1 use core::marker::PhantomData;
2 use core::ptr::NonNull;
4 /// Models a reborrow of some unique reference, when you know that the reborrow
5 /// and all its descendants (i.e., all pointers and references derived from it)
6 /// will not be used any more at some point, after which you want to use the
7 /// original unique reference again.
9 /// The borrow checker usually handles this stacking of borrows for you, but
10 /// some control flows that accomplish this stacking are too complicated for
11 /// the compiler to follow. A `DormantMutRef` allows you to check borrowing
12 /// yourself, while still expressing its stacked nature, and encapsulating
13 /// the raw pointer code needed to do this without undefined behavior.
14 pub struct DormantMutRef<'a, T> {
16 _marker: PhantomData<&'a mut T>,
19 unsafe impl<'a, T> Sync for DormantMutRef<'a, T> where &'a mut T: Sync {}
20 unsafe impl<'a, T> Send for DormantMutRef<'a, T> where &'a mut T: Send {}
22 impl<'a, T> DormantMutRef<'a, T> {
23 /// Capture a unique borrow, and immediately reborrow it. For the compiler,
24 /// the lifetime of the new reference is the same as the lifetime of the
25 /// original reference, but you promise to use it for a shorter period.
26 pub fn new(t: &'a mut T) -> (&'a mut T, Self) {
27 let ptr = NonNull::from(t);
28 // SAFETY: we hold the borrow throughout 'a via `_marker`, and we expose
29 // only this reference, so it is unique.
30 let new_ref = unsafe { &mut *ptr.as_ptr() };
31 (new_ref, Self { ptr, _marker: PhantomData })
34 /// Revert to the unique borrow initially captured.
38 /// The reborrow must have ended, i.e., the reference returned by `new` and
39 /// all pointers and references derived from it, must not be used anymore.
40 pub unsafe fn awaken(self) -> &'a mut T {
41 // SAFETY: our own safety conditions imply this reference is again unique.
42 unsafe { &mut *self.ptr.as_ptr() }