Function command_arg 

pub fn command_arg<'a>(cmd: &'a mut Command, arg: &str) -> &'a mut Command

Append a single positional arg to cmd, returning the same cmd for chaining.

Note: Command::arg is defined as &mut self -> &mut Command, which the named-arg lowering pass treats as a method shape. This free-fn form re-exposes it with explicit parameter names.