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

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

    /// 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 its contents. For example, initializing a `ManuallyDrop<&mut T>`
    /// with [`mem::zeroed`] is undefined behavior.
    /// If you need to handle uninitialized data, use [`MaybeUninit<T>`] instead.
    ///
    /// Note that accessing the value inside a `ManuallyDrop<T>` is safe.
    /// This means that a `ManuallyDrop<T>` whose content has been dropped must not
    /// be exposed through a public safe API.
    /// Correspondingly, `ManuallyDrop::drop` is unsafe.
    ///
    /// # `ManuallyDrop` and drop order.
    ///
    /// Rust has a well-defined [drop order] of values. To make sure that fields or
    /// locals are dropped in a specific order, reorder the declarations such that
    /// the implicit drop order is the correct one.
    ///
    /// It is possible to use `ManuallyDrop` to control the drop order, but this
    /// requires unsafe code and is hard to do correctly in the presence of
    /// unwinding.
    ///
    /// For example, if you want to make sure that a specific field is dropped after
    /// the others, make it the last field of a struct:
    ///
    /// ```
    /// struct Context;
    ///
    /// struct Widget {
    ///     children: Vec<Widget>,
    ///     // `context` will be dropped after `children`.
    ///     // Rust guarantees that fields are dropped in the order of declaration.
    ///     context: Context,
    /// }
    /// ```
    ///
    /// [drop order]: https://doc.rust-lang.org/reference/destructors.html
    /// [`mem::zeroed`]: crate::mem::zeroed
    /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
    #[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;
        /// let mut x = ManuallyDrop::new(String::from("Hello World!"));
        /// x.truncate(5); // You can still safely operate on the value
        /// assert_eq!(*x, "Hello");
        /// // But `Drop` will not be run here
        /// ```
        #[must_use = "if you don't need the wrapper, you can use `mem::forget` instead"]
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[rustc_const_stable(feature = "const_manually_drop", since = "1.36.0")]
        #[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")]
        #[rustc_const_stable(feature = "const_manually_drop", since = "1.36.0")]
        #[inline(always)]
        pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
            slot.value
        }

        /// Takes the value from the `ManuallyDrop<T>` container 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.
        ///
        /// Whenever possible, it is preferable to use [`into_inner`][`ManuallyDrop::into_inner`]
        /// instead, which prevents duplicating the content of the `ManuallyDrop<T>`.
        ///
        /// # Safety
        ///
        /// This function semantically moves out the contained value without preventing further usage,
        /// leaving the state of this container unchanged.
        /// It is your responsibility to ensure that this `ManuallyDrop` is not used again.
        ///
        #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"]
        #[stable(feature = "manually_drop_take", since = "1.42.0")]
        #[inline]
        pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
            // SAFETY: we are reading from a reference, which is guaranteed
            // to be valid for reads.
            unsafe { ptr::read(&slot.value) }
        }
    }

    impl<T: ?Sized> ManuallyDrop<T> {
        /// Manually drops the contained value. This is exactly equivalent to calling
        /// [`ptr::drop_in_place`] with a pointer to the contained value. As such, unless
        /// the contained value is a packed struct, the destructor will be called in-place
        /// without moving the value, and thus can be used to safely drop [pinned] data.
        ///
        /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead.
        ///
        /// # Safety
        ///
        /// This function runs the destructor of the contained value. Other than changes made by
        /// the destructor itself, the memory is left unchanged, and so as far as the compiler is
        /// concerned still holds a bit-pattern which is valid for the type `T`.
        ///
        /// However, this "zombie" value should not be exposed to safe code, and this function
        /// should not be called more than once. To use a value after it's been dropped, or drop
        /// a value multiple times, can cause Undefined Behavior (depending on what `drop` does).
        /// This is normally prevented by the type system, but users of `ManuallyDrop` must
        /// uphold those guarantees without assistance from the compiler.
        ///
        /// [pinned]: crate::pin
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline]
        pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
            // SAFETY: we are dropping the value pointed to by a mutable reference
            // which is guaranteed to be valid for writes.
            // It is up to the caller to make sure that `slot` isn't dropped again.
            unsafe { 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
        }
    }