Scoped formatting

[jamesmunns]

Hey all,

It would be nice to have an option that would allow me to only format a subset of a project. Typically this comes into play when making changes to a larger project which may either: 1. never have had formatting applied, or 2. was formatted using a different version of rustfmt with different rules, or 3. has components that do not format deterministically.

In these cases, a small change + a format can end up touching nearly every file (or large parts of a single file) in the project, which is not great for maintainers, as it makes it difficult to review changes.

Ideally, I would like to do something like this:

# check out the `master` branch of a project
git checkout <SOME PROJECT>

# make some changes
nano src/main.rs

# This part could be optional (if diffing against the working directory)
git checkout -b some-patch
git add .
git commit -m "This is a helpful patch!"

# Now the magic
cargo fmt --only-changes-since-git-revision="master"

This would apply formatting changes ONLY to some subset of the code, probably one of the following:

• Only the changed lines
• Only the nearest block containing changed lines (e.g. function, closure, struct definition)

I am not familiar with the inner workings of rustfmt, and I saw the --file-lines option, however I couldn't find any docs regarding that, and I wasn't sure if it was possible (within rustfmt) to figure out how far a block stretches when a line number is specified.

If this sounds feasible, I would be interested in trying to work on this, if there is a mentor I can ask some spinning-up questions to.

Let me know what you think!

[jamesmunns]

Whoops, after searching a little more, I believe this is a duplicate of #1324.

Please feel free to close this issue, and let me know if you would like me to add the body of the issue as a comment elsewhere.

[jamesmunns]

I'd be especially interested in hearing if anyone has a good way to convert "lines touched" to "AST nodes touched", maybe back to "lines to format".

[topecongiro]

Closing in favor of #1324.