that it is brittle and relying on the operational semantics being a certain way, rather than being a "necessarily true" interpretation of Rust's semantics. Programs which are slight modifications easily become unsound, and libstd unfortunately has slight modifications done to it all the time, so...

aiui this code currently passes miri because we do run miri on libstd's tests, but is not what I would consider "durable".

( probably. )