Macro thread_local_cache 

macro_rules! thread_local_cache {
    (static $name:ident : $ty:ty) => { ... };
}

Declare a thread-local slot for use with Cached.

thread_local_cache!(static SLOT: MyType);

Expands to a thread_local! declaration wrapping RefCell<(bool, Option<MyType>)> where:

• (false, None) means uninitialized
• (false, Some(_)) means available
• (true, None) means held