Add feature to only changes that were already modified (git)

[vinipsmaker]

Hi, could you add a feature to not produce noisy git diffs? Thanks.

[edited for tone, by nrc]

[nrc]

If somebody wanted to work on this, then I'd be happy to take a PR. But it would be an interesting (big) project. It would mean adding knowledge of Git (and presumably Hg and other VCSs) to Rustfmt and would mean making the Rustfmt work on only sections of code (which it can't do today). It would also be imperfect because at a minimum we have to operate on a whole AST node, not literally just a chunk of text.

[vinipsmaker]

It would also be imperfect because at a minimum we have to operate on a whole AST node, not literally just a chunk of text.

Why would it be imperfect? I don't get. You still should work on the AST. Just don't change the tokens/text that match the "sacred" lines.

[nrc]

Why would it be imperfect?

Because Rustfmt might change the number of lines or the content of lines outside the diff. In the former case, this makes it hard (perhaps not impossible, but hard - consider that git diff is trying to solve the same problem (matching changes to lines) and often gets it quite wrong. It is easier for Rustfmt since it is only a single change, but still very hard). The second change means it might not be possible to do this. Consider the following:

foo_bar(arg1, arg2,
        arg3, arg4);  // Only this line is in the diff.

Rustfmt has to work on the node whole node, and makes:

foo_bar(
    arg1,
    arg2,
    arg3,
    arg4,
};

If we restrict to the changed lines we get:

foo_bar(arg1, arg2,
    arg3,

Which doesn't compile. One can create examples that do compile but have different behaviour. You might suppose we could track the tokens we actually change, but this is not done and would be a huge change (and since we can move tokens as well as change the whitespace around them, is much harder than it appears). Finally, even if we managed, some how, to achieve all this, then hopefully the changing indents in the above example shows how by formatting only some lines in a node, we might actually make formatting worse instead of better.

[garyyu]

I love rustfmt, it's great 👍 But indeed, rustfmt's code changing on untouched code of a git commit is an evil for code review (for this git commit), heavy headache on this.

Hope somebody can take this issue and let rustfmt have this feature!

[scampi]

For the git we could leverage https://crates.io/crates/git2. In addition, this feature could rely on the file_lines option to offload most of the work:

• get the diff from git
• turn the diff into the format understood by file_lines

[zroug]

I use a script that is very similar to this approach for myself. It's far from perfect, both because the script itself isn't perfect and because file_lines doesn't always work perfectly. But sometimes I find it very useful as a temporary workaround. I haven't added error handling or anything.

https://gist.github.com/zroug/28605f45a662b483fb4a7c3545627f66

[eddyb]

@zroug That works great, I just had to add "--edition=2018" to the args list.