114 lines
3.4 KiB
Rust
114 lines
3.4 KiB
Rust
//! Specialized methods for `Cell` of some specific `!Copy` types,
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//! allowing limited access to a value without moving it of the cell.
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//!
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//!
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//! # Soundness
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//!
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//! These methods use and `Cell::as_ptr` and `unsafe`.
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//! Their soundness lies in that:
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//!
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//! * `Cell<T>: !Sync` for any `T`, so no other thread is accessing this cell.
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//! * For the duration of the raw pointer access,
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//! this thread only runs code that is known to not access the same cell again.
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//! In particular, no method of a type paramater is called.
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//! For example, `clone_inner` would be unsound to generalize to any `Cell<T>`
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//! because it would involve running arbitrary code through `T::clone`
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//! and provide that code with a reference to the inside of the cell.
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//!
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//! ```rust
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//! struct Evil(Box<u32>, Rc<Cell<Option<Evil>>>);
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//! impl Clone for Evil {
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//! fn clone(&self) -> Self {
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//! mem::drop(self.1.take()); // Mess with the "other" node, which might be `self`.
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//! Evil(
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//! self.0.clone(), // possible use after free!
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//! Rc::new(Cell::new(None))
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//! )
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//! }
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//! }
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//! let a = Rc::new(Cell::new(None));
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//! a.set(Some(Evil(Box::new(5), a.clone()))); // Make a reference cycle.
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//! a.clone_inner();
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//! ```
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//!
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//! `Rc<T>::clone` and `Weak<T>::clone` do not have this problem
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//! as they only increment reference counts and never call `T::clone`.
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//!
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//!
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//! # Alternative
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//!
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//! To avoid using `unsafe` entirely, operating on a `T: !Copy` value inside a `Cell<T>`
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//! would require temporarily replacing it with a default value:
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//!
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//! ```rust
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//! fn option_dance<T, F, R>(cell: &Cell<T>, f: F) -> R
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//! where T: Default, F: FnOnce(&mut T) -> R
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//! {
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//! let mut value = cell.take();
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//! let result = f(&mut value);
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//! cell.set(value);
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//! result
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//! }
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//! ```
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//!
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//! It would be worth exploring whether LLVM can reliably optimize away these extra moves
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//! and compile the `Option` dance to assembly similar to that of the `unsafe` operation.
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use std::cell::Cell;
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use std::rc::{Rc, Weak};
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pub trait CellOption {
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fn is_none(&self) -> bool;
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}
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impl<T> CellOption for Cell<Option<T>> {
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#[inline]
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fn is_none(&self) -> bool {
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unsafe { (*self.as_ptr()).is_none() }
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}
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}
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pub trait CellOptionWeak<T> {
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fn upgrade(&self) -> Option<Rc<T>>;
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fn clone_inner(&self) -> Option<Weak<T>>;
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}
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impl<T> CellOptionWeak<T> for Cell<Option<Weak<T>>> {
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#[inline]
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fn upgrade(&self) -> Option<Rc<T>> {
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unsafe { (*self.as_ptr()).as_ref().and_then(Weak::upgrade) }
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}
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#[inline]
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fn clone_inner(&self) -> Option<Weak<T>> {
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unsafe { (*self.as_ptr()).clone() }
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}
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}
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pub trait CellOptionRc<T> {
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/// Return `Some` if this `Rc` is the only strong reference count,
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/// even if there are weak references.
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fn take_if_unique_strong(&self) -> Option<Rc<T>>;
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fn clone_inner(&self) -> Option<Rc<T>>;
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}
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impl<T> CellOptionRc<T> for Cell<Option<Rc<T>>> {
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#[inline]
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fn take_if_unique_strong(&self) -> Option<Rc<T>> {
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unsafe {
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match *self.as_ptr() {
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None => None,
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Some(ref rc) if Rc::strong_count(rc) > 1 => None,
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// Not borrowing the `Rc<T>` here
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// as we would be invalidating that borrow while it is outstanding:
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Some(_) => self.take(),
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}
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}
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}
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#[inline]
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fn clone_inner(&self) -> Option<Rc<T>> {
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unsafe { (*self.as_ptr()).clone() }
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}
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}
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