Module once 

Asynchronous primitives for one-time coordination.

Choosing a primitive

• Once runs a side-effecting initializer successfully once without storing a value. A cancelled or panicked attempt leaves it available for another caller to retry.
• OnceCell stores one value from an initializer supplied at access time. Failed, cancelled, or panicked attempts leave the cell empty and retryable.
• LazyCell owns one initializer and preserves the same in-flight future across caller cancellation. A panic poisons the cell instead of starting another attempt.
• OnceMap applies retryable OnceCell initialization per key and retains successful values until they are explicitly removed.

These primitives retain completed state. Use singleflight::Group when only overlapping calls for a key should share work and later calls should execute again.

Structs

OnceCell

A thread-safe cell whose value is asynchronously initialized at most once.