Type Alias Once 

pub type Once = OnceCell<()>;

A synchronization primitive which can be used to run a one-time global initialization.

This type uses spin-locks if the initialization is contended and is thus #[no_std] compatible.

For the API of this type alias, see the generic OnceCell type. This is a specialization with T = ().

Aliased Type

pub struct Once { /* private fields */ }