Type Alias futures_util::future::BoxFuture
source · pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
Expand description
An owned dynamically typed Future
for use in cases where you can’t
statically type your result or need to add some indirection.
Aliased Type§
struct BoxFuture<'a, T> {}
Implementations
source§impl<Ptr> Pin<Ptr>
impl<Ptr> Pin<Ptr>
1.33.0 (const: unstable) · sourcepub fn new(pointer: Ptr) -> Pin<Ptr>
pub fn new(pointer: Ptr) -> Pin<Ptr>
Construct a new Pin<Ptr>
around a pointer to some data of a type that
implements Unpin
.
Unlike Pin::new_unchecked
, this method is safe because the pointer
Ptr
dereferences to an Unpin
type, which cancels the pinning guarantees.
§Examples
use std::pin::Pin;
let mut val: u8 = 5;
// Since `val` doesn't care about being moved, we can safely create a "facade" `Pin`
// which will allow `val` to participate in `Pin`-bound apis without checking that
// pinning guarantees are actually upheld.
let mut pinned: Pin<&mut u8> = Pin::new(&mut val);
1.39.0 (const: unstable) · sourcepub fn into_inner(pin: Pin<Ptr>) -> Ptr
pub fn into_inner(pin: Pin<Ptr>) -> Ptr
Unwraps this Pin<Ptr>
, returning the underlying pointer.
Doing this operation safely requires that the data pointed at by this pinning pointer
implements Unpin
so that we can ignore the pinning invariants when unwrapping it.
§Examples
use std::pin::Pin;
let mut val: u8 = 5;
let pinned: Pin<&mut u8> = Pin::new(&mut val);
// Unwrap the pin to get the underlying mutable reference to the value. We can do
// this because `val` doesn't care about being moved, so the `Pin` was just
// a "facade" anyway.
let r = Pin::into_inner(pinned);
assert_eq!(*r, 5);
source§impl<Ptr> Pin<Ptr>where
Ptr: Deref,
impl<Ptr> Pin<Ptr>where
Ptr: Deref,
1.33.0 (const: unstable) · sourcepub unsafe fn new_unchecked(pointer: Ptr) -> Pin<Ptr>
pub unsafe fn new_unchecked(pointer: Ptr) -> Pin<Ptr>
Construct a new Pin<Ptr>
around a reference to some data of a type that
may or may not implement Unpin
.
If pointer
dereferences to an Unpin
type, Pin::new
should be used
instead.
§Safety
This constructor is unsafe because we cannot guarantee that the data
pointed to by pointer
is pinned. At its core, pinning a value means making the
guarantee that the value’s data will not be moved nor have its storage invalidated until
it gets dropped. For a more thorough explanation of pinning, see the pin
module docs.
If the caller that is constructing this Pin<Ptr>
does not ensure that the data Ptr
points to is pinned, that is a violation of the API contract and may lead to undefined
behavior in later (even safe) operations.
By using this method, you are also making a promise about the Deref
and
DerefMut
implementations of Ptr
, if they exist. Most importantly, they
must not move out of their self
arguments: Pin::as_mut
and Pin::as_ref
will call DerefMut::deref_mut
and Deref::deref
on the pointer type Ptr
and expect these methods to uphold the pinning invariants.
Moreover, by calling this method you promise that the reference Ptr
dereferences to will not be moved out of again; in particular, it
must not be possible to obtain a &mut Ptr::Target
and then
move out of that reference (using, for example mem::swap
).
For example, calling Pin::new_unchecked
on an &'a mut T
is unsafe because
while you are able to pin it for the given lifetime 'a
, you have no control
over whether it is kept pinned once 'a
ends, and therefore cannot uphold the
guarantee that a value, once pinned, remains pinned until it is dropped:
use std::mem;
use std::pin::Pin;
fn move_pinned_ref<T>(mut a: T, mut b: T) {
unsafe {
let p: Pin<&mut T> = Pin::new_unchecked(&mut a);
// This should mean the pointee `a` can never move again.
}
mem::swap(&mut a, &mut b); // Potential UB down the road ⚠️
// The address of `a` changed to `b`'s stack slot, so `a` got moved even
// though we have previously pinned it! We have violated the pinning API contract.
}
A value, once pinned, must remain pinned until it is dropped (unless its type implements
Unpin
). Because Pin<&mut T>
does not own the value, dropping the Pin
will not drop
the value and will not end the pinning contract. So moving the value after dropping the
Pin<&mut T>
is still a violation of the API contract.
Similarly, calling Pin::new_unchecked
on an Rc<T>
is unsafe because there could be
aliases to the same data that are not subject to the pinning restrictions:
use std::rc::Rc;
use std::pin::Pin;
fn move_pinned_rc<T>(mut x: Rc<T>) {
// This should mean the pointee can never move again.
let pin = unsafe { Pin::new_unchecked(Rc::clone(&x)) };
{
let p: Pin<&T> = pin.as_ref();
// ...
}
drop(pin);
let content = Rc::get_mut(&mut x).unwrap(); // Potential UB down the road ⚠️
// Now, if `x` was the only reference, we have a mutable reference to
// data that we pinned above, which we could use to move it as we have
// seen in the previous example. We have violated the pinning API contract.
}
§Pinning of closure captures
Particular care is required when using Pin::new_unchecked
in a closure:
Pin::new_unchecked(&mut var)
where var
is a by-value (moved) closure capture
implicitly makes the promise that the closure itself is pinned, and that all uses
of this closure capture respect that pinning.
use std::pin::Pin;
use std::task::Context;
use std::future::Future;
fn move_pinned_closure(mut x: impl Future, cx: &mut Context<'_>) {
// Create a closure that moves `x`, and then internally uses it in a pinned way.
let mut closure = move || unsafe {
let _ignore = Pin::new_unchecked(&mut x).poll(cx);
};
// Call the closure, so the future can assume it has been pinned.
closure();
// Move the closure somewhere else. This also moves `x`!
let mut moved = closure;
// Calling it again means we polled the future from two different locations,
// violating the pinning API contract.
moved(); // Potential UB ⚠️
}
When passing a closure to another API, it might be moving the closure any time, so
Pin::new_unchecked
on closure captures may only be used if the API explicitly documents
that the closure is pinned.
The better alternative is to avoid all that trouble and do the pinning in the outer function
instead (here using the pin!
macro):
use std::pin::pin;
use std::task::Context;
use std::future::Future;
fn move_pinned_closure(mut x: impl Future, cx: &mut Context<'_>) {
let mut x = pin!(x);
// Create a closure that captures `x: Pin<&mut _>`, which is safe to move.
let mut closure = move || {
let _ignore = x.as_mut().poll(cx);
};
// Call the closure, so the future can assume it has been pinned.
closure();
// Move the closure somewhere else.
let mut moved = closure;
// Calling it again here is fine (except that we might be polling a future that already
// returned `Poll::Ready`, but that is a separate problem).
moved();
}
1.33.0 · sourcepub fn as_ref(&self) -> Pin<&<Ptr as Deref>::Target>
pub fn as_ref(&self) -> Pin<&<Ptr as Deref>::Target>
Gets a shared reference to the pinned value this Pin
points to.
This is a generic method to go from &Pin<Pointer<T>>
to Pin<&T>
.
It is safe because, as part of the contract of Pin::new_unchecked
,
the pointee cannot move after Pin<Pointer<T>>
got created.
“Malicious” implementations of Pointer::Deref
are likewise
ruled out by the contract of Pin::new_unchecked
.
1.39.0 (const: unstable) · sourcepub unsafe fn into_inner_unchecked(pin: Pin<Ptr>) -> Ptr
pub unsafe fn into_inner_unchecked(pin: Pin<Ptr>) -> Ptr
Unwraps this Pin<Ptr>
, returning the underlying Ptr
.
§Safety
This function is unsafe. You must guarantee that you will continue to
treat the pointer Ptr
as pinned after you call this function, so that
the invariants on the Pin
type can be upheld. If the code using the
resulting Ptr
does not continue to maintain the pinning invariants that
is a violation of the API contract and may lead to undefined behavior in
later (safe) operations.
Note that you must be able to guarantee that the data pointed to by Ptr
will be treated as pinned all the way until its drop
handler is complete!
For more information, see the pin
module docs
If the underlying data is Unpin
, Pin::into_inner
should be used
instead.
source§impl<Ptr> Pin<Ptr>where
Ptr: DerefMut,
impl<Ptr> Pin<Ptr>where
Ptr: DerefMut,
1.33.0 · sourcepub fn as_mut(&mut self) -> Pin<&mut <Ptr as Deref>::Target>
pub fn as_mut(&mut self) -> Pin<&mut <Ptr as Deref>::Target>
Gets a mutable reference to the pinned value this Pin<Ptr>
points to.
This is a generic method to go from &mut Pin<Pointer<T>>
to Pin<&mut T>
.
It is safe because, as part of the contract of Pin::new_unchecked
,
the pointee cannot move after Pin<Pointer<T>>
got created.
“Malicious” implementations of Pointer::DerefMut
are likewise
ruled out by the contract of Pin::new_unchecked
.
This method is useful when doing multiple calls to functions that consume the pinning pointer.
§Example
use std::pin::Pin;
impl Type {
fn method(self: Pin<&mut Self>) {
// do something
}
fn call_method_twice(mut self: Pin<&mut Self>) {
// `method` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
self.as_mut().method();
self.as_mut().method();
}
}
1.33.0 · sourcepub fn set(&mut self, value: <Ptr as Deref>::Target)
pub fn set(&mut self, value: <Ptr as Deref>::Target)
Assigns a new value to the memory location pointed to by the Pin<Ptr>
.
This overwrites pinned data, but that is okay: the original pinned value’s destructor gets
run before being overwritten and the new value is also a valid value of the same type, so
no pinning invariant is violated. See the pin
module documentation
for more information on how this upholds the pinning invariants.
§Example
use std::pin::Pin;
let mut val: u8 = 5;
let mut pinned: Pin<&mut u8> = Pin::new(&mut val);
println!("{}", pinned); // 5
pinned.set(10);
println!("{}", pinned); // 10
Trait Implementations
source§impl<P> AsyncBufRead for Pin<P>
impl<P> AsyncBufRead for Pin<P>
source§impl<P> AsyncIterator for Pin<P>
impl<P> AsyncIterator for Pin<P>
§type Item = <<P as Deref>::Target as AsyncIterator>::Item
type Item = <<P as Deref>::Target as AsyncIterator>::Item
async_iterator
)source§fn poll_next(
self: Pin<&mut Pin<P>>,
cx: &mut Context<'_>,
) -> Poll<Option<<Pin<P> as AsyncIterator>::Item>>
fn poll_next( self: Pin<&mut Pin<P>>, cx: &mut Context<'_>, ) -> Poll<Option<<Pin<P> as AsyncIterator>::Item>>
async_iterator
)None
if the async iterator is exhausted. Read moresource§impl<P> AsyncRead for Pin<P>
impl<P> AsyncRead for Pin<P>
source§impl<P> AsyncWrite for Pin<P>
impl<P> AsyncWrite for Pin<P>
source§fn poll_write(
self: Pin<&mut Pin<P>>,
cx: &mut Context<'_>,
buf: &[u8],
) -> Poll<Result<usize, Error>>
fn poll_write( self: Pin<&mut Pin<P>>, cx: &mut Context<'_>, buf: &[u8], ) -> Poll<Result<usize, Error>>
buf
into the object. Read moresource§fn poll_write_vectored(
self: Pin<&mut Pin<P>>,
cx: &mut Context<'_>,
bufs: &[IoSlice<'_>],
) -> Poll<Result<usize, Error>>
fn poll_write_vectored( self: Pin<&mut Pin<P>>, cx: &mut Context<'_>, bufs: &[IoSlice<'_>], ) -> Poll<Result<usize, Error>>
bufs
into the object using vectored
IO operations. Read moresource§impl<G, R, A> Coroutine<R> for Pin<Box<G, A>>
impl<G, R, A> Coroutine<R> for Pin<Box<G, A>>
§type Yield = <G as Coroutine<R>>::Yield
type Yield = <G as Coroutine<R>>::Yield
coroutine_trait
)1.33.0 · source§impl<T, A> From<Box<T, A>> for Pin<Box<T, A>>
impl<T, A> From<Box<T, A>> for Pin<Box<T, A>>
source§fn from(boxed: Box<T, A>) -> Pin<Box<T, A>>
fn from(boxed: Box<T, A>) -> Pin<Box<T, A>>
Converts a Box<T>
into a Pin<Box<T>>
. If T
does not implement Unpin
, then
*boxed
will be pinned in memory and unable to be moved.
This conversion does not allocate on the heap and happens in place.
This is also available via Box::into_pin
.
Constructing and pinning a Box
with <Pin<Box<T>>>::from(Box::new(x))
can also be written more concisely using Box::pin(x)
.
This From
implementation is useful if you already have a Box<T>
, or you are
constructing a (pinned) Box
in a different way than with Box::new
.
source§impl<P> FusedFuture for Pin<P>
impl<P> FusedFuture for Pin<P>
source§fn is_terminated(&self) -> bool
fn is_terminated(&self) -> bool
true
if the underlying future should no longer be polled.source§impl<P> FusedStream for Pin<P>
impl<P> FusedStream for Pin<P>
source§fn is_terminated(&self) -> bool
fn is_terminated(&self) -> bool
true
if the stream should no longer be polled.1.41.0 · source§impl<Ptr> Ord for Pin<Ptr>
impl<Ptr> Ord for Pin<Ptr>
1.41.0 · source§impl<Ptr, Q> PartialOrd<Pin<Q>> for Pin<Ptr>
impl<Ptr, Q> PartialOrd<Pin<Q>> for Pin<Ptr>
source§impl<P, Item> Sink<Item> for Pin<P>
impl<P, Item> Sink<Item> for Pin<P>
§type Error = <<P as Deref>::Target as Sink<Item>>::Error
type Error = <<P as Deref>::Target as Sink<Item>>::Error
source§fn poll_ready(
self: Pin<&mut Pin<P>>,
cx: &mut Context<'_>,
) -> Poll<Result<(), <Pin<P> as Sink<Item>>::Error>>
fn poll_ready( self: Pin<&mut Pin<P>>, cx: &mut Context<'_>, ) -> Poll<Result<(), <Pin<P> as Sink<Item>>::Error>>
Sink
to receive a value. Read moresource§fn start_send(
self: Pin<&mut Pin<P>>,
item: Item,
) -> Result<(), <Pin<P> as Sink<Item>>::Error>
fn start_send( self: Pin<&mut Pin<P>>, item: Item, ) -> Result<(), <Pin<P> as Sink<Item>>::Error>
poll_ready
which returned Poll::Ready(Ok(()))
. Read more