Struct OnceSlot 

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

Zero-initialized slot for one-time initialization of a T.

The slot itself starts zeroed (state UNINIT), which is what makes it usable with generic_static. The T value is written exactly once by the thread that wins the claim race; other threads block until INIT (or POISONED) becomes visible.

With the std feature, waiters block on a Mutex/Condvar pair (no spinning); without it they spin with core::hint::spin_loop.

Panics

get_or_init must not be called reentrantly with the same slot (the initializer calling back into the same slot). That deadlocks: the thread already holds the BUSY claim, so the inner call blocks forever waiting for itself.

If the initializer panics, the slot is poisoned.

Implementations

impl<T> OnceSlot<T>

pub const fn new() -> Self

Create a new uninitialized slot.

pub fn get(&'static self) -> Option<&'static T>

Returns the value if already initialized, otherwise None.

pub fn is_completed(&'static self) -> bool

Returns true if initialization completed successfully.

pub fn is_poisoned(&'static self) -> bool

Returns true if the initializer panicked and poisoned the slot.

pub fn get_or_init(&'static self, init: impl FnOnce() -> T) -> &'static T

Returns the value, running init exactly once to produce it.

Concurrent callers block (on a condvar with std, by spinning without it) until the winner finishes. If init panics, the slot is poisoned and every caller — past, present, and future — panics.

Panics

Panics if the slot is poisoned (including when a concurrent initializer panics while this call is blocked). Must not be called reentrantly with the same slot.

Trait Implementations

impl<T> Default for OnceSlot<T>

fn default() -> Self

Returns the “default value” for a type.

impl<T: Send> Send for OnceSlot<T>

impl<T: Send + Sync> Sync for OnceSlot<T>

impl<T> Zeroable for OnceSlot<T>

fn zeroed() -> Self

Calls zeroed.