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use crate::loom::cell::UnsafeCell;
use std::rc::Rc;
/// This is exactly like `Cell<Option<Rc<T>>>`, except that it provides a `get`
/// method even though `Rc` is not `Copy`.
pub(crate) struct RcCell<T> {
inner: UnsafeCell<Option<Rc<T>>>,
}
impl<T> RcCell<T> {
#[cfg(not(all(loom, test)))]
pub(crate) const fn new() -> Self {
Self {
inner: UnsafeCell::new(None),
}
}
// The UnsafeCell in loom does not have a const `new` fn.
#[cfg(all(loom, test))]
pub(crate) fn new() -> Self {
Self {
inner: UnsafeCell::new(None),
}
}
/// Safety: This method may not be called recursively.
#[inline]
unsafe fn with_inner<F, R>(&self, f: F) -> R
where
F: FnOnce(&mut Option<Rc<T>>) -> R,
{
// safety: This type is not Sync, so concurrent calls of this method
// cannot happen. Furthermore, the caller guarantees that the method is
// not called recursively. Finally, this is the only place that can
// create mutable references to the inner Rc. This ensures that any
// mutable references created here are exclusive.
self.inner.with_mut(|ptr| f(&mut *ptr))
}
pub(crate) fn get(&self) -> Option<Rc<T>> {
// safety: The `Rc::clone` method will not call any unknown user-code,
// so it will not result in a recursive call to `with_inner`.
unsafe { self.with_inner(|rc| rc.clone()) }
}
pub(crate) fn replace(&self, val: Option<Rc<T>>) -> Option<Rc<T>> {
// safety: No destructors or other unknown user-code will run inside the
// `with_inner` call, so no recursive call to `with_inner` can happen.
unsafe { self.with_inner(|rc| std::mem::replace(rc, val)) }
}
pub(crate) fn set(&self, val: Option<Rc<T>>) {
let old = self.replace(val);
drop(old);
}
}