Proposal: add the `--diff` option to `zig fmt`

[perillo]

I propose to add the --diff option to the zig fmt command. When it is set, it will display diffs instead of rewriting files.

The --diff and --check options can be used together.

Rationale

Several modern code formatting tools have an option to display diffs instead of rewriting files.

I discovered this option when using ruff (a formatter for Python code written in Rust) and I find it is very convenient.

Examples:

Go

gofmt has the options:

  -d	display diffs instead of rewriting files
  -l	list files whose formatting differs from gofmt's

Python

black has the options:

  --check                         Don't write the files back, just return the
                                  status. Return code 0 means nothing would
                                  change. Return code 1 means some files would
                                  be reformatted. Return code 123 means there
                                  was an internal error.
  --diff                          Don't write the files back, just output a
                                  diff for each file on stdout.

Javascript

prettier has the options

  -c, --check              Check if the given files are formatted, print a human-friendly summary
                           message and paths to unformatted files (see also --list-different).
  -l, --list-different     Print the names of files that are different from Prettier's formatting (see also --check).
  -w, --write              Edit files in-place. (Beware!)

Note: prettier does not support setting -c and -l together.

Thanks.

[perillo]

This is the implementation in Go (I linked the Go implementation because the code is probably easier to read and the algorithm also guarantees to run in O(n log n) time instead of the standard O(n²) time.): https://github.com/golang/go/blob/master/src/internal/diff/diff.go

[FnControlOption]

I wrote a Rust port of diff.go that may or may not be easier to understand: https://gist.github.com/FnControlOption/4d36dd2043ec86bc877beff114821def

[Arnau478]

This would be really easy to implement if #18360 was accepted

[perillo]

This would be really easy to implement if #18360 was accepted

Thanks for the link.

I avoided creating a PR (by converting the Go code to Zig) since one of the issues was where to place the diff implementation.