external diff-editors: allow 3-pane diff, set up meld-3.

[ilyagr]

Implementation for 3-pane diff as discussed in #1905 (reply in thread).

To test it out, patch this PR and try

cargo run -- --config-toml 'ui.diff-editor="meld-3"' diffedit -r @-

If you'd like to try programs other than meld, just use a merge tool config that has $output in its edit-args.

I am also thinking about trying out a conflict resolution interface based on this.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (src/config-schema.json)
• I have added tests to cover my changes

[ilyagr]

Re the HELP WANTED commit: @yuja suggested I simply have a separate function to check if a variable is in the args. I'll probably do that at some point.

The biggest issue slowing down this PR is now the lack of tests.

[Dr-Emann]

Wild idea, we could do something like

enum InterpolationChunk<'a> {
    Literal(&'a str),
    Variable {
        name: &'a str,  // e.g. "output"
        original: &'a str, // e.g. "$output", maybe someday "${output}"
    }
}

fn interpolated_variables(s: &'_ str) -> impl Iterator<Item=InterpolationChunk<'_>> {}

(of course with better names. VariableStrChunk? TemplateChunk?)

Which would allow doing the replacements and querying if a variable is present efficiently.

[ilyagr]

@Dr-Emann That's a nice idea. If we start doing something more complicated with arguments, we should try it. For now, I'll just follow Yuya's simple suggestion.

[ilyagr]

This PR is a bit more polished now. I did not completely adapt JJ-INSTRUCTIONS text to the 3-pane diff, but I hope that the preambles I added will eliminate confusion.

I may add tests for JJ-INSTRUCTIONS to this in the future or in a separate PR.

I am not sure whether to remove the (currently unused) possibility to start out $output with the left side of the diff. I am considering using that for conflict resolution, if I ever get to that.

[martinvonz]

Thanks! I'm curious to hear feedback about this once people start using it. I think it sounds like a big improvement.

[ilyagr]

Yeah, I've been using this and I like it quite a bit!

I'm not super-happy about it from the technical perspective: I can't get this to work nicely with anything other than meld (vimdiff will likely work, but it takes some work), and this should really be a difftool's job. A script calling meld would have worked, but not on Windows. Overall, I think this is the best we can do right now.

[yuja]

Just fyi, kdiff3's behavior would be close to meld-3 since the diff panes are readonly.

[ilyagr]

Thanks, Yuya!

Just fyi, kdiff3's behavior would be close to meld-3 since the diff panes are readonly.

It is similar, even without any need for the trickery with meld. However, it's frustratingly disfunctional. Let me know if you know how to get it to work.

The issues are:

1. The output window starts with a merge conflict for every diff.
2. There seems to be a command designed to fix it: "Merge=> Select B everywhere". However, it doesn't work for me. Kdiff3 starts telling me weird things I don't understand about "losing unsaved merge". I think that command saves the file in addition to selecting B everywhere? In any case, I couldn't get it to work. Let me know if you can.

I was thinking of filing a feature request to execute "Select B everywhere" after starting a "diff merge" (in KDiff3 parlance), but issue number 2 made me throw my hands up in frustration.

[yuja]

it's frustratingly disfunctional.

Yep. I don't recommend kdiff3 over meld for diff editing. I just mean it's similar to meld ... -o $right.

1. The output window starts with a merge conflict for every diff.

2. There seems to be a command designed to fix it: "Merge=> Select B everywhere". However, it doesn't work for me. Kdiff3 starts telling me weird things I don't understand about "losing unsaved merge". I think that command saves the file in addition to selecting B everywhere? In any case, I couldn't get it to work. Let me know if you can.

I think you could do that in "Directory merge" pane (select A or B, then "Run Operation for Current Item), but it's not intuitive. I don't know a better option.