pub struct ManuallyDrop<T: ?Sized> { /* private fields */ }
Expand description
A wrapper to inhibit the compiler from automatically calling T
’s
destructor. This wrapper is 0-cost.
ManuallyDrop<T>
is guaranteed to have the same layout and bit validity as
T
, and 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,
}
§Interaction with Box
Currently, if you have a ManuallyDrop<T>
, where the type T
is a Box
or
contains a Box
inside, then dropping the T
followed by moving the
ManuallyDrop<T>
is considered to be undefined
behavior.
That is, the following code causes undefined behavior:
use std::mem::ManuallyDrop;
let mut x = ManuallyDrop::new(Box::new(42));
unsafe {
ManuallyDrop::drop(&mut x);
}
let y = x; // Undefined behavior!
This is likely to change in the
future. In the
meantime, consider using MaybeUninit
instead.
§Safety hazards when storing ManuallyDrop
in a struct or an enum.
Special care is needed when all of the conditions below are met:
- A struct or enum contains a
ManuallyDrop
. - The
ManuallyDrop
is not inside aunion
. - The struct or enum is part of public API, or is stored in a struct or an enum that is part of public API.
- There is code that drops the contents of the
ManuallyDrop
field, and this code is outside the struct or enum’sDrop
implementation.
In particular, the following hazards may occur:
§Storing generic types
If the ManuallyDrop
contains a client-supplied generic type, the client
might provide a Box
as that type. This would cause undefined behavior when
the struct or enum is later moved, as mentioned in the previous section. For
example, the following code causes undefined behavior:
use std::mem::ManuallyDrop;
pub struct BadOption<T> {
// Invariant: Has been dropped iff `is_some` is false.
value: ManuallyDrop<T>,
is_some: bool,
}
impl<T> BadOption<T> {
pub fn new(value: T) -> Self {
Self { value: ManuallyDrop::new(value), is_some: true }
}
pub fn change_to_none(&mut self) {
if self.is_some {
self.is_some = false;
unsafe {
// SAFETY: `value` hasn't been dropped yet, as per the invariant
// (This is actually unsound!)
ManuallyDrop::drop(&mut self.value);
}
}
}
}
// In another crate:
let mut option = BadOption::new(Box::new(42));
option.change_to_none();
let option2 = option; // Undefined behavior!
§Deriving traits
Deriving Debug
, Clone
, PartialEq
, PartialOrd
, Ord
, or Hash
on
the struct or enum could be unsound, since the derived implementations of
these traits would access the ManuallyDrop
field. For example, the
following code causes undefined behavior:
use std::mem::ManuallyDrop;
// This derive is unsound in combination with the `ManuallyDrop::drop` call.
#[derive(Debug)]
pub struct Foo {
value: ManuallyDrop<String>,
}
impl Foo {
pub fn new() -> Self {
let mut temp = Self {
value: ManuallyDrop::new(String::from("Unsafe rust is hard."))
};
unsafe {
// SAFETY: `value` hasn't been dropped yet.
ManuallyDrop::drop(&mut temp.value);
}
temp
}
}
// In another crate:
let foo = Foo::new();
println!("{:?}", foo); // Undefined behavior!
Implementations§
source§impl<T> ManuallyDrop<T>
impl<T> ManuallyDrop<T>
1.20.0 (const: 1.32.0) · sourcepub const fn new(value: T) -> ManuallyDrop<T>
pub const fn new(value: T) -> ManuallyDrop<T>
Wrap a value to be manually dropped.
§Examples
1.20.0 (const: 1.32.0) · sourcepub const fn into_inner(slot: ManuallyDrop<T>) -> T
pub const fn into_inner(slot: ManuallyDrop<T>) -> T
Extracts the value from the ManuallyDrop
container.
This allows the value to be dropped again.
§Examples
1.42.0 · sourcepub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T
pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T
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
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.
source§impl<T: ?Sized> ManuallyDrop<T>
impl<T: ?Sized> ManuallyDrop<T>
1.20.0 · sourcepub unsafe fn drop(slot: &mut ManuallyDrop<T>)
pub unsafe fn drop(slot: &mut 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.
Trait Implementations§
1.20.0 · source§impl<T: Clone + ?Sized> Clone for ManuallyDrop<T>
impl<T: Clone + ?Sized> Clone for ManuallyDrop<T>
source§fn clone(&self) -> ManuallyDrop<T>
fn clone(&self) -> ManuallyDrop<T>
1.6.0 · source§fn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
source
. Read more