Function fmt 

pub fn fmt<F, S>(fun: F, out: Arc<Mutex<S>>) -> Box<Closure>
where
    F: Fn(&mut S, &Level, &str, &str) -> Result + Send + Sync + Copy + 'static,
    S: Write + Send + 'static,

Builds a capture callback around a compatible formatting function.

The output is locked for each event before the formatter is called. The returned callback panics if the lock is poisoned or the formatter returns an error.