manually_drop.rs.html -- source
Description: Source to the Rust file `src/libcore/mem/manually_drop.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::ptr;
    use crate::ops::{Deref, DerefMut};

    /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
    ///
    /// This wrapper is 0-cost.
    ///
    /// `ManuallyDrop<T>` is subject to the same layout optimizations as `T`.
    /// As a consequence, it has *no effect* on the assumptions that the compiler makes
    /// about all values being initialized at their type.  In particular, initializing
    /// a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined behavior.
    /// If you need to handle uninitialized data, use [`MaybeUninit<T>`] instead.
    ///
    /// # Examples
    ///
    /// This wrapper helps with explicitly documenting the drop order dependencies between fields of
    /// the type:
    ///
    /// ```rust
    /// use std::mem::ManuallyDrop;
    /// struct Peach;
    /// struct Banana;
    /// struct Melon;
    /// struct FruitBox {
    ///     // Immediately clear there’s something non-trivial going on with these fields.
    ///     peach: ManuallyDrop<Peach>,
    ///     melon: Melon, // Field that’s independent of the other two.
    ///     banana: ManuallyDrop<Banana>,
    /// }
    ///
    /// impl Drop for FruitBox {
    ///     fn drop(&mut self) {
    ///         unsafe {
    ///             // Explicit ordering in which field destructors are run specified in the intuitive
    ///             // location – the destructor of the structure containing the fields.
    ///             // Moreover, one can now reorder fields within the struct however much they want.
    ///             ManuallyDrop::drop(&mut self.peach);
    ///             ManuallyDrop::drop(&mut self.banana);
    ///         }
    ///         // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
    ///         // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
    ///     }
    /// }
    /// ```
    ///
    /// [`mem::zeroed`]: fn.zeroed.html
    /// [`MaybeUninit<T>`]: union.MaybeUninit.html
    #[stable(feature = "manually_drop", since = "1.20.0")]
    #[lang = "manually_drop"]
    #[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
    #[repr(transparent)]
    pub struct ManuallyDrop<T: ?Sized> {
        value: T,
    }

    impl<T> ManuallyDrop<T> {
        /// Wrap a value to be manually dropped.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::mem::ManuallyDrop;
        /// ManuallyDrop::new(Box::new(()));
        /// ```
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline(always)]
        pub const fn new(value: T) -> ManuallyDrop<T> {
            ManuallyDrop { value }
        }

        /// Extracts the value from the `ManuallyDrop` container.
        ///
        /// This allows the value to be dropped again.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::mem::ManuallyDrop;
        /// let x = ManuallyDrop::new(Box::new(()));
        /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
        /// ```
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline(always)]
        pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
            slot.value
        }

        /// Takes the contained value out.
        ///
        /// This method is primarily intended for moving out values in drop.
        /// Instead of using [`ManuallyDrop::drop`] to manually drop the value,
        /// you can use this method to take the value and use it however desired.
        /// `Drop` will be invoked on the returned value following normal end-of-scope rules.
        ///
        /// If you have ownership of the container, you can use [`ManuallyDrop::into_inner`] instead.
        ///
        /// # Safety
        ///
        /// This function semantically moves out the contained value without preventing further usage.
        /// It is up to the user of this method to ensure that this container is not used again.
        ///
        /// [`ManuallyDrop::drop`]: #method.drop
        /// [`ManuallyDrop::into_inner`]: #method.into_inner
        #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"]
        #[unstable(feature = "manually_drop_take", issue = "55422")]
        #[inline]
        pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
            ManuallyDrop::into_inner(ptr::read(slot))
        }
    }

    impl<T: ?Sized> ManuallyDrop<T> {
        /// Manually drops the contained value.
        ///
        /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead.
        ///
        /// # Safety
        ///
        /// This function runs the destructor of the contained value and thus the wrapped value
        /// now represents uninitialized data. It is up to the user of this method to ensure the
        /// uninitialized data is not actually used.
        /// In particular, this function can only be called called at most once
        /// for a given instance of `ManuallyDrop<T>`.
        ///
        /// [`ManuallyDrop::into_inner`]: #method.into_inner
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline]
        pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
            ptr::drop_in_place(&mut slot.value)
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: ?Sized> Deref for ManuallyDrop<T> {
        type Target = T;
        #[inline(always)]
        fn deref(&self) -> &T {
            &self.value
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
        #[inline(always)]
        fn deref_mut(&mut self) -> &mut T {
            &mut self.value
        }
    }