Struct elrond_wasm::Box
1.0.0· source · [−]Expand description
A pointer type for heap allocation.
See the module-level documentation for more.
Implementations
Allocates memory on the heap and then places x
into it.
This doesn’t actually allocate if T
is zero-sized.
Examples
let five = Box::new(5);
pub fn new_uninit() -> Box<MaybeUninit<T>, Global>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub fn new_uninit() -> Box<MaybeUninit<T>, Global>ⓘ
new_uninit
)Constructs a new box with uninitialized contents.
Examples
#![feature(new_uninit)]
let mut five = Box::<u32>::new_uninit();
let five = unsafe {
// Deferred initialization:
five.as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5)
pub fn new_zeroed() -> Box<MaybeUninit<T>, Global>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub fn new_zeroed() -> Box<MaybeUninit<T>, Global>ⓘ
new_uninit
)Constructs a new Box
with uninitialized contents, with the memory
being filled with 0
bytes.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(new_uninit)]
let zero = Box::<u32>::new_zeroed();
let zero = unsafe { zero.assume_init() };
assert_eq!(*zero, 0)
Constructs a new Pin<Box<T>>
. If T
does not implement Unpin
, then
x
will be pinned in memory and unable to be moved.
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Allocates memory on the heap then places x
into it,
returning an error if the allocation fails
This doesn’t actually allocate if T
is zero-sized.
Examples
#![feature(allocator_api)]
let five = Box::try_new(5)?;
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Constructs a new box with uninitialized contents on the heap, returning an error if the allocation fails
Examples
#![feature(allocator_api, new_uninit)]
let mut five = Box::<u32>::try_new_uninit()?;
let five = unsafe {
// Deferred initialization:
five.as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5);
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Constructs a new Box
with uninitialized contents, with the memory
being filled with 0
bytes on the heap
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(allocator_api, new_uninit)]
let zero = Box::<u32>::try_new_zeroed()?;
let zero = unsafe { zero.assume_init() };
assert_eq!(*zero, 0);
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Allocates memory in the given allocator then places x
into it.
This doesn’t actually allocate if T
is zero-sized.
Examples
#![feature(allocator_api)]
use std::alloc::System;
let five = Box::new_in(5, System);
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Allocates memory in the given allocator then places x
into it,
returning an error if the allocation fails
This doesn’t actually allocate if T
is zero-sized.
Examples
#![feature(allocator_api)]
use std::alloc::System;
let five = Box::try_new_in(5, System)?;
pub fn new_uninit_in(alloc: A) -> Box<MaybeUninit<T>, A>ⓘ where
A: Allocator,
🔬 This is a nightly-only experimental API. (allocator_api
)
pub fn new_uninit_in(alloc: A) -> Box<MaybeUninit<T>, A>ⓘ where
A: Allocator,
allocator_api
)Constructs a new box with uninitialized contents in the provided allocator.
Examples
#![feature(allocator_api, new_uninit)]
use std::alloc::System;
let mut five = Box::<u32, _>::new_uninit_in(System);
let five = unsafe {
// Deferred initialization:
five.as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5)
pub fn try_new_uninit_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError> where
A: Allocator,
🔬 This is a nightly-only experimental API. (allocator_api
)
pub fn try_new_uninit_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError> where
A: Allocator,
allocator_api
)Constructs a new box with uninitialized contents in the provided allocator, returning an error if the allocation fails
Examples
#![feature(allocator_api, new_uninit)]
use std::alloc::System;
let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
let five = unsafe {
// Deferred initialization:
five.as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5);
pub fn new_zeroed_in(alloc: A) -> Box<MaybeUninit<T>, A>ⓘ where
A: Allocator,
🔬 This is a nightly-only experimental API. (allocator_api
)
pub fn new_zeroed_in(alloc: A) -> Box<MaybeUninit<T>, A>ⓘ where
A: Allocator,
allocator_api
)Constructs a new Box
with uninitialized contents, with the memory
being filled with 0
bytes in the provided allocator.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(allocator_api, new_uninit)]
use std::alloc::System;
let zero = Box::<u32, _>::new_zeroed_in(System);
let zero = unsafe { zero.assume_init() };
assert_eq!(*zero, 0)
pub fn try_new_zeroed_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError> where
A: Allocator,
🔬 This is a nightly-only experimental API. (allocator_api
)
pub fn try_new_zeroed_in(alloc: A) -> Result<Box<MaybeUninit<T>, A>, AllocError> where
A: Allocator,
allocator_api
)Constructs a new Box
with uninitialized contents, with the memory
being filled with 0
bytes in the provided allocator,
returning an error if the allocation fails,
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(allocator_api, new_uninit)]
use std::alloc::System;
let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
let zero = unsafe { zero.assume_init() };
assert_eq!(*zero, 0);
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Constructs a new Pin<Box<T, A>>
. If T
does not implement Unpin
, then
x
will be pinned in memory and unable to be moved.
pub fn into_boxed_slice(boxed: Box<T, A>) -> Box<[T], A>ⓘ
🔬 This is a nightly-only experimental API. (box_into_boxed_slice
)
pub fn into_boxed_slice(boxed: Box<T, A>) -> Box<[T], A>ⓘ
box_into_boxed_slice
)Converts a Box<T>
into a Box<[T]>
This conversion does not allocate on the heap and happens in place.
🔬 This is a nightly-only experimental API. (box_into_inner
)
box_into_inner
)Consumes the Box
, returning the wrapped value.
Examples
#![feature(box_into_inner)]
let c = Box::new(5);
assert_eq!(Box::into_inner(c), 5);
pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>], Global>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>], Global>ⓘ
new_uninit
)Constructs a new boxed slice with uninitialized contents.
Examples
#![feature(new_uninit)]
let mut values = Box::<[u32]>::new_uninit_slice(3);
let values = unsafe {
// Deferred initialization:
values[0].as_mut_ptr().write(1);
values[1].as_mut_ptr().write(2);
values[2].as_mut_ptr().write(3);
values.assume_init()
};
assert_eq!(*values, [1, 2, 3])
pub fn new_zeroed_slice(len: usize) -> Box<[MaybeUninit<T>], Global>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub fn new_zeroed_slice(len: usize) -> Box<[MaybeUninit<T>], Global>ⓘ
new_uninit
)Constructs a new boxed slice with uninitialized contents, with the memory
being filled with 0
bytes.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(new_uninit)]
let values = Box::<[u32]>::new_zeroed_slice(3);
let values = unsafe { values.assume_init() };
assert_eq!(*values, [0, 0, 0])
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Constructs a new boxed slice with uninitialized contents. Returns an error if the allocation fails
Examples
#![feature(allocator_api, new_uninit)]
let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
let values = unsafe {
// Deferred initialization:
values[0].as_mut_ptr().write(1);
values[1].as_mut_ptr().write(2);
values[2].as_mut_ptr().write(3);
values.assume_init()
};
assert_eq!(*values, [1, 2, 3]);
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Constructs a new boxed slice with uninitialized contents, with the memory
being filled with 0
bytes. Returns an error if the allocation fails
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(allocator_api, new_uninit)]
let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
let values = unsafe { values.assume_init() };
assert_eq!(*values, [0, 0, 0]);
pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[MaybeUninit<T>], A>ⓘ
🔬 This is a nightly-only experimental API. (allocator_api
)
pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[MaybeUninit<T>], A>ⓘ
allocator_api
)Constructs a new boxed slice with uninitialized contents in the provided allocator.
Examples
#![feature(allocator_api, new_uninit)]
use std::alloc::System;
let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
let values = unsafe {
// Deferred initialization:
values[0].as_mut_ptr().write(1);
values[1].as_mut_ptr().write(2);
values[2].as_mut_ptr().write(3);
values.assume_init()
};
assert_eq!(*values, [1, 2, 3])
pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[MaybeUninit<T>], A>ⓘ
🔬 This is a nightly-only experimental API. (allocator_api
)
pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[MaybeUninit<T>], A>ⓘ
allocator_api
)Constructs a new boxed slice with uninitialized contents in the provided allocator,
with the memory being filled with 0
bytes.
See MaybeUninit::zeroed
for examples of correct and incorrect usage
of this method.
Examples
#![feature(allocator_api, new_uninit)]
use std::alloc::System;
let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
let values = unsafe { values.assume_init() };
assert_eq!(*values, [0, 0, 0])
pub unsafe fn assume_init(self) -> Box<T, A>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub unsafe fn assume_init(self) -> Box<T, A>ⓘ
new_uninit
)Converts to Box<T, A>
.
Safety
As with MaybeUninit::assume_init
,
it is up to the caller to guarantee that the value
really is in an initialized state.
Calling this when the content is not yet fully initialized
causes immediate undefined behavior.
Examples
#![feature(new_uninit)]
let mut five = Box::<u32>::new_uninit();
let five: Box<u32> = unsafe {
// Deferred initialization:
five.as_mut_ptr().write(5);
five.assume_init()
};
assert_eq!(*five, 5)
pub fn write(boxed: Box<MaybeUninit<T>, A>, value: T) -> Box<T, A>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub fn write(boxed: Box<MaybeUninit<T>, A>, value: T) -> Box<T, A>ⓘ
new_uninit
)Writes the value and converts to Box<T, A>
.
This method converts the box similarly to Box::assume_init
but
writes value
into it before conversion thus guaranteeing safety.
In some scenarios use of this method may improve performance because
the compiler may be able to optimize copying from stack.
Examples
#![feature(new_uninit)]
let big_box = Box::<[usize; 1024]>::new_uninit();
let mut array = [0; 1024];
for (i, place) in array.iter_mut().enumerate() {
*place = i;
}
// The optimizer may be able to elide this copy, so previous code writes
// to heap directly.
let big_box = Box::write(big_box, array);
for (i, x) in big_box.iter().enumerate() {
assert_eq!(*x, i);
}
pub unsafe fn assume_init(self) -> Box<[T], A>ⓘ
🔬 This is a nightly-only experimental API. (new_uninit
)
pub unsafe fn assume_init(self) -> Box<[T], A>ⓘ
new_uninit
)Converts to Box<[T], A>
.
Safety
As with MaybeUninit::assume_init
,
it is up to the caller to guarantee that the values
really are in an initialized state.
Calling this when the content is not yet fully initialized
causes immediate undefined behavior.
Examples
#![feature(new_uninit)]
let mut values = Box::<[u32]>::new_uninit_slice(3);
let values = unsafe {
// Deferred initialization:
values[0].as_mut_ptr().write(1);
values[1].as_mut_ptr().write(2);
values[2].as_mut_ptr().write(3);
values.assume_init()
};
assert_eq!(*values, [1, 2, 3])
Constructs a box from a raw pointer.
After calling this function, the raw pointer is owned by the
resulting Box
. Specifically, the Box
destructor will call
the destructor of T
and free the allocated memory. For this
to be safe, the memory must have been allocated in accordance
with the memory layout used by Box
.
Safety
This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.
The safety conditions are described in the memory layout section.
Examples
Recreate a Box
which was previously converted to a raw pointer
using Box::into_raw
:
let x = Box::new(5);
let ptr = Box::into_raw(x);
let x = unsafe { Box::from_raw(ptr) };
Manually create a Box
from scratch by using the global allocator:
use std::alloc::{alloc, Layout};
unsafe {
let ptr = alloc(Layout::new::<i32>()) as *mut i32;
// In general .write is required to avoid attempting to destruct
// the (uninitialized) previous contents of `ptr`, though for this
// simple example `*ptr = 5` would have worked as well.
ptr.write(5);
let x = Box::from_raw(ptr);
}
pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Box<T, A>ⓘ
🔬 This is a nightly-only experimental API. (allocator_api
)
pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Box<T, A>ⓘ
allocator_api
)Constructs a box from a raw pointer in the given allocator.
After calling this function, the raw pointer is owned by the
resulting Box
. Specifically, the Box
destructor will call
the destructor of T
and free the allocated memory. For this
to be safe, the memory must have been allocated in accordance
with the memory layout used by Box
.
Safety
This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.
Examples
Recreate a Box
which was previously converted to a raw pointer
using Box::into_raw_with_allocator
:
#![feature(allocator_api)]
use std::alloc::System;
let x = Box::new_in(5, System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
let x = unsafe { Box::from_raw_in(ptr, alloc) };
Manually create a Box
from scratch by using the system allocator:
#![feature(allocator_api, slice_ptr_get)]
use std::alloc::{Allocator, Layout, System};
unsafe {
let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
// In general .write is required to avoid attempting to destruct
// the (uninitialized) previous contents of `ptr`, though for this
// simple example `*ptr = 5` would have worked as well.
ptr.write(5);
let x = Box::from_raw_in(ptr, System);
}
Consumes the Box
, returning a wrapped raw pointer.
The pointer will be properly aligned and non-null.
After calling this function, the caller is responsible for the
memory previously managed by the Box
. In particular, the
caller should properly destroy T
and release the memory, taking
into account the memory layout used by Box
. The easiest way to
do this is to convert the raw pointer back into a Box
with the
Box::from_raw
function, allowing the Box
destructor to perform
the cleanup.
Note: this is an associated function, which means that you have
to call it as Box::into_raw(b)
instead of b.into_raw()
. This
is so that there is no conflict with a method on the inner type.
Examples
Converting the raw pointer back into a Box
with Box::from_raw
for automatic cleanup:
let x = Box::new(String::from("Hello"));
let ptr = Box::into_raw(x);
let x = unsafe { Box::from_raw(ptr) };
Manual cleanup by explicitly running the destructor and deallocating the memory:
use std::alloc::{dealloc, Layout};
use std::ptr;
let x = Box::new(String::from("Hello"));
let p = Box::into_raw(x);
unsafe {
ptr::drop_in_place(p);
dealloc(p as *mut u8, Layout::new::<String>());
}
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Consumes the Box
, returning a wrapped raw pointer and the allocator.
The pointer will be properly aligned and non-null.
After calling this function, the caller is responsible for the
memory previously managed by the Box
. In particular, the
caller should properly destroy T
and release the memory, taking
into account the memory layout used by Box
. The easiest way to
do this is to convert the raw pointer back into a Box
with the
Box::from_raw_in
function, allowing the Box
destructor to perform
the cleanup.
Note: this is an associated function, which means that you have
to call it as Box::into_raw_with_allocator(b)
instead of b.into_raw_with_allocator()
. This
is so that there is no conflict with a method on the inner type.
Examples
Converting the raw pointer back into a Box
with Box::from_raw_in
for automatic cleanup:
#![feature(allocator_api)]
use std::alloc::System;
let x = Box::new_in(String::from("Hello"), System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
let x = unsafe { Box::from_raw_in(ptr, alloc) };
Manual cleanup by explicitly running the destructor and deallocating the memory:
#![feature(allocator_api)]
use std::alloc::{Allocator, Layout, System};
use std::ptr::{self, NonNull};
let x = Box::new_in(String::from("Hello"), System);
let (ptr, alloc) = Box::into_raw_with_allocator(x);
unsafe {
ptr::drop_in_place(ptr);
let non_null = NonNull::new_unchecked(ptr);
alloc.deallocate(non_null.cast(), Layout::new::<String>());
}
🔬 This is a nightly-only experimental API. (allocator_api
)
allocator_api
)Returns a reference to the underlying allocator.
Note: this is an associated function, which means that you have
to call it as Box::allocator(&b)
instead of b.allocator()
. This
is so that there is no conflict with a method on the inner type.
Consumes and leaks the Box
, returning a mutable reference,
&'a mut T
. Note that the type T
must outlive the chosen lifetime
'a
. If the type has only static references, or none at all, then this
may be chosen to be 'static
.
This function is mainly useful for data that lives for the remainder of
the program’s life. Dropping the returned reference will cause a memory
leak. If this is not acceptable, the reference should first be wrapped
with the Box::from_raw
function producing a Box
. This Box
can
then be dropped which will properly destroy T
and release the
allocated memory.
Note: this is an associated function, which means that you have
to call it as Box::leak(b)
instead of b.leak()
. This
is so that there is no conflict with a method on the inner type.
Examples
Simple usage:
let x = Box::new(41);
let static_ref: &'static mut usize = Box::leak(x);
*static_ref += 1;
assert_eq!(*static_ref, 42);
Unsized data:
let x = vec![1, 2, 3].into_boxed_slice();
let static_ref = Box::leak(x);
static_ref[0] = 4;
assert_eq!(*static_ref, [4, 2, 3]);
Attempt to downcast the box to a concrete type.
Examples
use std::any::Any;
fn print_if_string(value: Box<dyn Any>) {
if let Ok(string) = value.downcast::<String>() {
println!("String ({}): {}", string.len(), string);
}
}
let my_string = "Hello World".to_string();
print_if_string(Box::new(my_string));
print_if_string(Box::new(0i8));
pub unsafe fn downcast_unchecked<T>(self) -> Box<T, A>ⓘ where
T: Any,
🔬 This is a nightly-only experimental API. (downcast_unchecked
)
pub unsafe fn downcast_unchecked<T>(self) -> Box<T, A>ⓘ where
T: Any,
downcast_unchecked
)Downcasts the box to a concrete type.
For a safe alternative see downcast
.
Examples
#![feature(downcast_unchecked)]
use std::any::Any;
let x: Box<dyn Any> = Box::new(1_usize);
unsafe {
assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}
Safety
The contained value must be of type T
. Calling this method
with the incorrect type is undefined behavior.
Attempt to downcast the box to a concrete type.
Examples
use std::any::Any;
fn print_if_string(value: Box<dyn Any + Send>) {
if let Ok(string) = value.downcast::<String>() {
println!("String ({}): {}", string.len(), string);
}
}
let my_string = "Hello World".to_string();
print_if_string(Box::new(my_string));
print_if_string(Box::new(0i8));
pub unsafe fn downcast_unchecked<T>(self) -> Box<T, A>ⓘ where
T: Any,
🔬 This is a nightly-only experimental API. (downcast_unchecked
)
pub unsafe fn downcast_unchecked<T>(self) -> Box<T, A>ⓘ where
T: Any,
downcast_unchecked
)Downcasts the box to a concrete type.
For a safe alternative see downcast
.
Examples
#![feature(downcast_unchecked)]
use std::any::Any;
let x: Box<dyn Any + Send> = Box::new(1_usize);
unsafe {
assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}
Safety
The contained value must be of type T
. Calling this method
with the incorrect type is undefined behavior.
Attempt to downcast the box to a concrete type.
Examples
use std::any::Any;
fn print_if_string(value: Box<dyn Any + Send + Sync>) {
if let Ok(string) = value.downcast::<String>() {
println!("String ({}): {}", string.len(), string);
}
}
let my_string = "Hello World".to_string();
print_if_string(Box::new(my_string));
print_if_string(Box::new(0i8));
pub unsafe fn downcast_unchecked<T>(self) -> Box<T, A>ⓘ where
T: Any,
🔬 This is a nightly-only experimental API. (downcast_unchecked
)
pub unsafe fn downcast_unchecked<T>(self) -> Box<T, A>ⓘ where
T: Any,
downcast_unchecked
)Downcasts the box to a concrete type.
For a safe alternative see downcast
.
Examples
#![feature(downcast_unchecked)]
use std::any::Any;
let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
unsafe {
assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}
Safety
The contained value must be of type T
. Calling this method
with the incorrect type is undefined behavior.
Trait Implementations
Mutably borrows from an owned value. Read more
Returns a new box with a clone()
of this box’s contents.
Examples
let x = Box::new(5);
let y = x.clone();
// The value is the same
assert_eq!(x, y);
// But they are unique objects
assert_ne!(&*x as *const i32, &*y as *const i32);
Copies source
’s contents into self
without creating a new allocation.
Examples
let x = Box::new(5);
let mut y = Box::new(10);
let yp: *const i32 = &*y;
y.clone_from(&x);
// The value is the same
assert_eq!(x, y);
// And no allocation occurred
assert_eq!(yp, &*y);
impl<I, A> DoubleEndedIterator for Box<I, A> where
I: DoubleEndedIterator + ?Sized,
A: Allocator,
impl<I, A> DoubleEndedIterator for Box<I, A> where
I: DoubleEndedIterator + ?Sized,
A: Allocator,
Removes and returns an element from the end of the iterator. Read more
Returns the n
th element from the end of the iterator. Read more
iter_advance_by
)Advances the iterator from the back by n
elements. Read more
This is the reverse version of Iterator::try_fold()
: it takes
elements starting from the back of the iterator. Read more
An iterator method that reduces the iterator’s elements to a single, final value, starting from the back. Read more
Extends a collection with the contents of an iterator. Read more
extend_one
)Extends a collection with exactly one element.
extend_one
)Reserves capacity in a collection for the given number of additional elements. Read more
Converts a &[T]
into a Box<[T]>
This conversion allocates on the heap
and performs a copy of slice
.
Examples
// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice: Box<[u8]> = Box::from(slice);
println!("{:?}", boxed_slice);
Converts a Box<str>
into a Box<[u8]>
This conversion does not allocate on the heap and happens in place.
Examples
// create a Box<str> which will be used to create a Box<[u8]>
let boxed: Box<str> = Box::from("hello");
let boxed_str: Box<[u8]> = Box::from(boxed);
// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice = Box::from(slice);
assert_eq!(boxed_slice, boxed_str);
Converts a Cow<'_, str>
into a Box<str>
When cow
is the Cow::Borrowed
variant, this
conversion allocates on the heap and copies the
underlying str
. Otherwise, it will try to reuse the owned
String
’s allocation.
Examples
use std::borrow::Cow;
let unboxed = Cow::Borrowed("hello");
let boxed: Box<str> = Box::from(unboxed);
println!("{}", boxed);
let unboxed = Cow::Owned("hello".to_string());
let boxed: Box<str> = Box::from(unboxed);
println!("{}", boxed);
Writes a single u128
into this hasher.
Writes a single usize
into this hasher.
Writes a single i128
into this hasher.
Writes a single isize
into this hasher.
Advances the iterator and returns the next value. Read more
Returns the bounds on the remaining length of the iterator. Read more
Returns the n
th element of the iterator. Read more
Consumes the iterator, returning the last element. Read more
Consumes the iterator, counting the number of iterations and returning it. Read more
iter_advance_by
)Advances the iterator by n
elements. Read more
Creates an iterator starting at the same point, but stepping by the given amount at each iteration. Read more
fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter> where
U: IntoIterator<Item = Self::Item>,
fn chain<U>(self, other: U) -> Chain<Self, <U as IntoIterator>::IntoIter> where
U: IntoIterator<Item = Self::Item>,
Takes two iterators and creates a new iterator over both in sequence. Read more
‘Zips up’ two iterators into a single iterator of pairs. Read more
iter_intersperse
)Creates a new iterator which places a copy of separator
between adjacent
items of the original iterator. Read more
fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G> where
G: FnMut() -> Self::Item,
fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G> where
G: FnMut() -> Self::Item,
iter_intersperse
)Creates a new iterator which places an item generated by separator
between adjacent items of the original iterator. Read more
Takes a closure and creates an iterator which calls that closure on each element. Read more
Calls a closure on each element of an iterator. Read more
Creates an iterator which uses a closure to determine if an element should be yielded. Read more
Creates an iterator that both filters and maps. Read more
Creates an iterator which gives the current iteration count as well as the next value. Read more
Creates an iterator that yields elements based on a predicate. Read more
Creates an iterator that both yields elements based on a predicate and maps. Read more
Creates an iterator that skips the first n
elements. Read more
Creates an iterator that yields the first n
elements, or fewer
if the underlying iterator ends sooner. Read more
Creates an iterator that works like map, but flattens nested structure. Read more
Creates an iterator that flattens nested structure. Read more
Does something with each element of an iterator, passing the value on. Read more
Transforms an iterator into a collection. Read more
Consumes an iterator, creating two collections from it. Read more
fn partition_in_place<'a, T, P>(self, predicate: P) -> usize where
T: 'a,
Self: DoubleEndedIterator<Item = &'a mut T>,
P: FnMut(&T) -> bool,
fn partition_in_place<'a, T, P>(self, predicate: P) -> usize where
T: 'a,
Self: DoubleEndedIterator<Item = &'a mut T>,
P: FnMut(&T) -> bool,
iter_partition_in_place
)Reorders the elements of this iterator in-place according to the given predicate,
such that all those that return true
precede all those that return false
.
Returns the number of true
elements found. Read more
iter_is_partitioned
)Checks if the elements of this iterator are partitioned according to the given predicate,
such that all those that return true
precede all those that return false
. Read more
An iterator method that applies a function as long as it returns successfully, producing a single, final value. Read more
An iterator method that applies a fallible function to each item in the iterator, stopping at the first error and returning that error. Read more
Folds every element into an accumulator by applying an operation, returning the final result. Read more
Reduces the elements to a single one, by repeatedly applying a reducing operation. Read more
iterator_try_reduce
)Reduces the elements to a single one by repeatedly applying a reducing operation. If the closure returns a failure, the failure is propagated back to the caller immediately. Read more
Tests if every element of the iterator matches a predicate. Read more
Tests if any element of the iterator matches a predicate. Read more
Searches for an element of an iterator that satisfies a predicate. Read more
Applies function to the elements of iterator and returns the first non-none result. Read more
try_find
)Applies function to the elements of iterator and returns the first true result or the first error. Read more
Searches for an element in an iterator, returning its index. Read more
fn rposition<P>(&mut self, predicate: P) -> Option<usize> where
P: FnMut(Self::Item) -> bool,
Self: ExactSizeIterator + DoubleEndedIterator,
fn rposition<P>(&mut self, predicate: P) -> Option<usize> where
P: FnMut(Self::Item) -> bool,
Self: ExactSizeIterator + DoubleEndedIterator,
Searches for an element in an iterator from the right, returning its index. Read more
Returns the maximum element of an iterator. Read more
Returns the minimum element of an iterator. Read more
Returns the element that gives the maximum value from the specified function. Read more
Returns the element that gives the maximum value with respect to the specified comparison function. Read more
Returns the element that gives the minimum value from the specified function. Read more
Returns the element that gives the minimum value with respect to the specified comparison function. Read more
Reverses an iterator’s direction. Read more
Converts an iterator of pairs into a pair of containers. Read more
Creates an iterator which copies all of its elements. Read more
Sums the elements of an iterator. Read more
Iterates over the entire iterator, multiplying all the elements Read more
Lexicographically compares the elements of this Iterator
with those
of another. Read more
fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering where
I: IntoIterator,
F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Ordering,
fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering where
I: IntoIterator,
F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Ordering,
iter_order_by
)Lexicographically compares the elements of this Iterator
with those
of another with respect to the specified comparison function. Read more
1.5.0 · sourcefn partial_cmp<I>(self, other: I) -> Option<Ordering> where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
fn partial_cmp<I>(self, other: I) -> Option<Ordering> where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
Lexicographically compares the elements of this Iterator
with those
of another. Read more
fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering> where
I: IntoIterator,
F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Option<Ordering>,
fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering> where
I: IntoIterator,
F: FnMut(Self::Item, <I as IntoIterator>::Item) -> Option<Ordering>,
iter_order_by
)Lexicographically compares the elements of this Iterator
with those
of another with respect to the specified comparison function. Read more
1.5.0 · sourcefn eq<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<<I as IntoIterator>::Item>,
fn eq<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<<I as IntoIterator>::Item>,
fn eq_by<I, F>(self, other: I, eq: F) -> bool where
I: IntoIterator,
F: FnMut(Self::Item, <I as IntoIterator>::Item) -> bool,
fn eq_by<I, F>(self, other: I, eq: F) -> bool where
I: IntoIterator,
F: FnMut(Self::Item, <I as IntoIterator>::Item) -> bool,
iter_order_by
)1.5.0 · sourcefn ne<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<<I as IntoIterator>::Item>,
fn ne<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialEq<<I as IntoIterator>::Item>,
1.5.0 · sourcefn lt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
fn lt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
Determines if the elements of this Iterator
are lexicographically
less than those of another. Read more
1.5.0 · sourcefn le<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
fn le<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
Determines if the elements of this Iterator
are lexicographically
less or equal to those of another. Read more
1.5.0 · sourcefn gt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
fn gt<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
Determines if the elements of this Iterator
are lexicographically
greater than those of another. Read more
1.5.0 · sourcefn ge<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
fn ge<I>(self, other: I) -> bool where
I: IntoIterator,
Self::Item: PartialOrd<<I as IntoIterator>::Item>,
Determines if the elements of this Iterator
are lexicographically
greater than or equal to those of another. Read more
is_sorted
)Checks if the elements of this iterator are sorted. Read more
is_sorted
)Checks if the elements of this iterator are sorted using the given comparator function. Read more
fn is_sorted_by_key<F, K>(self, f: F) -> bool where
F: FnMut(Self::Item) -> K,
K: PartialOrd<K>,
fn is_sorted_by_key<F, K>(self, f: F) -> bool where
F: FnMut(Self::Item) -> K,
K: PartialOrd<K>,
is_sorted
)Checks if the elements of this iterator are sorted using the given key extraction function. Read more
pub fn dep_decode<I>(input: &mut I) -> Result<Box<T, Global>, DecodeError> where
I: NestedDecodeInput,
pub fn dep_decode<I>(input: &mut I) -> Result<Box<T, Global>, DecodeError> where
I: NestedDecodeInput,
Attempt to deserialise the value from input, using the format of an object nested inside another structure. In case of success returns the deserialized value and the number of bytes consumed during the operation. Read more
pub fn dep_decode_or_exit<I, ExitCtx>(
input: &mut I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<T, Global>ⓘ where
I: NestedDecodeInput,
ExitCtx: Clone,
pub fn dep_decode_or_exit<I, ExitCtx>(
input: &mut I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<T, Global>ⓘ where
I: NestedDecodeInput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
pub fn dep_decode<I>(input: &mut I) -> Result<Box<str, Global>, DecodeError> where
I: NestedDecodeInput,
pub fn dep_decode<I>(input: &mut I) -> Result<Box<str, Global>, DecodeError> where
I: NestedDecodeInput,
Attempt to deserialise the value from input, using the format of an object nested inside another structure. In case of success returns the deserialized value and the number of bytes consumed during the operation. Read more
pub fn dep_decode_or_exit<I, ExitCtx>(
input: &mut I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<str, Global>ⓘ where
I: NestedDecodeInput,
ExitCtx: Clone,
pub fn dep_decode_or_exit<I, ExitCtx>(
input: &mut I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<str, Global>ⓘ where
I: NestedDecodeInput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
pub fn dep_encode<O>(&self, dest: &mut O) -> Result<(), EncodeError> where
O: NestedEncodeOutput,
pub fn dep_encode<O>(&self, dest: &mut O) -> Result<(), EncodeError> where
O: NestedEncodeOutput,
NestedEncode to output, using the format of an object nested inside another structure. Does not provide compact version. Read more
pub fn dep_encode_or_exit<O, ExitCtx>(
&self,
dest: &mut O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: NestedEncodeOutput,
ExitCtx: Clone,
pub fn dep_encode_or_exit<O, ExitCtx>(
&self,
dest: &mut O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: NestedEncodeOutput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
pub fn dep_encode<O>(&self, dest: &mut O) -> Result<(), EncodeError> where
O: NestedEncodeOutput,
pub fn dep_encode<O>(&self, dest: &mut O) -> Result<(), EncodeError> where
O: NestedEncodeOutput,
NestedEncode to output, using the format of an object nested inside another structure. Does not provide compact version. Read more
pub fn dep_encode_or_exit<O, ExitCtx>(
&self,
dest: &mut O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: NestedEncodeOutput,
ExitCtx: Clone,
pub fn dep_encode_or_exit<O, ExitCtx>(
&self,
dest: &mut O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: NestedEncodeOutput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
pub fn dep_encode<O>(&self, dest: &mut O) -> Result<(), EncodeError> where
O: NestedEncodeOutput,
pub fn dep_encode<O>(&self, dest: &mut O) -> Result<(), EncodeError> where
O: NestedEncodeOutput,
NestedEncode to output, using the format of an object nested inside another structure. Does not provide compact version. Read more
pub fn dep_encode_or_exit<O, ExitCtx>(
&self,
dest: &mut O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: NestedEncodeOutput,
ExitCtx: Clone,
pub fn dep_encode_or_exit<O, ExitCtx>(
&self,
dest: &mut O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: NestedEncodeOutput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
This method returns an ordering between self
and other
values if one exists. Read more
This method tests less than (for self
and other
) and is used by the <
operator. Read more
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
This method tests greater than or equal to (for self
and other
) and is used by the >=
operator. Read more
async_stream
)The type of items yielded by the stream.
async_stream
)Attempt to pull out the next value of this stream, registering the
current task for wakeup if the value is not yet available, and returning
None
if the stream is exhausted. Read more
pub fn top_decode_or_exit<I, ExitCtx>(
input: I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<[T], Global>ⓘ where
I: TopDecodeInput,
ExitCtx: Clone,
pub fn top_decode_or_exit<I, ExitCtx>(
input: I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<[T], Global>ⓘ where
I: TopDecodeInput,
ExitCtx: Clone,
Quick exit for any of the contained types
pub fn top_decode<I>(input: I) -> Result<Box<[T], Global>, DecodeError> where
I: TopDecodeInput,
pub fn top_decode<I>(input: I) -> Result<Box<[T], Global>, DecodeError> where
I: TopDecodeInput,
Attempt to deserialize the value from input.
pub fn top_decode<I>(input: I) -> Result<Box<T, Global>, DecodeError> where
I: TopDecodeInput,
pub fn top_decode<I>(input: I) -> Result<Box<T, Global>, DecodeError> where
I: TopDecodeInput,
Attempt to deserialize the value from input.
pub fn top_decode_or_exit<I, ExitCtx>(
input: I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<T, Global>ⓘ where
I: TopDecodeInput,
ExitCtx: Clone,
pub fn top_decode_or_exit<I, ExitCtx>(
input: I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<T, Global>ⓘ where
I: TopDecodeInput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
pub fn top_decode<I>(input: I) -> Result<Box<str, Global>, DecodeError> where
I: TopDecodeInput,
pub fn top_decode<I>(input: I) -> Result<Box<str, Global>, DecodeError> where
I: TopDecodeInput,
Attempt to deserialize the value from input.
pub fn top_decode_or_exit<I, ExitCtx>(
input: I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<str, Global>ⓘ where
I: TopDecodeInput,
ExitCtx: Clone,
pub fn top_decode_or_exit<I, ExitCtx>(
input: I,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !
) -> Box<str, Global>ⓘ where
I: TopDecodeInput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller implementations
in cases where the application is supposed to exit directly on decode error. Read more
Provides the underlying data as an owned byte slice box. Consumes the input object in the process. Read more
pub fn into_nested_buffer(
self
) -> <Box<[u8], Global> as TopDecodeInput>::NestedBufferⓘ
Retrieves the underlying data as a pre-parsed u64. Expected to panic if the conversion is not possible. Read more
Retrieves the underlying data as a pre-parsed i64. Expected to panic if the conversion is not possible. Read more
fn into_specialized<T, F>(self, else_deser: F) -> Result<T, DecodeError> where
T: TryStaticCast,
F: FnOnce(Self) -> Result<T, DecodeError>,
fn into_specialized_or_exit<T, F, ExitCtx>(
self,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !,
else_deser: F
) -> T where
T: TryStaticCast,
ExitCtx: Clone,
F: FnOnce(Self, ExitCtx, fn(ExitCtx, DecodeError) -> !) -> T,
fn into_specialized_or_exit<T, F, ExitCtx>(
self,
c: ExitCtx,
exit: fn(ExitCtx, DecodeError) -> !,
else_deser: F
) -> T where
T: TryStaticCast,
ExitCtx: Clone,
F: FnOnce(Self, ExitCtx, fn(ExitCtx, DecodeError) -> !) -> T,
Note: currently not in use.
Attempt to serialize the value to ouput.
pub fn top_encode_or_exit<O, ExitCtx>(
&self,
output: O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: TopEncodeOutput,
ExitCtx: Clone,
pub fn top_encode_or_exit<O, ExitCtx>(
&self,
output: O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: TopEncodeOutput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller bytecode implementations
in cases where the application is supposed to exit directly on decode error. Read more
Attempt to serialize the value to ouput.
pub fn top_encode_or_exit<O, ExitCtx>(
&self,
output: O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: TopEncodeOutput,
ExitCtx: Clone,
pub fn top_encode_or_exit<O, ExitCtx>(
&self,
output: O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: TopEncodeOutput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller bytecode implementations
in cases where the application is supposed to exit directly on decode error. Read more
Attempt to serialize the value to ouput.
pub fn top_encode_or_exit<O, ExitCtx>(
&self,
output: O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: TopEncodeOutput,
ExitCtx: Clone,
pub fn top_encode_or_exit<O, ExitCtx>(
&self,
output: O,
c: ExitCtx,
exit: fn(ExitCtx, EncodeError) -> !
) where
O: TopEncodeOutput,
ExitCtx: Clone,
Version of top_decode
that exits quickly in case of error.
Its purpose is to create smaller bytecode implementations
in cases where the application is supposed to exit directly on decode error. Read more
Attempts to convert a Box<[T]>
into a Box<[T; N]>
.
The conversion occurs in-place and does not require a new memory allocation.
Errors
Returns the old Box<[T]>
in the Err
variant if
boxed_slice.len()
does not equal N
.
A type can provide more than its own description. For instance, a struct can also provide the descriptions of the type of its fields. TypeAbi doesn’t care for the exact accumulator type, which is abstracted by the TypeDescriptionContainer trait. Read more
A type can provide more than its own description. For instance, a struct can also provide the descriptions of the type of its fields. TypeAbi doesn’t care for the exact accumulator type, which is abstracted by the TypeDescriptionContainer trait. Read more
A type can provide more than its own description. For instance, a struct can also provide the descriptions of the type of its fields. TypeAbi doesn’t care for the exact accumulator type, which is abstracted by the TypeDescriptionContainer trait. Read more
impl<T, U, A> CoerceUnsized<Box<U, A>> for Box<T, A> where
T: Unsize<U> + ?Sized,
A: Allocator,
U: ?Sized,
impl<T, U> DispatchFromDyn<Box<U, Global>> for Box<T, Global> where
T: Unsize<U> + ?Sized,
U: ?Sized,
impl<I, A> FusedIterator for Box<I, A> where
I: FusedIterator + ?Sized,
A: Allocator,
Auto Trait Implementations
impl<T: ?Sized, A> RefUnwindSafe for Box<T, A> where
A: RefUnwindSafe,
T: RefUnwindSafe,
impl<T: ?Sized, A> UnwindSafe for Box<T, A> where
A: UnwindSafe,
T: UnwindSafe,
Blanket Implementations
Mutably borrows from an owned value. Read more
into_future
)The output that the future will produce on completion.
type Future = F
type Future = F
into_future
)Which kind of future are we turning this into?
into_future
)Creates a future from a value.
type Searcher = CharPredicateSearcher<'a, F>
type Searcher = CharPredicateSearcher<'a, F>
pattern
)Associated searcher for this pattern
pattern
)Constructs the associated searcher from
self
and the haystack
to search in. Read more
pattern
)Checks whether the pattern matches anywhere in the haystack
pattern
)Checks whether the pattern matches at the front of the haystack
pattern
)Removes the pattern from the front of haystack, if it matches.
pub fn is_suffix_of(self, haystack: &'a str) -> bool where
CharPredicateSearcher<'a, F>: ReverseSearcher<'a>,
pub fn is_suffix_of(self, haystack: &'a str) -> bool where
CharPredicateSearcher<'a, F>: ReverseSearcher<'a>,
pattern
)Checks whether the pattern matches at the back of the haystack
pub fn strip_suffix_of(self, haystack: &'a str) -> Option<&'a str> where
CharPredicateSearcher<'a, F>: ReverseSearcher<'a>,
pub fn strip_suffix_of(self, haystack: &'a str) -> Option<&'a str> where
CharPredicateSearcher<'a, F>: ReverseSearcher<'a>,
pattern
)Removes the pattern from the back of haystack, if it matches.