cell.rs

//! Copy pasted from std::cell::SyncUnsafeCell
//! can be removed once the feature stabilizes.
use std::cell::UnsafeCell;

/// [`UnsafeCell`], but [`Sync`].
///
/// This is just an [`UnsafeCell`], except it implements [`Sync`]
/// if `T` implements [`Sync`].
///
/// [`UnsafeCell`] doesn't implement [`Sync`], to prevent accidental misuse.
/// You can use [`SyncUnsafeCell`] instead of [`UnsafeCell`] to allow it to be
/// shared between threads, if that's intentional.
/// Providing proper synchronization is still the task of the user,
/// making this type just as unsafe to use.
///
/// See [`UnsafeCell`] for details.
#[repr(transparent)]
pub struct SyncUnsafeCell<T: ?Sized> {
    value: UnsafeCell<T>,
}

unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}

impl<T> SyncUnsafeCell<T> {
    /// Constructs a new instance of [`SyncUnsafeCell`] which will wrap the specified value.
    #[inline]
    pub fn new(value: T) -> Self {
        Self {
            value: UnsafeCell::new(value),
        }
    }

    /// Unwraps the value.
    #[inline]
    pub fn into_inner(self) -> T {
        self.value.into_inner()
    }
}

impl<T: ?Sized> SyncUnsafeCell<T> {
    /// Gets a mutable pointer to the wrapped value.
    ///
    /// This can be cast to a pointer of any kind.
    /// Ensure that the access is unique (no active references, mutable or not)
    /// when casting to `&mut T`, and ensure that there are no mutations
    /// or mutable aliases going on when casting to `&T`
    #[inline]
    pub fn get(&self) -> *mut T {
        self.value.get()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows the [`SyncUnsafeCell`] mutably (at compile-time) which
    /// guarantees that we possess the only reference.
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        self.value.get_mut()
    }

    /// Gets a mutable pointer to the wrapped value.
    ///
    /// See [`UnsafeCell::get`] for details.
    #[inline]
    pub fn raw_get(this: *const Self) -> *mut T {
        // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
        // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
        // See UnsafeCell::raw_get.
        this as *const T as *mut T
    }
}

impl<T: Default> Default for SyncUnsafeCell<T> {
    /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
    fn default() -> SyncUnsafeCell<T> {
        SyncUnsafeCell::new(Default::default())
    }
}

impl<T> From<T> for SyncUnsafeCell<T> {
    /// Creates a new [`SyncUnsafeCell<T>`] containing the given value.
    fn from(t: T) -> SyncUnsafeCell<T> {
        SyncUnsafeCell::new(t)
    }
}