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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.
#[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))
}
}
|
