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

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.
    ///
    /// [`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
    }
}