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
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")]
#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
impl<T: ?Sized> const Deref for ManuallyDrop<T> {
    type Target = T;
    #[inline(always)]
    fn deref(&self) -> &T {
        &self.value
    }
}

#[stable(feature = "manually_drop", since = "1.20.0")]
#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
impl<T: ?Sized> const DerefMut for ManuallyDrop<T> {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut T {
        &mut self.value
    }
}