completion-hilit-commonality is a compiled Lisp function in `minibuffer.el'.

(completion-hilit-commonality COMPLETIONS PREFIX-LEN &optional BASE-SIZE)

Apply font-lock highlighting to a list of completions, COMPLETIONS.
PREFIX-LEN is an integer. BASE-SIZE is an integer or nil (meaning zero).

This adds the face `completions-common-part' to the first
(PREFIX-LEN - BASE-SIZE) characters of each completion, and the face
`completions-first-difference' to the first character after that.

It returns a list with font-lock properties applied to each element,
and with BASE-SIZE appended as the last element.