miri accepts when embedded UnsafeCell in mutable Pin reference is changed #1860
Description
It's my understanding that changing a memory location through a pointer that is an alias of a mutable reference is disallowed in Rust, and that miri could/should help detecting such scenarios.
Consider this playground reproduced here:
use std::ptr;
use std::pin::Pin;
use std::cell::UnsafeCell;
struct A {
x: UnsafeCell<isize>,
ptr_x: *mut isize,
}
impl A {
fn new(x: isize) -> Pin<Box<A>> {
let mut a = Box::pin(A { x: UnsafeCell::new(x),
ptr_x: ptr::null_mut()
});
a.ptr_x = a.x.get_mut();
a
}
fn change_x(self: &mut Pin<Box<A>>) {
unsafe {
self.ptr_x.write(0);
}
}
#[inline(never)]
fn wait_for_x(self: &mut Pin<Box<A>>) {
while unsafe { *self.x.get() == 42 } {
self.change_x();
}
}
}
fn main()
{
let mut a = Box::new(A::new(42));
println!("waiting for Godot");
a.wait_for_x();
println!("done");
}Should miri flag UB in this code?
ps: compare to this similar playground that does not use Pin. Here, miri flags UB and the release/nightly compiler correctly exploits the UB by emitting an infinite loop.