1use crate::ops::{Deref, DerefMut, DerefPure};
2use crate::ptr;
3
4/// A wrapper to inhibit the compiler from automatically calling `T`’s
5/// destructor. This wrapper is 0-cost.
6///
7/// `ManuallyDrop<T>` is guaranteed to have the same layout and bit validity as
8/// `T`, and is subject to the same layout optimizations as `T`. As a
9/// consequence, it has *no effect* on the assumptions that the compiler makes
10/// about its contents. For example, initializing a `ManuallyDrop<&mut T>` with
11/// [`mem::zeroed`] is undefined behavior. If you need to handle uninitialized
12/// data, use [`MaybeUninit<T>`] instead.
13///
14/// Note that accessing the value inside a `ManuallyDrop<T>` is safe. This means
15/// that a `ManuallyDrop<T>` whose content has been dropped must not be exposed
16/// through a public safe API. Correspondingly, `ManuallyDrop::drop` is unsafe.
17///
18/// # `ManuallyDrop` and drop order
19///
20/// Rust has a well-defined [drop order] of values. To make sure that fields or
21/// locals are dropped in a specific order, reorder the declarations such that
22/// the implicit drop order is the correct one.
23///
24/// It is possible to use `ManuallyDrop` to control the drop order, but this
25/// requires unsafe code and is hard to do correctly in the presence of
26/// unwinding.
27///
28/// For example, if you want to make sure that a specific field is dropped after
29/// the others, make it the last field of a struct:
30///
31/// ```
32/// struct Context;
33///
34/// struct Widget {
35/// children: Vec<Widget>,
36/// // `context` will be dropped after `children`.
37/// // Rust guarantees that fields are dropped in the order of declaration.
38/// context: Context,
39/// }
40/// ```
41///
42/// # Interaction with `Box`
43///
44/// Currently, if you have a `ManuallyDrop<T>`, where the type `T` is a `Box` or
45/// contains a `Box` inside, then dropping the `T` followed by moving the
46/// `ManuallyDrop<T>` is [considered to be undefined
47/// behavior](https://github.com/rust-lang/unsafe-code-guidelines/issues/245).
48/// That is, the following code causes undefined behavior:
49///
50/// ```no_run
51/// use std::mem::ManuallyDrop;
52///
53/// let mut x = ManuallyDrop::new(Box::new(42));
54/// unsafe {
55/// ManuallyDrop::drop(&mut x);
56/// }
57/// let y = x; // Undefined behavior!
58/// ```
59///
60/// This is [likely to change in the
61/// future](https://rust-lang.github.io/rfcs/3336-maybe-dangling.html). In the
62/// meantime, consider using [`MaybeUninit`] instead.
63///
64/// # Safety hazards when storing `ManuallyDrop` in a struct or an enum.
65///
66/// Special care is needed when all of the conditions below are met:
67/// * A struct or enum contains a `ManuallyDrop`.
68/// * The `ManuallyDrop` is not inside a `union`.
69/// * The struct or enum is part of public API, or is stored in a struct or an
70/// enum that is part of public API.
71/// * There is code that drops the contents of the `ManuallyDrop` field, and
72/// this code is outside the struct or enum's `Drop` implementation.
73///
74/// In particular, the following hazards may occur:
75///
76/// #### Storing generic types
77///
78/// If the `ManuallyDrop` contains a client-supplied generic type, the client
79/// might provide a `Box` as that type. This would cause undefined behavior when
80/// the struct or enum is later moved, as mentioned in the previous section. For
81/// example, the following code causes undefined behavior:
82///
83/// ```no_run
84/// use std::mem::ManuallyDrop;
85///
86/// pub struct BadOption<T> {
87/// // Invariant: Has been dropped if `is_some` is false.
88/// value: ManuallyDrop<T>,
89/// is_some: bool,
90/// }
91/// impl<T> BadOption<T> {
92/// pub fn new(value: T) -> Self {
93/// Self { value: ManuallyDrop::new(value), is_some: true }
94/// }
95/// pub fn change_to_none(&mut self) {
96/// if self.is_some {
97/// self.is_some = false;
98/// unsafe {
99/// // SAFETY: `value` hasn't been dropped yet, as per the invariant
100/// // (This is actually unsound!)
101/// ManuallyDrop::drop(&mut self.value);
102/// }
103/// }
104/// }
105/// }
106///
107/// // In another crate:
108///
109/// let mut option = BadOption::new(Box::new(42));
110/// option.change_to_none();
111/// let option2 = option; // Undefined behavior!
112/// ```
113///
114/// #### Deriving traits
115///
116/// Deriving `Debug`, `Clone`, `PartialEq`, `PartialOrd`, `Ord`, or `Hash` on
117/// the struct or enum could be unsound, since the derived implementations of
118/// these traits would access the `ManuallyDrop` field. For example, the
119/// following code causes undefined behavior:
120///
121/// ```no_run
122/// use std::mem::ManuallyDrop;
123///
124/// // This derive is unsound in combination with the `ManuallyDrop::drop` call.
125/// #[derive(Debug)]
126/// pub struct Foo {
127/// value: ManuallyDrop<String>,
128/// }
129/// impl Foo {
130/// pub fn new() -> Self {
131/// let mut temp = Self {
132/// value: ManuallyDrop::new(String::from("Unsafe rust is hard."))
133/// };
134/// unsafe {
135/// // SAFETY: `value` hasn't been dropped yet.
136/// ManuallyDrop::drop(&mut temp.value);
137/// }
138/// temp
139/// }
140/// }
141///
142/// // In another crate:
143///
144/// let foo = Foo::new();
145/// println!("{:?}", foo); // Undefined behavior!
146/// ```
147///
148/// [drop order]: https://doc.rust-lang.org/reference/destructors.html
149/// [`mem::zeroed`]: crate::mem::zeroed
150/// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
151/// [`MaybeUninit`]: crate::mem::MaybeUninit
152#[stable(feature = "manually_drop", since = "1.20.0")]
153#[lang = "manually_drop"]
154#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
155#[repr(transparent)]
156#[rustc_pub_transparent]
157pub struct ManuallyDrop<T: ?Sized> {
158 value: T,
159}
160
161impl<T> ManuallyDrop<T> {
162 /// Wrap a value to be manually dropped.
163 ///
164 /// # Examples
165 ///
166 /// ```rust
167 /// use std::mem::ManuallyDrop;
168 /// let mut x = ManuallyDrop::new(String::from("Hello World!"));
169 /// x.truncate(5); // You can still safely operate on the value
170 /// assert_eq!(*x, "Hello");
171 /// // But `Drop` will not be run here
172 /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
173 /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
174 /// # let _ = ManuallyDrop::into_inner(x);
175 /// ```
176 #[must_use = "if you don't need the wrapper, you can use `mem::forget` instead"]
177 #[stable(feature = "manually_drop", since = "1.20.0")]
178 #[rustc_const_stable(feature = "const_manually_drop", since = "1.32.0")]
179 #[inline(always)]
180 pub const fn new(value: T) -> ManuallyDrop<T> {
181 ManuallyDrop { value }
182 }
183
184 /// Extracts the value from the `ManuallyDrop` container.
185 ///
186 /// This allows the value to be dropped again.
187 ///
188 /// # Examples
189 ///
190 /// ```rust
191 /// use std::mem::ManuallyDrop;
192 /// let x = ManuallyDrop::new(Box::new(()));
193 /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
194 /// ```
195 #[stable(feature = "manually_drop", since = "1.20.0")]
196 #[rustc_const_stable(feature = "const_manually_drop", since = "1.32.0")]
197 #[inline(always)]
198 pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
199 slot.value
200 }
201
202 /// Takes the value from the `ManuallyDrop<T>` container out.
203 ///
204 /// This method is primarily intended for moving out values in drop.
205 /// Instead of using [`ManuallyDrop::drop`] to manually drop the value,
206 /// you can use this method to take the value and use it however desired.
207 ///
208 /// Whenever possible, it is preferable to use [`into_inner`][`ManuallyDrop::into_inner`]
209 /// instead, which prevents duplicating the content of the `ManuallyDrop<T>`.
210 ///
211 /// # Safety
212 ///
213 /// This function semantically moves out the contained value without preventing further usage,
214 /// leaving the state of this container unchanged.
215 /// It is your responsibility to ensure that this `ManuallyDrop` is not used again.
216 ///
217 #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"]
218 #[stable(feature = "manually_drop_take", since = "1.42.0")]
219 #[inline]
220 pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
221 // SAFETY: we are reading from a reference, which is guaranteed
222 // to be valid for reads.
223 unsafe { ptr::read(&slot.value) }
224 }
225}
226
227impl<T: ?Sized> ManuallyDrop<T> {
228 /// Manually drops the contained value.
229 ///
230 /// This is exactly equivalent to calling [`ptr::drop_in_place`] with a
231 /// pointer to the contained value. As such, unless the contained value is a
232 /// packed struct, the destructor will be called in-place without moving the
233 /// value, and thus can be used to safely drop [pinned] data.
234 ///
235 /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead.
236 ///
237 /// # Safety
238 ///
239 /// This function runs the destructor of the contained value. Other than changes made by
240 /// the destructor itself, the memory is left unchanged, and so as far as the compiler is
241 /// concerned still holds a bit-pattern which is valid for the type `T`.
242 ///
243 /// However, this "zombie" value should not be exposed to safe code, and this function
244 /// should not be called more than once. To use a value after it's been dropped, or drop
245 /// a value multiple times, can cause Undefined Behavior (depending on what `drop` does).
246 /// This is normally prevented by the type system, but users of `ManuallyDrop` must
247 /// uphold those guarantees without assistance from the compiler.
248 ///
249 /// [pinned]: crate::pin
250 #[stable(feature = "manually_drop", since = "1.20.0")]
251 #[inline]
252 pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
253 // SAFETY: we are dropping the value pointed to by a mutable reference
254 // which is guaranteed to be valid for writes.
255 // It is up to the caller to make sure that `slot` isn't dropped again.
256 unsafe { ptr::drop_in_place(&mut slot.value) }
257 }
258}
259
260#[stable(feature = "manually_drop", since = "1.20.0")]
261impl<T: ?Sized> Deref for ManuallyDrop<T> {
262 type Target = T;
263 #[inline(always)]
264 fn deref(&self) -> &T {
265 &self.value
266 }
267}
268
269#[stable(feature = "manually_drop", since = "1.20.0")]
270impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
271 #[inline(always)]
272 fn deref_mut(&mut self) -> &mut T {
273 &mut self.value
274 }
275}
276
277#[unstable(feature = "deref_pure_trait", issue = "87121")]
278unsafe impl<T: ?Sized> DerefPure for ManuallyDrop<T> {}
279