Struct nom::lib::std::mem::ManuallyDrop

#[lang = "manually_drop"]
#[repr(transparent)]
pub struct ManuallyDrop<T> where
    T: ?Sized,  { /* fields omitted */ }

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 all values being initialized at their type. In particular, initializing a ManuallyDrop<&mut T> with mem::zeroed is undefined behavior. If you need to handle uninitialized data, use MaybeUninit<T> instead.

Examples

This wrapper helps with explicitly documenting the drop order dependencies between fields of the type:

use std::mem::ManuallyDrop;
struct Peach;
struct Banana;
struct Melon;
struct FruitBox {
    // Immediately clear there’s something non-trivial going on with these fields.
    peach: ManuallyDrop<Peach>,
    melon: Melon, // Field that’s independent of the other two.
    banana: ManuallyDrop<Banana>,
}

impl Drop for FruitBox {
    fn drop(&mut self) {
        unsafe {
            // Explicit ordering in which field destructors are run specified in the intuitive
            // location – the destructor of the structure containing the fields.
            // Moreover, one can now reorder fields within the struct however much they want.
            ManuallyDrop::drop(&mut self.peach);
            ManuallyDrop::drop(&mut self.banana);
        }
        // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
        // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
    }
}

Methods

impl<T> ManuallyDrop<T>

pub const fn new(value: T) -> ManuallyDrop<T>

Wrap a value to be manually dropped.

Examples

use std::mem::ManuallyDrop;
ManuallyDrop::new(Box::new(()));

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

use std::mem::ManuallyDrop;
let x = ManuallyDrop::new(Box::new(()));
let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.

#[must_use = "if you don\'t need the value, you can use `ManuallyDrop::drop` instead"] 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 preferrable 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.

impl<T> ManuallyDrop<T> where
    T: ?Sized, 

pub unsafe fn drop(slot: &mut ManuallyDrop<T>)

Manually drops the contained value.

If you have ownership of the value, you can use ManuallyDrop::into_inner instead.

Safety

This function runs the destructor of the contained value and thus the wrapped value now represents uninitialized data. It is up to the user of this method to ensure the uninitialized data is not actually used. In particular, this function can only be called at most once for a given instance of ManuallyDrop<T>.

Trait Implementations

impl<T> Clone for ManuallyDrop<T> where
    T: Clone + ?Sized, 

fn clone(&self) -> ManuallyDrop<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Copy for ManuallyDrop<T> where
    T: Copy + ?Sized, 

impl<T> Debug for ManuallyDrop<T> where
    T: Debug + ?Sized, 

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for ManuallyDrop<T> where
    T: Default + ?Sized, 

fn default() -> ManuallyDrop<T>

Returns the "default value" for a type.

impl<T> Deref for ManuallyDrop<T> where
    T: ?Sized, 

type Target = T

The resulting type after dereferencing.

Important traits for &'_ mut I

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;

fn deref(&self) -> &T

Dereferences the value.

impl<T> DerefMut for ManuallyDrop<T> where
    T: ?Sized, 

Important traits for &'_ mut I

impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;

fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.

impl<T> Eq for ManuallyDrop<T> where
    T: Eq + ?Sized, 

impl<T> Hash for ManuallyDrop<T> where
    T: Hash + ?Sized, 

fn hash<__H>(&self, state: &mut __H) where
    __H: Hasher, 

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].

impl<T> Ord for ManuallyDrop<T> where
    T: Ord + ?Sized, 

fn cmp(&self, other: &ManuallyDrop<T>) -> Ordering

This method returns an [Ordering] between self and other.

#[must_use] fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

#[must_use] fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

#[must_use] fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp)

Restrict a value to a certain interval.

impl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where
    T: PartialEq<T> + ?Sized, 

fn eq(&self, other: &ManuallyDrop<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &ManuallyDrop<T>) -> bool

This method tests for !=.

impl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where
    T: PartialOrd<T> + ?Sized, 

fn partial_cmp(&self, other: &ManuallyDrop<T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &ManuallyDrop<T>) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &ManuallyDrop<T>) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &ManuallyDrop<T>) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &ManuallyDrop<T>) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T> StructuralEq for ManuallyDrop<T> where
    T: ?Sized, 

impl<T> StructuralPartialEq for ManuallyDrop<T> where
    T: ?Sized,