tokio/util/
rc_cell.rs

1use crate::loom::cell::UnsafeCell;
2
3use std::rc::Rc;
4
5/// This is exactly like `Cell<Option<Rc<T>>>`, except that it provides a `get`
6/// method even though `Rc` is not `Copy`.
7pub(crate) struct RcCell<T> {
8    inner: UnsafeCell<Option<Rc<T>>>,
9}
10
11impl<T> RcCell<T> {
12    #[cfg(not(all(loom, test)))]
13    pub(crate) const fn new() -> Self {
14        Self {
15            inner: UnsafeCell::new(None),
16        }
17    }
18
19    // The UnsafeCell in loom does not have a const `new` fn.
20    #[cfg(all(loom, test))]
21    pub(crate) fn new() -> Self {
22        Self {
23            inner: UnsafeCell::new(None),
24        }
25    }
26
27    /// Safety: This method may not be called recursively.
28    #[inline]
29    unsafe fn with_inner<F, R>(&self, f: F) -> R
30    where
31        F: FnOnce(&mut Option<Rc<T>>) -> R,
32    {
33        // safety: This type is not Sync, so concurrent calls of this method
34        // cannot happen. Furthermore, the caller guarantees that the method is
35        // not called recursively. Finally, this is the only place that can
36        // create mutable references to the inner Rc. This ensures that any
37        // mutable references created here are exclusive.
38        self.inner.with_mut(|ptr| f(&mut *ptr))
39    }
40
41    pub(crate) fn get(&self) -> Option<Rc<T>> {
42        // safety: The `Rc::clone` method will not call any unknown user-code,
43        // so it will not result in a recursive call to `with_inner`.
44        unsafe { self.with_inner(|rc| rc.clone()) }
45    }
46
47    pub(crate) fn replace(&self, val: Option<Rc<T>>) -> Option<Rc<T>> {
48        // safety: No destructors or other unknown user-code will run inside the
49        // `with_inner` call, so no recursive call to `with_inner` can happen.
50        unsafe { self.with_inner(|rc| std::mem::replace(rc, val)) }
51    }
52
53    pub(crate) fn set(&self, val: Option<Rc<T>>) {
54        let old = self.replace(val);
55        drop(old);
56    }
57}
tokio/util/rc_cell.rs

tokio/util/
rc_cell.rs
