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

use std::cell::Cell;
use std::rc::{Rc, Weak};

pub trait CellOption {
    fn is_none(&self) -> bool;
}

impl<T> CellOption for Cell<Option<T>> {
    #[inline]
    fn is_none(&self) -> bool {
        unsafe { (*self.as_ptr()).is_none() }
    }
}

pub trait CellOptionWeak<T> {
    fn upgrade(&self) -> Option<Rc<T>>;
    fn clone_inner(&self) -> Option<Weak<T>>;
}

impl<T> CellOptionWeak<T> for Cell<Option<Weak<T>>> {
    #[inline]
    fn upgrade(&self) -> Option<Rc<T>> {
        unsafe { (*self.as_ptr()).as_ref().and_then(Weak::upgrade) }
    }

    #[inline]
    fn clone_inner(&self) -> Option<Weak<T>> {
        unsafe { (*self.as_ptr()).clone() }
    }
}

pub trait CellOptionRc<T> {
    /// Return `Some` if this `Rc` is the only strong reference count,
    /// even if there are weak references.
    fn take_if_unique_strong(&self) -> Option<Rc<T>>;
    fn clone_inner(&self) -> Option<Rc<T>>;
}

impl<T> CellOptionRc<T> for Cell<Option<Rc<T>>> {
    #[inline]
    fn take_if_unique_strong(&self) -> Option<Rc<T>> {
        unsafe {
            match *self.as_ptr() {
                None => None,
                Some(ref rc) if Rc::strong_count(rc) > 1 => None,
                // Not borrowing the `Rc<T>` here
                // as we would be invalidating that borrow while it is outstanding:
                Some(_) => self.take(),
            }
        }
    }

    #[inline]
    fn clone_inner(&self) -> Option<Rc<T>> {
        unsafe { (*self.as_ptr()).clone() }
    }
}