ZSH completion of `nix shell <flake-pkg-ref> --command <cmd>` should suggest `_files`

[doronbehar]

On ZSH, I noticed that querying tab completions for the following kind of commands:

nix shell <flake-pkg-ref> --command <cmd>

Gives no suggestions. IMO since we don't dynamically load the completion for <cmd>, the least we can do is to complete _files in this case - as it will most likely work for most commands. At the moment, nothing is being completed in this case, and if you want to give a file as an argument, it is pretty annoying to not have even a file completion.

I managed to trace how Nix' ZSH completions are generated, and found the completion.zsh script, and the NIX_GET_COMPLETIONS support done by all Nix commands. Indeed (example):

NIX_GET_COMPLETIONS=4 nix shell nixpkgs-current#texliveFull --command xelatex

Prints:

normal

And that's it - no file names are printed there.

I managed to workaround the issue with the following diff to the file:

22c22,27
<   compadd -J nix "${args[@]}" -a suggestions
---
>   if [[ "$tpe" == normal ]] && (( ${#suggestions} == 0 )); then
>     # In case there are no suggestions by Nix,
>     _files
>   else
>     compadd -J nix "${args[@]}" -a suggestions
>   fi

But I wonder. Is it the correct way to go? Maybe the CPP code that handles NIX_GET_COMPLETIONS should be changed? If so, should it be changed for many more cases? My solution is pretty generic, but perhaps too absolute?

I think that in this specific case of completing external commands, it should be possible to detect a --command is being used and to edit the ZSH completion script to fully complete xelatex - as _xelatex is a completion function already distributed by ZSH:

$ whence -v _tex
_tex is a shell function from /run/current-system/sw/share/zsh/5.9/functions/_tex
$ head -1 $(whence -v _tex | awk '{print $NF}')
#compdef tex latex slitex pdftex pdflatex jadetex pdfjadetex xetex=tex xelatex=latex latexmk

CCing people who were involved in the ZSH completion:

• @bryango
• @matthewbauer
• @Ma27
• @thufschmitt