What are the important factors to consider when deciding whether to
use symbols or strings for hash keys?

I ask b/c I noticed that fileutils.rb uses strings[1], though the keys
represent method names. Seems to me that symbols would be more
appropriate.

Even so it would be good to know the general criteria to consider.

[1]https://github.com/ruby/ruby/blob/trunk/lib/fileutils.rb
(OPT_TABLE)