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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:02:58 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:02:58 +0000 |
commit | 698f8c2f01ea549d77d7dc3338a12e04c11057b9 (patch) | |
tree | 173a775858bd501c378080a10dca74132f05bc50 /library/alloc/src/collections/btree/borrow.rs | |
parent | Initial commit. (diff) | |
download | rustc-698f8c2f01ea549d77d7dc3338a12e04c11057b9.tar.xz rustc-698f8c2f01ea549d77d7dc3338a12e04c11057b9.zip |
Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | library/alloc/src/collections/btree/borrow.rs | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/library/alloc/src/collections/btree/borrow.rs b/library/alloc/src/collections/btree/borrow.rs new file mode 100644 index 000000000..016f139a5 --- /dev/null +++ b/library/alloc/src/collections/btree/borrow.rs @@ -0,0 +1,47 @@ +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; |