Struct rustc_data_structures::sync::OnceLock

1.70.0 · source ·
pub struct OnceLock<T> {
    once: Once,
    value: UnsafeCell<MaybeUninit<T>>,
    _marker: PhantomData<T>,
}
Expand description

A synchronization primitive which can nominally be written to only once.

This type is a thread-safe OnceCell, and can be used in statics. In many simple cases, you can use LazyLock<T, F> instead to get the benefits of this type with less effort: LazyLock<T, F> “looks like” &T because it initializes with F on deref! Where OnceLock shines is when LazyLock is too simple to support a given case, as LazyLock doesn’t allow additional inputs to its function after you call LazyLock::new(|| ...).

§Examples

Writing to a OnceLock from a separate thread:

use std::sync::OnceLock;

static CELL: OnceLock<usize> = OnceLock::new();

// `OnceLock` has not been written to yet.
assert!(CELL.get().is_none());

// Spawn a thread and write to `OnceLock`.
std::thread::spawn(|| {
    let value = CELL.get_or_init(|| 12345);
    assert_eq!(value, &12345);
})
.join()
.unwrap();

// `OnceLock` now contains the value.
assert_eq!(
    CELL.get(),
    Some(&12345),
);

You can use OnceLock to implement a type that requires “append-only” logic:

use std::sync::{OnceLock, atomic::{AtomicU32, Ordering}};
use std::thread;

struct OnceList<T> {
    data: OnceLock<T>,
    next: OnceLock<Box<OnceList<T>>>,
}
impl<T> OnceList<T> {
    const fn new() -> OnceList<T> {
        OnceList { data: OnceLock::new(), next: OnceLock::new() }
    }
    fn push(&self, value: T) {
        // FIXME: this impl is concise, but is also slow for long lists or many threads.
        // as an exercise, consider how you might improve on it while preserving the behavior
        if let Err(value) = self.data.set(value) {
            let next = self.next.get_or_init(|| Box::new(OnceList::new()));
            next.push(value)
        };
    }
    fn contains(&self, example: &T) -> bool
    where
        T: PartialEq,
    {
        self.data.get().map(|item| item == example).filter(|v| *v).unwrap_or_else(|| {
            self.next.get().map(|next| next.contains(example)).unwrap_or(false)
        })
    }
}

// Let's exercise this new Sync append-only list by doing a little counting
static LIST: OnceList<u32> = OnceList::new();
static COUNTER: AtomicU32 = AtomicU32::new(0);

let vec = (0..thread::available_parallelism().unwrap().get()).map(|_| thread::spawn(|| {
    while let i @ 0..=1000 = COUNTER.fetch_add(1, Ordering::Relaxed) {
        LIST.push(i);
    }
})).collect::<Vec<thread::JoinHandle<_>>>();
vec.into_iter().for_each(|handle| handle.join().unwrap());

for i in 0..=1000 {
    assert!(LIST.contains(&i));
}

Fields§

§once: Once§value: UnsafeCell<MaybeUninit<T>>§_marker: PhantomData<T>

Implementations§

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impl<T> OnceLock<T>

1.70.0 (const: 1.70.0) · source

pub const fn new() -> OnceLock<T>

Creates a new empty cell.

1.70.0 · source

pub fn get(&self) -> Option<&T>

Gets the reference to the underlying value.

Returns None if the cell is empty, or being initialized. This method never blocks.

1.70.0 · source

pub fn get_mut(&mut self) -> Option<&mut T>

Gets the mutable reference to the underlying value.

Returns None if the cell is empty. This method never blocks.

1.70.0 · source

pub fn set(&self, value: T) -> Result<(), T>

Sets the contents of this cell to value.

May block if another thread is currently attempting to initialize the cell. The cell is guaranteed to contain a value when set returns, though not necessarily the one provided.

Returns Ok(()) if the cell’s value was set by this call.

§Examples
use std::sync::OnceLock;

static CELL: OnceLock<i32> = OnceLock::new();

fn main() {
    assert!(CELL.get().is_none());

    std::thread::spawn(|| {
        assert_eq!(CELL.set(92), Ok(()));
    }).join().unwrap();

    assert_eq!(CELL.set(62), Err(62));
    assert_eq!(CELL.get(), Some(&92));
}
source

pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)>

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

Sets the contents of this cell to value if the cell was empty, then returns a reference to it.

May block if another thread is currently attempting to initialize the cell. The cell is guaranteed to contain a value when set returns, though not necessarily the one provided.

Returns Ok(&value) if the cell was empty and Err(&current_value, value) if it was full.

§Examples
#![feature(once_cell_try_insert)]

use std::sync::OnceLock;

static CELL: OnceLock<i32> = OnceLock::new();

fn main() {
    assert!(CELL.get().is_none());

    std::thread::spawn(|| {
        assert_eq!(CELL.try_insert(92), Ok(&92));
    }).join().unwrap();

    assert_eq!(CELL.try_insert(62), Err((&92, 62)));
    assert_eq!(CELL.get(), Some(&92));
}
1.70.0 · source

pub fn get_or_init<F>(&self, f: F) -> &T
where F: FnOnce() -> T,

Gets the contents of the cell, initializing it with f if the cell was empty.

Many threads may call get_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.

§Panics

If f panics, the panic is propagated to the caller, and the cell remains uninitialized.

It is an error to reentrantly initialize the cell from f. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

§Examples
use std::sync::OnceLock;

let cell = OnceLock::new();
let value = cell.get_or_init(|| 92);
assert_eq!(value, &92);
let value = cell.get_or_init(|| unreachable!());
assert_eq!(value, &92);
source

pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut T
where F: FnOnce() -> T,

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

Gets the mutable reference of the contents of the cell, initializing it with f if the cell was empty.

Many threads may call get_mut_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.

§Panics

If f panics, the panic is propagated to the caller, and the cell remains uninitialized.

§Examples
#![feature(once_cell_get_mut)]

use std::sync::OnceLock;

let mut cell = OnceLock::new();
let value = cell.get_mut_or_init(|| 92);
assert_eq!(*value, 92);

*value += 2;
assert_eq!(*value, 94);

let value = cell.get_mut_or_init(|| unreachable!());
assert_eq!(*value, 94);
source

pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where F: FnOnce() -> Result<T, E>,

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

Gets the contents of the cell, initializing it with f if the cell was empty. If the cell was empty and f failed, an error is returned.

§Panics

If f panics, the panic is propagated to the caller, and the cell remains uninitialized.

It is an error to reentrantly initialize the cell from f. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

§Examples
#![feature(once_cell_try)]

use std::sync::OnceLock;

let cell = OnceLock::new();
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert!(cell.get().is_none());
let value = cell.get_or_try_init(|| -> Result<i32, ()> {
    Ok(92)
});
assert_eq!(value, Ok(&92));
assert_eq!(cell.get(), Some(&92))
source

pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
where F: FnOnce() -> Result<T, E>,

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

Gets the mutable reference of the contents of the cell, initializing it with f if the cell was empty. If the cell was empty and f failed, an error is returned.

§Panics

If f panics, the panic is propagated to the caller, and the cell remains uninitialized.

§Examples
#![feature(once_cell_get_mut)]

use std::sync::OnceLock;

let mut cell: OnceLock<u32> = OnceLock::new();

// Failed initializers do not change the value
assert!(cell.get_mut_or_try_init(|| "not a number!".parse()).is_err());
assert!(cell.get().is_none());

let value = cell.get_mut_or_try_init(|| "1234".parse());
assert_eq!(value, Ok(&mut 1234));
*value.unwrap() += 2;
assert_eq!(cell.get(), Some(&1236))
1.70.0 · source

pub fn into_inner(self) -> Option<T>

Consumes the OnceLock, returning the wrapped value. Returns None if the cell was empty.

§Examples
use std::sync::OnceLock;

let cell: OnceLock<String> = OnceLock::new();
assert_eq!(cell.into_inner(), None);

let cell = OnceLock::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));
1.70.0 · source

pub fn take(&mut self) -> Option<T>

Takes the value out of this OnceLock, moving it back to an uninitialized state.

Has no effect and returns None if the OnceLock hasn’t been initialized.

Safety is guaranteed by requiring a mutable reference.

§Examples
use std::sync::OnceLock;

let mut cell: OnceLock<String> = OnceLock::new();
assert_eq!(cell.take(), None);

let mut cell = OnceLock::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);

Trait Implementations§

1.70.0 · source§

impl<T> Clone for OnceLock<T>
where T: Clone,

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

Returns a copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from source. Read more
1.70.0 · source§

impl<T> Debug for OnceLock<T>
where T: Debug,

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

Formats the value using the given formatter. Read more
1.70.0 · source§

impl<T> Default for OnceLock<T>

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fn default() -> OnceLock<T>

Creates a new empty cell.

§Example
use std::sync::OnceLock;

fn main() {
    assert_eq!(OnceLock::<()>::new(), OnceLock::default());
}
1.70.0 · source§

impl<T> Drop for OnceLock<T>

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fn drop(&mut self)

Executes the destructor for this type. Read more
1.70.0 · source§

impl<T> From<T> for OnceLock<T>

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fn from(value: T) -> OnceLock<T>

Create a new cell with its contents set to value.

§Example
use std::sync::OnceLock;

let a = OnceLock::from(3);
let b = OnceLock::new();
b.set(3)?;
assert_eq!(a, b);
Ok(())
1.70.0 · source§

impl<T> PartialEq for OnceLock<T>
where T: PartialEq,

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

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

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl<T: DynSend + DynSync> DynSync for OnceLock<T>

1.70.0 · source§

impl<T> Eq for OnceLock<T>
where T: Eq,

1.70.0 · source§

impl<T> RefUnwindSafe for OnceLock<T>

1.70.0 · source§

impl<T> Send for OnceLock<T>
where T: Send,

1.70.0 · source§

impl<T> Sync for OnceLock<T>
where T: Sync + Send,

1.70.0 · source§

impl<T> UnwindSafe for OnceLock<T>
where T: UnwindSafe,

Auto Trait Implementations§

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impl<T> DynSend for OnceLock<T>
where T: DynSend,

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impl<T> !Freeze for OnceLock<T>

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impl<T> Unpin for OnceLock<T>
where T: Unpin,

Blanket Implementations§

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impl<T> Aligned for T

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const ALIGN: Alignment = const ALIGN: Alignment = Alignment::of::<Self>();

Alignment of Self.
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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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

Mutably borrows from an owned value. Read more
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impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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fn equivalent(&self, key: &K) -> bool

Compare self to key and return true if they are equal.
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impl<T> From<!> for T

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fn from(t: !) -> T

Converts to this type from the input type.
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T> Instrument for T

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fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
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fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current Span, returning an Instrumented wrapper. Read more
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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T> IntoEither for T

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fn into_either(self, into_left: bool) -> Either<Self, Self>

Converts self into a Left variant of Either<Self, Self> if into_left is true. Converts self into a Right variant of Either<Self, Self> otherwise. Read more
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fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
where F: FnOnce(&Self) -> bool,

Converts self into a Left variant of Either<Self, Self> if into_left(&self) returns true. Converts self into a Right variant of Either<Self, Self> otherwise. Read more
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impl<T> Pointable for T

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const ALIGN: usize = _

The alignment of pointer.
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type Init = T

The type for initializers.
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unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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impl<T> ToOwned for T
where T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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impl<T> WithSubscriber for T

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fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided Subscriber to this type, returning a WithDispatch wrapper. Read more
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fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more
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impl<'a, T> Captures<'a> for T
where T: ?Sized,

Layout§

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.