Function std::local_data::modify
pub fn modify<T: 'static>(key: Key<T>, f: |Option<T>| -> Option<T>)
Modifies a task-local value by temporarily removing it from task-local
storage and then re-inserting if Some
is returned from the closure.
This function will have the same runtime errors as generated from pop
and
set
(the key must not currently be on loan