Trait nom::lib::std::prelude::v1::rust_2021::Drop

pub trait Drop {
    pub fn drop(&mut self);
}

[−] Expand description

Custom code within the destructor.

When a value is no longer needed, Rust will run a “destructor” on that value. The most common way that a value is no longer needed is when it goes out of scope. Destructors may still run in other circumstances, but we’re going to focus on scope for the examples here. To learn about some of those other cases, please see the reference section on destructors.

This destructor consists of two components:

• A call to Drop::drop for that value, if this special Drop trait is implemented for its type.
• The automatically generated “drop glue” which recursively calls the destructors of the all fields of this value.

As Rust automatically calls the destructors of all contained fields, you don’t have to implement Drop in most cases. But there are some cases where it is useful, for example for types which directly manage a resource. That resource may be memory, it may be a file descriptor, it may be a network socket. Once a value of that type is no longer going to be used, it should “clean up” its resource by freeing the memory or closing the file or socket. This is the job of a destructor, and therefore the job of Drop::drop.

Examples

To see destructors in action, let’s take a look at the following program:

struct HasDrop;

impl Drop for HasDrop {
    fn drop(&mut self) {
        println!("Dropping HasDrop!");
    }
}

struct HasTwoDrops {
    one: HasDrop,
    two: HasDrop,
}

impl Drop for HasTwoDrops {
    fn drop(&mut self) {
        println!("Dropping HasTwoDrops!");
    }
}

fn main() {
    let _x = HasTwoDrops { one: HasDrop, two: HasDrop };
    println!("Running!");
}

Rust will first call Drop::drop for _x and then for both _x.one and _x.two, meaning that running this will print

Running!
Dropping HasTwoDrops!
Dropping HasDrop!
Dropping HasDrop!

Even if we remove the implementation of Drop for HasTwoDrop, the destructors of its fields are still called. This would result in

Running!
Dropping HasDrop!
Dropping HasDrop!

You cannot call Drop::drop yourself

Because Drop::drop is used to clean up a value, it may be dangerous to use this value after the method has been called. As Drop::drop does not take ownership of its input, Rust prevents misuse by not allowing you to call Drop::drop directly.

In other words, if you tried to explicitly call Drop::drop in the above example, you’d get a compiler error.

If you’d like to explicitly call the destructor of a value, mem::drop can be used instead.

Drop order

Which of our two HasDrop drops first, though? For structs, it’s the same order that they’re declared: first one, then two. If you’d like to try this yourself, you can modify HasDrop above to contain some data, like an integer, and then use it in the println! inside of Drop. This behavior is guaranteed by the language.

Unlike for structs, local variables are dropped in reverse order:

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Dropping Foo!")
    }
}

struct Bar;

impl Drop for Bar {
    fn drop(&mut self) {
        println!("Dropping Bar!")
    }
}

fn main() {
    let _foo = Foo;
    let _bar = Bar;
}

This will print

Dropping Bar!
Dropping Foo!

Please see the reference for the full rules.

Copy and Drop are exclusive

You cannot implement both Copy and Drop on the same type. Types that are Copy get implicitly duplicated by the compiler, making it very hard to predict when, and how often destructors will be executed. As such, these types cannot have destructors.

Required methods

pub fn drop(&mut self)[−]

Executes the destructor for this type.

This method is called implicitly when the value goes out of scope, and cannot be called explicitly (this is compiler error E0040). However, the mem::drop function in the prelude can be used to call the argument’s Drop implementation.

When this method has been called, self has not yet been deallocated. That only happens after the method is over. If this wasn’t the case, self would be a dangling reference.

Panics

Given that a panic! will call drop as it unwinds, any panic! in a drop implementation will likely abort.

Note that even if this panics, the value is considered to be dropped; you must not cause drop to be called again. This is normally automatically handled by the compiler, but when using unsafe code, can sometimes occur unintentionally, particularly when using ptr::drop_in_place.

Implementations on Foreign Types

impl<'_, T> Drop for MutexGuard<'_, T> where
    T: ?Sized, 

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pub fn drop(&mut self)

impl<T> Drop for SyncSender<T>

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pub fn drop(&mut self)

impl<'_, T> Drop for RwLockWriteGuard<'_, T> where
    T: ?Sized, 

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pub fn drop(&mut self)

impl<T> Drop for RwLock<T> where
    T: ?Sized, 

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pub fn drop(&mut self)

impl Drop for CString

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pub fn drop(&mut self)

impl<T> Drop for SyncOnceCell<T>

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pub fn drop(&mut self)

impl<T> Drop for Sender<T>

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pub fn drop(&mut self)

impl<T> Drop for Receiver<T>

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pub fn drop(&mut self)

impl<W> Drop for BufWriter<W> where
    W: Write, 

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pub fn drop(&mut self)

impl<'_, T> Drop for RwLockReadGuard<'_, T> where
    T: ?Sized, 

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pub fn drop(&mut self)

impl<'f> Drop for VaListImpl<'f>

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pub fn drop(&mut self)

impl<T, const N: usize> Drop for IntoIter<T, N>

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pub fn drop(&mut self)

impl Drop for Waker

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pub fn drop(&mut self)

impl<T> Drop for Rc<T> where
    T: ?Sized, 

pub fn drop(&mut self)[−]

Drops the Rc.

This will decrement the strong reference count. If the strong reference count reaches zero then the only other references (if any) are Weak, so we drop the inner value.

Examples

use std::rc::Rc;

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo  = Rc::new(Foo);
let foo2 = Rc::clone(&foo);

drop(foo);    // Doesn't print anything
drop(foo2);   // Prints "dropped!"

impl<T> Drop for Weak<T> where
    T: ?Sized, 

pub fn drop(&mut self)[−]

Drops the Weak pointer.

Examples

use std::sync::{Arc, Weak};

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo = Arc::new(Foo);
let weak_foo = Arc::downgrade(&foo);
let other_weak_foo = Weak::clone(&weak_foo);

drop(weak_foo);   // Doesn't print anything
drop(foo);        // Prints "dropped!"

assert!(other_weak_foo.upgrade().is_none());

impl<T> Drop for Weak<T> where
    T: ?Sized, 

pub fn drop(&mut self)[−]

Drops the Weak pointer.

Examples

use std::rc::{Rc, Weak};

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo = Rc::new(Foo);
let weak_foo = Rc::downgrade(&foo);
let other_weak_foo = Weak::clone(&weak_foo);

drop(weak_foo);   // Doesn't print anything
drop(foo);        // Prints "dropped!"

assert!(other_weak_foo.upgrade().is_none());

impl<T> Drop for Arc<T> where
    T: ?Sized, 

pub fn drop(&mut self)[−]

Drops the Arc.

This will decrement the strong reference count. If the strong reference count reaches zero then the only other references (if any) are Weak, so we drop the inner value.

Examples

use std::sync::Arc;

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo  = Arc::new(Foo);
let foo2 = Arc::clone(&foo);

drop(foo);    // Doesn't print anything
drop(foo2);   // Prints "dropped!"

Implementors

impl<'_> Drop for nom::lib::std::string::Drain<'_>

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pub fn drop(&mut self)

impl<'_, I, A> Drop for Splice<'_, I, A> where
    I: Iterator,
    A: Allocator, 

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pub fn drop(&mut self)

impl<'_, K, V, F> Drop for nom::lib::std::collections::btree_map::DrainFilter<'_, K, V, F> where
    F: FnMut(&K, &mut V) -> bool, 

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pub fn drop(&mut self)

impl<'_, T> Drop for PeekMut<'_, T> where
    T: Ord, 

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pub fn drop(&mut self)

impl<'_, T> Drop for nom::lib::std::collections::vec_deque::Drain<'_, T>

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pub fn drop(&mut self)

impl<'_, T, A> Drop for nom::lib::std::vec::Drain<'_, T, A> where
    A: Allocator, 

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pub fn drop(&mut self)

impl<'_, T, F> Drop for nom::lib::std::collections::btree_set::DrainFilter<'_, T, F> where
    F: FnMut(&T) -> bool, 

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pub fn drop(&mut self)

impl<'_, T, F> Drop for nom::lib::std::collections::linked_list::DrainFilter<'_, T, F> where
    F: FnMut(&mut T) -> bool, 

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pub fn drop(&mut self)

impl<'_, T, F, A> Drop for nom::lib::std::vec::DrainFilter<'_, T, F, A> where
    F: FnMut(&mut T) -> bool,
    A: Allocator, 

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pub fn drop(&mut self)

impl<'a, T> Drop for DrainSorted<'a, T> where
    T: Ord, 

pub fn drop(&mut self)[−]

Removes heap elements in heap order.

impl<K, V> Drop for nom::lib::std::collections::btree_map::IntoIter<K, V>

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pub fn drop(&mut self)

impl<K, V> Drop for BTreeMap<K, V>

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pub fn drop(&mut self)

impl<T> Drop for LinkedList<T>

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pub fn drop(&mut self)

impl<T> Drop for VecDeque<T>

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pub fn drop(&mut self)

impl<T, A> Drop for Box<T, A> where
    T: ?Sized,
    A: Allocator, 

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pub fn drop(&mut self)

impl<T, A> Drop for nom::lib::std::vec::IntoIter<T, A> where
    A: Allocator, 

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pub fn drop(&mut self)

impl<T, A> Drop for Vec<T, A> where
    A: Allocator, 

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pub fn drop(&mut self)

impl Drop for BacktraceFrameFmt<'_, '_, '_>

impl<T> Drop for Sender<T>

impl<T> Drop for Receiver<T>

impl Drop for SelectedOperation<'_>

impl<T> Drop for Injector<T>

impl<T: ?Sized + Pointable> Drop for Owned<T>

impl Drop for LocalHandle

impl Drop for Guard

impl<T> Drop for ArrayQueue<T>

impl<T> Drop for SegQueue<T>

impl<K, V> Drop for SkipList<K, V>

impl<K, V> Drop for IntoIter<K, V>

impl<K, V> Drop for Entry<'_, K, V>

impl<T: ?Sized> Drop for ShardedLockWriteGuard<'_, T>

impl Drop for WaitGroup

impl<W: Write> Drop for DeflateEncoder<W>

impl<W: Write> Drop for ZlibEncoder<W>

impl Drop for Bitmap

impl Drop for TextureFont

impl<T> Drop for Receiver<T>

impl<T> Drop for UnboundedReceiver<T>

impl<T> Drop for Sender<T>

impl<T> Drop for Receiver<T>

impl Drop for Enter

impl<T> Drop for LocalFutureObj<'_, T>

impl<Fut> Drop for Shared<Fut> where     Fut: Future,

impl<Fut> Drop for FuturesUnordered<Fut>

impl<T: ?Sized> Drop for MutexLockFuture<'_, T>

impl<T: ?Sized> Drop for MutexGuard<'_, T>

impl<T: ?Sized, U: ?Sized> Drop for MappedMutexGuard<'_, T, U>

impl<T, N> Drop for GenericArrayIter<T, N> where     N: ArrayLength<T>,

impl<W: Write> Drop for Encoder<W>

impl Drop for Cursor

impl Drop for Glfw

impl Drop for Window

impl Drop for Texture

impl<'a, T: 'static, B: WithBackend> Drop for MapRead<'a, T, B>

impl Drop for RenderBuffer

impl Drop for Vao

impl Drop for Sampler

impl Drop for Fence

impl Drop for DebugGroup

impl Drop for Buffer

impl<'a> Drop for Blob<'a>

impl<T> Drop for RawTable<T>

impl<T> Drop for RawIntoIter<T>

impl<T> Drop for RawDrain<'_, T>

impl<T> Drop for RawParDrain<'_, T>

impl<'a, K, V, F> Drop for DrainFilter<'a, K, V, F> where     F: FnMut(&K, &mut V) -> bool,

impl<'a, K, F> Drop for DrainFilter<'a, K, F> where     F: FnMut(&K) -> bool,

impl<'a, K, I, F> Drop for Group<'a, K, I, F> where     I: Iterator,     I::Item: 'a,

impl<'a, I> Drop for Chunk<'a, I> where     I: Iterator,     I::Item: 'a,

impl<'a, R: RawMutex + 'a, T: ?Sized + 'a> Drop for MutexGuard<'a, R, T>

impl<'a, R: RawMutex + 'a, T: ?Sized + 'a> Drop for MappedMutexGuard<'a, R, T>

impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Drop for ReentrantMutexGuard<'a, R, G, T>

impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Drop for MappedReentrantMutexGuard<'a, R, G, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for RwLockReadGuard<'a, R, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for RwLockWriteGuard<'a, R, T>

impl<'a, R: RawRwLockUpgrade + 'a, T: ?Sized + 'a> Drop for RwLockUpgradableReadGuard<'a, R, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for MappedRwLockReadGuard<'a, R, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for MappedRwLockWriteGuard<'a, R, T>

impl<W: Write> Drop for Writer<W>

impl<'a, W: Write> Drop for StreamWriter<'a, W>

impl<T> Drop for Polygon<T>

impl<'a, T: Ord + Send> Drop for Drain<'a, T>

impl<'a, T: Send> Drop for Drain<'a, T>

impl<'a> Drop for Drain<'a>

impl<'data, T: Send> Drop for Drain<'data, T>

impl Drop for ThreadPool

impl Drop for Ast

impl Drop for ClassSet

impl Drop for Hir

impl Drop for Window

impl<T> Drop for VecStorage<T>

impl<T, F, S> Drop for ScopeGuard<T, F, S> where     F: FnOnce(T),     S: Strategy,

impl<'a, T2> Drop for StreamRc<'a, T2>

impl<'a, T2> Drop for Stream<'a, T2>

impl<'a, K: Key, V: Slottable> Drop for Drain<'a, K, V>

impl<'a, K: Key, V> Drop for Drain<'a, K, V>

impl<'a, K: Key, V: Slottable> Drop for Drain<'a, K, V>

impl<'a, K: Key, V> Drop for Drain<'a, K, V>

impl<'a, K: Key, V> Drop for Drain<'a, K, V>

impl<'a, T: 'a + Array> Drop for Drain<'a, T>

impl<A: Array> Drop for SmallVec<A>

impl<A: Array> Drop for IntoIter<A>

impl<'a> Drop for StructBuilder<'a>

impl<'a> Drop for ParseBuffer<'a>

impl<T: Send> Drop for ThreadLocal<T>

impl<'a, W: Write + Seek> Drop for DirectoryEncoder<'a, W>

impl<'a, W: Write + Seek, C: ColorType> Drop for ImageEncoder<'a, W, C>