make failure modes better when struct names are wrong about docopt.rs

[BurntSushi]

This is actually working as expected. The problem here is that the failure mode is terrible and that should be fixed (I've changed the title of your issue to reflect that).

The problem is how Docopt things are mapped to struct fields. It's documented, but it's buried in the README:

-g            => flag_g
--group       => flag_group
--group <arg> => flag_group
FILE          => arg_FILE
<file>        => arg_file
build         => cmd_build

So in your case, you'd want flag_port instead of arg_port. Indeed, swapping these makes your example work just fine (after swapping the rest too). The arg_ prefix is reserved for positional arguments.

Finally, if you change your usage string to use alternations, then you can get your originally desired behavior of catching the case where neither -4 or -6 are specified:

Usage: rtracker ((-4 [--port=<port> --ip=<ip>]) | (-6 [--port6=<port6> --ip6=<ip6>]))
       rtracker (--help)

Although, it would be understandable to continue with what you're doing and handle this case explicitly, because the error message for running your command without a -4 or a -6 is simply "Invalid arguments. Here's the usage again:" which isn't terribly helpful. (Basically, Docopt is saying that it can't find a match, but it doesn't know how to say why it can't find a match.)

[justinnoah]

@BurntSushi I read the readme many times, I can't believe I missed that. Thank you for pointing out that I needed flag_ rather than arg_. Your suggested usage string, however, does not allow for both -4 and -6 to be used simultaneously. 0.6.38 isn't building currently, so I have not tested the struct parameter prefix changes, but I believe that will be the change I need.