Static variables have upper case names conventionally.

warning: static variable `make_upper_case` should have an upper case name
   |
   | static make_upper_case: fn(&str) -> String = str::to_uppercase;
   |        ^^^^^^^^^^^^^^^^^^ help: convert the identifier to upper case: `MAKE_UPPER_CASE`
   |
   = note: `#[warn(non_upper_case_globals)]` on by default