Updates the derive example handling environment variables to illustrate
the case where it contains a sensitive value which should not be
displayed on the help screen.
Closes https://github.com/clap-rs/clap/issues/2101
This PR switches the Arg::Short macro to take a character instead of a string. It removes the hacky code in the Method to_token method and implements the logic for Short when parsing the clap derive arguments.
Fixes#1815.
