Constant MAX_UNSIGNED_DURATION_LEN 

const MAX_UNSIGNED_DURATION_LEN: usize = 190;

Defines the maximum possible length (in bytes) of a std::time::Duration printed in the friendly format.

See MAX_SPAN_LEN for how this was computed. In this case, std::time::Duration::MAX - std::time::Duration::from_secs(16) was used to find the maximal length. This was a little tricky because the maximal unsigned duration has minute units equivalent to 0, and is thus quite a bit shorter than the maximum possible length.

Note that this is shorter than MAX_SIGNED_DURATION because one cannot get a negative formatted duration from a std::time::Duration. So there’s no way to get an ago suffix (or a - prefix).