A rule that disallows variable names not in snake_case.

snake_case is when only lowercase letters are used, and words are separated with underscores, rather than spaces.

Good:

my_mood = :happy
[number_of_cats] = [3]
{function_name, _, other_stuff} = ast

Bad:

myMood = :sad
[numberOfCats] = [3]
{functionName, meta, otherStuff} = ast