Type Alias Formatter 

pub type Formatter = Arc<dyn Fn(&str, &str) -> Option<String> + Send + Sync>;

Formats one value the way one spec asks for. Both arguments are opaque to core: the spec is whatever a format attribute or a format_* builtin wrote, and what it means is the formatter’s business. Returns None when the spec is not one it knows or the value is not what that spec expects, so callers can leave the text alone.

Aliased Type

pub struct Formatter { /* private fields */ }