Auto-generated completion scripts add NuShell support

[hustcer]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

3.0.0-beta.4

Describe your use case

Auto-generated completion scripts add NuShell support

Describe the solution you'd like

Auto-generated completion scripts add NuShell support, just as clap support (Bash, Zsh, Fish, Elvish and PowerShell), Thx

Alternatives, if applicable

No response

Additional Context

nu --version
───┬────────────────────┬────────────────────────────────────────────────────────────────────────────────────
 # │      Column0       │                                      Column1
───┼────────────────────┼────────────────────────────────────────────────────────────────────────────────────
 0 │ version            │ 0.37.0
 1 │ build_os           │ macos-x86_64
 2 │ rust_version       │ rustc 1.55.0
 3 │ cargo_version      │ cargo 1.55.0
 4 │ pkg_version        │ 0.37.0
 5 │ build_time         │ 2021-09-14 19:45:30 +00:00
 6 │ build_rust_channel │ release
 7 │ features           │ clipboard-cli, ctrlc, dataframe, default, rustyline, term, trash, uuid, which, zip
 8 │ installed_plugins  │
───┴────────────────────┴────────────────────────────────────────────────────────────────────────────────────

[epage]

Somewhat unrelated but I wonder what the code size / compile time looks like for all of our generators and if there is anything we can help developers with development time as our generators expand in number of targets and capabilities.

[pksunkara]

That was the intention for extra generators. For the basic common ones, we have them in the package itself, but if we were to implement this, we would create a new package called clap_generator_nushell or something.

One thing I am not sure if we can do in Rust because of these new const stuff is, can a user extend the Shell enum through 3rd party code (traits)?

[epage]

A user could manually impl ArgEnum and flatten Shell into their enum or we could add flatten support but I'm not aware of a language feature that would help.

[epage]

If nushell works like bash where you have to write your own parser, #3166 could be a big help towards this.

[TornaxO7]

May I ask, what the state of this issue is?

[jokeyrhyme]

I'm a huge fan of nushell, but it seems to be in a state of flux (going off the last couple of releases): https://www.nushell.sh/blog/

How likely is it that there are significant changes in the syntax or other workings relating to completion over on the nushell side?

[epage]

Looks like someone created https://github.com/nibon7/clap_complete_nushell