Teach dynamic fish completions to expand variables

[krobelus]

In fish, the command line

cargo b --manifest-path $PWD/clap_complete/Cargo.toml --example

wrongly gets example-name completions from $PWD/Cargo.toml instead of
$PWD/clap_complete/Cargo.toml.

This is because fish's "commandline --tokenize" option produces tokens
like "$PWD/clap_complete/Cargo.toml", which clap cannot see through.

Starting in the upcoming fish version 4, there is a new option,
"--tokens-expanded"¹ to output the expanded arguments, matching
what the program would see during execution.

If an expansion of a token like {1,2,3}{1,2,3} would produce too
many results, the unexpanded token is forwarded, thus falling back to
existing behavior. Similarly, command substitutions ("$()") present
on the command line are forwarded as-is, because they are not deemed
safe to expand given that the user hasn't agreed to running them.

Use this option if available, to fix the above example.

I have not checked if any existing clap user tries to expand variables
on their own. If this is a real possibility, we could let users
opt into the new behavior.

While at it, break up this overlong line. Add redundant semicolons,
in case a preexisting completion accidentally joins these lines with
spaces², like

if set -l completions (COMPLETE=fish myapp 2>/dev/null)
	echo "$completions" | source
end

I have not updated clap_complete/tests/snapshots/register_dynamic.fish
(not sure how?) or tested this but I'm happy to do so.

Footnotes

1. https://github.com/fish-shell/fish-shell/pull/10212 ↩

2. https://github.com/fish-shell/fish-shell/pull/11046#discussion_r1917875912 ↩

[epage]

Could you split this into two commits

1. Reformats the output
2. Adds the new flag

[epage]

Could you add a test for this?

[epage]

In fish, the command line

cargo b --manifest-path $PWD/clap_complete/Cargo.toml --example

wrongly gets example-name completions from $PWD/Cargo.toml instead of $PWD/clap_complete/Cargo.toml.

A couple of cautions

• Fish has its own cargo completions, independent of Cargo's
• Even if you explicitly override Fish's completions, we don't currently offer support for context-sensitive completions (see Allow access to previous args in completion function for context-sensitive completions #5784, Don't show flags in native completion that are already present #5515 is also related)

If an expansion of a token like {1,2,3}{1,2,3} would produce too
many results, the unexpanded token is forwarded, thus falling back to
existing behavior.

I've been wondering about allowing completions for cases like file-name-{one,two}.[TAB] Would this change usually get in the way? The main way I see it getting in the way is dependent on what it does if no matches are present for it to expand the tokens.