rustup: add cargo completions; expand rustup completion shell support

[pantheraleo-7]

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• add rustup completion support for powershell and elvish
• add cargo completions to (upstream supported shells, namely,) bash and zsh

[pantheraleo-7]

I tried using generate_completions_from_executable DSL but there's just no way (at least to my knowledge) to insert the shell argument in the middle of the command (which rustup requires, there's no workaround). Hence the brew audit --strict rustup failure.

rustup
  * line 41, col 5: Use `generate_completions_from_executable` DSL instead of `(bash_completion/"cargo").write Utils.safe_popen_read(bin/"rustup", "completions", "bash", "cargo")`.
  * line 42, col 5: Use `generate_completions_from_executable` DSL instead of `(zsh_completion/"_cargo").write Utils.safe_popen_read(bin/"rustup", "completions", "zsh", "cargo")`.
Error: 2 problems in 1 formula detected.

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[daeho-ro]

hm... it seems that there cannot be two generate_completions_from_executable in a formula for now.

[pantheraleo-7]

Yeah... I tried [almost] everything I can think of. I hoped the DSL set $SHELL but it doesn't and the $ gets escaped anyway in the final command. #13536(comment) should've been implemented IMO. And Elvish support is missing too. All in all, the DSL needs updating.

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If I'm not mistaken, not using the DSL is a style error, right? If yes then, should it really be a blocker...?

[github-actions[bot]]

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