It's easier to read if you split it into tokens and qualifiers. The & is a reference. The '_ is a lifetime parameter. The things in the <>are types. So this is

It's easier to read if you split it into tokens and qualifiers. The & is a reference. The '_ is a lifetime parameter. The things in the <>are types. So this is a function which doesn't mutate self and takes a mutable reference to a formatter with a sensible lifetime and returns a result which will contain either nothing (or a unit) or an error if things fail. Thank you for posting this. I had to google and read up some things.