Sync BTreeMap with stdlib

src/btree/borrow.rs

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+use core::{marker::PhantomData, 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() }
+    }
+}