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,
}

§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 a safe function that drops the contents of the ManuallyDrop field, and it can be called outside the struct or enum’s Drop implementation.

In particular, 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, in the following code, derive(Debug) is unsound in combination with the ManuallyDrop::drop call in Foo::new:

#[derive(Debug)]
pub struct Foo {
    /// Invariant: this value may have been dropped!
    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
    }
}

As one could use the Debug implementation to access an already dropped field:

let foo = Foo::new();
println!("{foo:?}"); // Undefined behavior!

Note that similar unsoundness can arise without derive. The cause of the unsoundness are public APIs which allow to access an already dropped value inside ManuallyDrop.

§Pre-1.96 Interaction with Box

Before Rust 1.96.0, if you had a ManuallyDrop<T>, where the type T was a Box or contained a Box inside, then dropping the T followed by moving the ManuallyDrop<T> was considered to be undefined behavior. That is, the following code caused undefined behavior:

use std::mem::ManuallyDrop;

let mut x = ManuallyDrop::new(Box::new(42));
unsafe {
    ManuallyDrop::drop(&mut x);
}
let y = x; // Undefined behavior! (pre 1.96.0)

Note that this could also have happen with a generic type where the user of the library providing it could substitute the generic for a Box<_> and then move the library type:

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 pre rust 1.96.0!)
                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! (pre 1.96)