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

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174

    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.
    ///
    /// # Examples
    ///
    /// This wrapper can be used to enforce a particular drop order on fields, regardless
    /// of how they are defined in the struct:
    ///
    /// ```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`.
    ///     }
    /// }
    /// ```
    ///
    /// However, care should be taken when using this pattern as it can lead to *leak amplification*.
    /// In this example, if the `Drop` implementation for `Peach` were to panic, the `banana` field
    /// would also be leaked.
    ///
    /// In contrast, the automatically-generated compiler drop implementation would have ensured
    /// that all fields are dropped even in the presence of panics. This is especially important when
    /// working with [pinned] data, where reusing the memory without calling the destructor could lead
    /// to Undefined Behaviour.
    ///
    /// [`mem::zeroed`]: fn.zeroed.html
    /// [`MaybeUninit<T>`]: union.MaybeUninit.html
    /// [pinned]: ../pin/index.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")]
        #[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.
        ///
        /// [`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"]
        #[stable(feature = "manually_drop_take", since = "1.42.0")]
        #[inline]
        pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
            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.
        ///
        /// [`ManuallyDrop::into_inner`]: #method.into_inner
        /// [`ptr::drop_in_place`]: ../ptr/fn.drop_in_place.html
        /// [pinned]: ../pin/index.html
        #[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
        }
    }