| use core::marker::PhantomData; |
| use core::ptr::NonNull; |
| |
| /// Models a reborrow of some unique reference, when you know that the reborrow |
| /// and all its descendants (i.e., all pointers and references derived from it) |
| /// will not be used any more at some point, after which you want to use the |
| /// original unique reference again. |
| /// |
| /// The borrow checker usually handles this stacking of borrows for you, but |
| /// some control flows that accomplish this stacking are too complicated for |
| /// the compiler to follow. A `DormantMutRef` allows you to check borrowing |
| /// yourself, while still expressing its stacked nature, and encapsulating |
| /// the raw pointer code needed to do this without undefined behavior. |
| pub struct DormantMutRef<'a, T> { |
| ptr: NonNull<T>, |
| _marker: PhantomData<&'a mut T>, |
| } |
| |
| unsafe impl<'a, T> Sync for DormantMutRef<'a, T> where &'a mut T: Sync {} |
| unsafe impl<'a, T> Send for DormantMutRef<'a, T> where &'a mut T: Send {} |
| |
| impl<'a, T> DormantMutRef<'a, T> { |
| /// Capture a unique borrow, and immediately reborrow it. For the compiler, |
| /// the lifetime of the new reference is the same as the lifetime of the |
| /// original reference, but you promise to use it for a shorter period. |
| pub fn new(t: &'a mut T) -> (&'a mut T, Self) { |
| let ptr = NonNull::from(t); |
| // SAFETY: we hold the borrow throughout 'a via `_marker`, and we expose |
| // only this reference, so it is unique. |
| let new_ref = unsafe { &mut *ptr.as_ptr() }; |
| (new_ref, Self { ptr, _marker: PhantomData }) |
| } |
| |
| /// Revert to the unique borrow initially captured. |
| /// |
| /// # Safety |
| /// |
| /// The reborrow must have ended, i.e., the reference returned by `new` and |
| /// all pointers and references derived from it, must not be used anymore. |
| pub unsafe fn awaken(self) -> &'a mut T { |
| // SAFETY: our own safety conditions imply this reference is again unique. |
| unsafe { &mut *self.ptr.as_ptr() } |
| } |
| } |
| |
| #[cfg(test)] |
| mod tests; |