Constant AVG_PART_LENGTH 

const AVG_PART_LENGTH: usize = 8;

This is just a guess at the average length of a URL part, used for String::with_capacity calls in the FromIterator and Extend impls.

The value 8 was chosen for two main reasons:

• It seems like a good guess for the average part length.
• jemalloc’s size classes are all multiples of eight, which means that the amount of memory it allocates will often match the amount requested, avoiding wasted bytes.