Vendor things

third-party/vendor/triomphe/src/arc_borrow.rs

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+use core::mem;
+use core::mem::ManuallyDrop;
+use core::ops::Deref;
+use core::ptr;
+
+use super::Arc;
+
+/// A "borrowed `Arc`". This is a pointer to
+/// a T that is known to have been allocated within an
+/// `Arc`.
+///
+/// This is equivalent in guarantees to `&Arc<T>`, however it is
+/// a bit more flexible. To obtain an `&Arc<T>` you must have
+/// an `Arc<T>` instance somewhere pinned down until we're done with it.
+/// It's also a direct pointer to `T`, so using this involves less pointer-chasing
+///
+/// However, C++ code may hand us refcounted things as pointers to T directly,
+/// so we have to conjure up a temporary `Arc` on the stack each time. The
+/// same happens for when the object is managed by a `OffsetArc`.
+///
+/// `ArcBorrow` lets us deal with borrows of known-refcounted objects
+/// without needing to worry about where the `Arc<T>` is.
+#[derive(Debug, Eq, PartialEq)]
+#[repr(transparent)]
+pub struct ArcBorrow<'a, T: ?Sized + 'a>(pub(crate) &'a T);
+
+impl<'a, T> Copy for ArcBorrow<'a, T> {}
+impl<'a, T> Clone for ArcBorrow<'a, T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<'a, T> ArcBorrow<'a, T> {
+    /// Clone this as an `Arc<T>`. This bumps the refcount.
+    #[inline]
+    pub fn clone_arc(&self) -> Arc<T> {
+        let arc = unsafe { Arc::from_raw(self.0) };
+        // addref it!
+        mem::forget(arc.clone());
+        arc
+    }
+
+    /// For constructing from a reference known to be Arc-backed,
+    /// e.g. if we obtain such a reference over FFI
+    /// TODO: should from_ref be relaxed to unsized types? It can't be
+    /// converted back to an Arc right now for unsized types.
+    /// # Safety
+    /// - The reference to `T` must have come from a Triomphe Arc, UniqueArc, or ArcBorrow.
+    #[inline]
+    pub unsafe fn from_ref(r: &'a T) -> Self {
+        ArcBorrow(r)
+    }
+
+    /// Compare two `ArcBorrow`s via pointer equality. Will only return
+    /// true if they come from the same allocation
+    #[inline]
+    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
+        ptr::eq(this.0 as *const T, other.0 as *const T)
+    }
+
+    /// Temporarily converts |self| into a bonafide Arc and exposes it to the
+    /// provided callback. The refcount is not modified.
+    #[inline]
+    pub fn with_arc<F, U>(&self, f: F) -> U
+    where
+        F: FnOnce(&Arc<T>) -> U,
+        T: 'static,
+    {
+        // Synthesize transient Arc, which never touches the refcount.
+        let transient = unsafe { ManuallyDrop::new(Arc::from_raw(self.0)) };
+
+        // Expose the transient Arc to the callback, which may clone it if it wants
+        // and forward the result to the user
+        f(&transient)
+    }
+
+    /// Similar to deref, but uses the lifetime |a| rather than the lifetime of
+    /// self, which is incompatible with the signature of the Deref trait.
+    #[inline]
+    pub fn get(&self) -> &'a T {
+        self.0
+    }
+}
+
+impl<'a, T> Deref for ArcBorrow<'a, T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &T {
+        self.0
+    }
+}
+
+// Safety:
+// This implementation must guarantee that it is sound to call replace_ptr with an unsized variant
+// of the pointer retuned in `as_sized_ptr`. We leverage unsizing the contained reference. This
+// continues to point to the data of an ArcInner. The reference count remains untouched which is
+// correct since the number of owners did not change. This implies the returned instance fulfills
+// its safety invariants.
+#[cfg(feature = "unsize")]
+unsafe impl<'lt, T: 'lt, U: ?Sized + 'lt> unsize::CoerciblePtr<U> for ArcBorrow<'lt, T> {
+    type Pointee = T;
+    type Output = ArcBorrow<'lt, U>;
+
+    fn as_sized_ptr(&mut self) -> *mut T {
+        // Returns a pointer to the inner data. We do not need to care about any particular
+        // provenance here, only the pointer value, which we need to reconstruct the new pointer.
+        self.0 as *const T as *mut T
+    }
+
+    unsafe fn replace_ptr(self, new: *mut U) -> ArcBorrow<'lt, U> {
+        let inner = ManuallyDrop::new(self);
+        // Safety: backed by the same Arc that backed `self`.
+        ArcBorrow(inner.0.replace_ptr(new))
+    }
+}