std::ptr docs state:
The result of casting a reference to a pointer is valid for as long as the underlying allocation is live and no reference (just raw pointers) is used to access the same memory. That is, reference and pointer accesses cannot be interleaved.
This quote suggests that if I cast a reference to a pointer, I cannot use that reference if I'm also using the pointer.
But is this true if I'm only using shared references, casting them to *const ptrs, and only doing reads?
Above quote does not make that distinction and suggests that this would be invalid.
In the code below a_ref is cast to a_ptr and the rest of the code uses both of them. Miri does not raise ub for this.
use std::thread;
#[derive(Clone, Copy)]
struct Ptr<T>(*const T);
unsafe impl<T> Send for Ptr<T> {}
unsafe impl<T> Sync for Ptr<T> {}
fn main() {
unsafe {
let a = 1;
let a_ref = &a;
let a_ptr = Ptr(a_ref as *const i32);
let a_ptr2 = Ptr(&a as *const i32);
let a_ptr3 = Ptr(&raw const a);
println!("a_ptr: {:?}", *a_ptr.0);
println!("a_ref: {:?}", *a_ref);
println!("a_ptr2: {:?}", *a_ptr2.0);
println!("a_ptr3: {:?}", *a_ptr3.0);
// and again
println!("a_ptr: {:?}", *a_ptr.0);
println!("a_ref: {:?}", *a_ref);
println!("a_ptr2: {:?}", *a_ptr2.0);
println!("a_ptr3: {:?}", *a_ptr3.0);
// for good mesure do it also in parallel
thread::scope(move |s| {
for _ in 0..10 {
s.spawn(move || {
let a_ptr = a_ptr;
let a_ptr2 = a_ptr2;
let a_ptr3 = a_ptr3;
println!("a_ptr: {:?}", *a_ptr.0);
println!("a_ref: {:?}", *a_ref);
println!("a_ptr2: {:?}", *a_ptr2.0);
println!("a_ptr3: {:?}", *a_ptr3.0);
});
}
});
}
}