Function std::local_data::get_mut
pub fn get_mut<T: 'static, U>(key: Key<T>, f: |Option<&mut T>| -> U) -> U
Retrieves a mutable value from TLS. The closure provided is yielded Some
of a reference to the mutable value located in TLS if one exists, or None
if the key provided is not present in TLS currently.
It is considered a runtime error to attempt to get a value which is already
on loan via this or the get methods.