1 use std::cell::UnsafeCell;
2 use std::mem;
3 
4 // Like `&mut *x.get()`, but without intermediate raw pointers.
5 #[allow(mutable_transmutes)]
unsafe_cell_get<T>(x: &UnsafeCell<T>) -> &'static mut T6 unsafe fn unsafe_cell_get<T>(x: &UnsafeCell<T>) -> &'static mut T {
7     mem::transmute(x)
8 }
9 
main()10 fn main() { unsafe {
11     let c = &UnsafeCell::new(UnsafeCell::new(0));
12     let inner_uniq = &mut *c.get();
13     let inner_shr = &*inner_uniq;
14     // stack: [c: SharedReadWrite, inner_uniq: Unique, inner_shr: SharedReadWrite]
15 
16     let _val = c.get().read(); // invalidates inner_uniq
17     // stack: [c: SharedReadWrite, inner_uniq: Disabled, inner_shr: SharedReadWrite]
18 
19     // We have to be careful not to add any raw pointers above inner_uniq in
20     // the stack, hence the use of unsafe_cell_get.
21     let _val = *unsafe_cell_get(inner_shr); // this still works
22 
23     *c.get() = UnsafeCell::new(0); // now inner_shr gets invalidated
24     // stack: [c: SharedReadWrite]
25 
26     // now this does not work any more
27     let _val = *inner_shr.get(); //~ ERROR borrow stack
28 } }
29