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