This mostly exist for
- Knowing of the value came from the command-line but we now have
`ArgMatches::source`
- Counting the number of flags but we now have `ArgAction::Count`
`-h` (short help) still shows the same.
This gates it behind an `unstable-v4` feature flag to avoid disrupting users who set the help without knowing where all it shows up (particularly derive users where `ArgEnum` is automatically extracting the help).
Fixes#3312
When an Arg uses .min_values(0), that arg's value(s) are effectively
optional. This is conventionaly denoted in help messages by wrapping the
arg's values in square brackets. For example:
--foo[=value]
--bar [value]
This kind of argument can be seen in the wild in many git commands; e.g.
git-status(1).
Signed-off-by: Peter Grayson <pete@jpgrayson.net>
This is prep for moving the derive tests. Besides organizing the test
folder for each API, this should reduce link time at the cost of
re-compiling more when a test changes.
