Support dynamic value of argument completion

[shannmu]

Related issue #1232

Work in this PR

• Add CustomCompleter trait and ArgValueCompleter struct to provide users a way to add custom value of argument completion.
• Add test case to show the feat.

What may be left

Open question: does the completion function accept just the arg and current value or does it have access to ArgMatches for completing based on other data like in #1910

• Is there any way to access ArgMatches before the builder of the Command. We add custom value hint in builder now. If we really need this feat, we may need to do more things.

[shannmu]

Thanks for your review! Do you think it is necessary to prevent the spread of user code errors? I think it may be necessary, because the completion code is executed implicitly, and if the spread of errors is not prevented, it may cause confusion to users. Also, I think my current implementation is not good, maybe we can use other rust tools or crates to do this.

[epage]

I think it may be necessary, because the completion code is executed implicitly, and if the spread of errors is not prevented, it may cause confusion to user

If the user wants panic prevention, they can provide it. I don't think we need to be doing it for them

Also, I think my current implementation is not good, maybe we can use other rust tools or crates to do this.

Which parts are you concerned about? Do you have something in mind?

[shannmu]

Which parts are you concerned about? Do you have something in mind?

I mean the panic prevention part. I want to let the user code execute in a sandbox so as not to affect other possible completions.

[shannmu]

ArgExt has a restriction requiring the Debug trait, which means closures cannot be directly used (as closures cannot implement the Debug trait). Therefore, I added a wrapper called ClosureCompleter that allows users to customize the debug info. I'm not sure if this is the best approach, so if you have any other ideas, please let me know.