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/Rust

Struct ManuallyDrop

pub struct ManuallyDrop<T>where
    T: ?Sized,{ /* private fields */ }

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 a union.
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’s Drop 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 if `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!

Struct ManuallyDrop

ManuallyDrop and drop order

Interaction with Box

Safety hazards when storing ManuallyDrop in a struct or an enum.

Storing generic types

Deriving traits

Implementations

Examples

Examples

Safety

Safety

Trait Implementations

Auto Trait Implementations

Blanket Implementations

`ManuallyDrop` and drop order

Interaction with `Box`

Safety hazards when storing `ManuallyDrop` in a struct or an enum.