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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() }
}
/// Borrows a new mutable reference from 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 reborrow(&mut self) -> &'a mut T {
// SAFETY: our own safety conditions imply this reference is again unique.
unsafe { &mut *self.ptr.as_ptr() }
}
/// Borrows a new shared reference from 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 reborrow_shared(&self) -> &'a T {
// SAFETY: our own safety conditions imply this reference is again unique.
unsafe { &*self.ptr.as_ptr() }
}
}
#[cfg(test)]
mod tests;
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