Closing comment:

We currently provide ValueHint for leveraging shell logic, though it isn't customizable. For that, we have #1232.

I'm inclined to close this in favor of wrapping up this request with #1232. I'm hesitant for us to have two different solutions (a Rust driven and shell-specific snippets) and doing shell snippets would couple args to completion so they would know what shell they are completing for.

We can always expand how we are using ValueHint for the common cases and as a short term until we have full customization.

For some argument values, the bash-completions may want to include additional logic for what type of value to complete, rather than allowing arbitrary strings. For instance, an option might accept a path to a file that exists; bash has a mechanism for that. Or, an option might accept the name of a git ref that exists; that's something more easily implemented in the program in Rust. Either way, it makes sense to augment clap's completion logic.

This also argues for implementing the completions by calling the program at runtime, rather than via shell; that way, arbitrary logic in Rust can determine what to complete, rather than providing a template system to feed in shell code (ick).