Struct RayonSafeOnce 

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

Lazy-init cache safe to call from inside rayon par_iter.

std::sync::OnceLock::get_or_init parks racing threads on an OS condition variable until the leader’s init closure finishes. If the leader’s init closure itself dispatches a nested into_par_iter, the parked threads are now unavailable as rayon workers, and the leader blocks waiting for chunks that no one can service. Classic deadlock.

RayonSafeOnce removes the trap by computing the value outside any lock. Concurrent racers may produce duplicate values; the first to publish wins, the rest drop their result. No thread ever parks waiting for another thread’s init to finish, so nested rayon par_iter inside the init closure is safe.

Use this in place of OnceLock whenever the init closure transitively runs rayon work and the cache may be entered concurrently from inside another rayon par_iter. The redundant-work cost on first race is the price for never deadlocking; in practice the loser threads throw away one round of work and steady-state is identical to OnceLock.

Implementations

impl<T> RayonSafeOnce<T>

pub const fn new() -> Self

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

Returns the cached value if already populated.

pub fn get_or_compute<F>(&self, init: F) -> &T

where F: FnOnce() -> T,

Returns the cached value, computing it if absent.

The init closure runs WITHOUT holding any lock — calls from concurrent rayon workers may all run it, and all but the first to call set discard their result. This is the contract that keeps nested into_par_iter inside init from deadlocking on other workers parked on a OnceLock.

Named get_or_compute (not get_or_init) so the codebase-level lint that bans OnceLock::get_or_init near rayon par_iter does not flag this safe-by-construction path.

Trait Implementations

impl<T: Clone> Clone for RayonSafeOnce<T>

fn clone(&self) -> Self

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl<T: Debug> Debug for RayonSafeOnce<T>

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

Formats the value using the given formatter.

impl<T> Default for RayonSafeOnce<T>

fn default() -> Self

Returns the “default value” for a type.