[Bash completion]: Respects .file() extensions, uses _filedir

[gwynne]

This PR implements two TODOs left over in the Bash completion script generator code:

1. CompletionKind.file(extensions:) did not actually honor the extensions array. It now does so.
2. CompletionKind.file(extensions:) and .directory now both preferentially invoke the _filedir shell function from bash-completion (v1 for Bash 3.2 & v2 for Bash 4.2+), if it is available in the shell environment at the time, falling back on compgen if it is not.

Note: I could not find any open issues referring to these concerns.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rauhul]

@swift-ci please test

[rauhul]

I know I don't have enough experience with bash to say if these changes are the most correct implementation. the general shape looks good to me. If possible could you link to clap's or another language's arg parser script generation that does similar to compare?

[gwynne]

@rauhul I have to be honest; this is my first time tinkering with Bash completion 😅. Here are the references I used in coming up with this:

• The use of compgen -X !*.ext to handle extensions: _comp_compgen_filedir() guts on tip of trunk bash-completion 2.x - it seems that the maintainer has planned a truly massive overhaul for the "2.12" release.
• Making sure Bash 3.2 (for non-Homebrewed macOS bash users) works: The original _filedir from bash-completion 1.x.
• Also for extensions handling: A use example in the underlying completion generator used by yq
• Directory handling: The extra CDPATH handling built into bash-completion
• Use for files without extension matching: The builtin minimal fallback handler

[natecook1000]

I think we need an additional call to _filedir -d here – when I run completions on an extension-constrained .file item, it should show me the matching files plus the available directories, since I might need to navigate before finding the correct file.

[natecook1000]

bash-3.2$ math stats quantiles --file 
CHANGELOG.md        CMakeLists.txt      CODE_OF_CONDUCT.md  CONTRIBUTING.md     LICENSE.txt         README.md           
bash-3.2$ math stats quantiles --file Tes
<no output>
bash-3.2$ math stats quantiles --file Tests/
<auto-completes to:>
bash-3.2$ math stats quantiles --file Tests/CMakeLists.txt

Here, Tests and the other directories should be visible and autocomplete.

[gwynne]

Good catch, done and done! 👍

[natecook1000]

Thanks so much for this fix, @gwynne! One note about the files with extensions case, otherwise this looks great. 👏🏻👏🏻👏🏻👏🏻

[rauhul]

@swift-ci please test

[rauhul]

@gwynne it looks like some tests need to be updated:

             if declare -F _filedir >/dev/null; then
               _filedir 'txt'
               _filedir 'md'
               _filedir 'TXT'
               _filedir 'MD'
+              _filedir -d
             else

[gwynne]

@gwynne it looks like some tests need to be updated:

Whoops, sorry! Incoming momentarily 😅

[rauhul]

@swift-ci please test

[natecook1000]

Thanks again, @gwynne! 👏🏻👏🏻