Struct OnceLock

pub struct OnceLock<T> { /* private fields */ }

Wrapper for std::sync::OnceLock

The exact locking behaviour of std::sync::OnceLock is currently undefined, but may deadlock in the event of reentrant initialization attempts. This wrapper participates in cycle detection as normal and will therefore panic in the event of reentrancy.

Most of this primitive’s methods do not involve locking and as such are simply passed through to the inner implementation.

Examples

use tracing_mutex::stdsync::tracing::OnceLock;

static LOCK: OnceLock<i32> = OnceLock::new();
assert!(LOCK.get().is_none());

std::thread::spawn(|| {
   let value: &i32 = LOCK.get_or_init(|| 42);
   assert_eq!(value, &42);
}).join().unwrap();

let value: Option<&i32> = LOCK.get();
assert_eq!(value, Some(&42));

Implementations

impl<T> OnceLock<T>

pub const fn new() -> Self

Creates a new empty cell

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

Gets a reference to the underlying value.

This method does not attempt to lock and therefore does not participate in cycle detection.

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

Gets a mutable reference to the underlying value.

This method does not attempt to lock and therefore does not participate in cycle detection.

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

Sets the contents of this cell to the underlying value

As this method may block until initialization is complete, it participates in cycle detection.

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.

This method participates in cycle detection. Reentrancy is considered a cycle.

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

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

This method does not attempt to lock and therefore does not participate in cycle detection.

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

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

This method does not attempt to lock and therefore does not participate in cycle detection.

Trait Implementations

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

fn clone(&self) -> Self

Returns a copy of the value.

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

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

impl<T> Default for OnceLock<T>

fn default() -> Self

Returns the “default value” for a type.

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

fn from(value: T) -> Self

Converts to this type from the input type.

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

fn eq(&self, other: &Self) -> bool

Tests for self and other values to be equal, and is used by ==.

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

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

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