Dynamic, on-demand autocomplete

[Xion]

In Python, there is a package called argcomplete which provides very flexible autocompletion for apps that use the standard argparse module. What it allows is to implement a custom completion provider: essentially a piece of your own code that's executed when the the binary is invoked in a Special Way (tm) by the shell-specific completion script.

For an example, see here. The code is preparing completions dynamically from the filesystem, or even from a remote API (if certain flag isn't passed (flags are partially parsed at this point)).

Having something like this in clap would be very nice. I know this is a potentially complex subsystem so it'd be unreasonable to expect it implemented anytime, but I wanted to at least put this feature on the radar.

[kbknapp]

Thanks for taking the time to suggest this! It's been mentioned before in #568 but hasn't been implemented yet. There's still some design details that need to be figured out, but it's something I do want to add. I'm going to close this issue but copy your comments over to 568.