Macro singleton_token 

macro_rules! singleton_token {
    ($vis: vis $id: ident) => { ... };
    ($($vis: vis $id: ident),*) => { ... };
}

Produces tokens whose only identifier is their type, but is built such that only one instance of it can exist at any given time.

Looping on TokenTrait::new with a singleton token to access a TokenCell is equivalent to using a spin-lock.