add a check or diff mode to `bazel mod tidy`

[voxeljorge]

Description of the feature request:

The bazel mod tidy tool is very useful for keeping MODULE.bazel up to date, but it does require developers to be disciplined enough to remember to run this or some kind of hook to make sure changes get checked in. A common pattern for ensuring files like this are up to date is to use a check or diff mode which prints changes it would make and exits with an error status code. Gazelle for example has gazelle -mode diff which makes no changes on disk and prints them as a diff instead.

A similar feature for bazel mod tidy would be very useful (especially since one common use is to run it right after gazelle).

Which category does this issue belong to?

bzlmod

What underlying problem are you trying to solve with this feature?

We would like to add a CI check that fails if MODULE.bazel has not been updated with bazel mod tidy correctly.

Which operating system are you running Bazel on?

Linux

What is the output of bazel info release?

release 7.4.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

I've tried to look through bazel issues and the docs for bazel mod tidy and don't see anything like this.

Any other information, logs, or outputs that you want to share?

No response

[aaomidi]

Buildozer does have the -stdout flag, which can be used to create a --validate option.

[meteorcloudy]

/cc @fmeum

[fmeum]

This seems useful and shouldn't be too hard based on what @aaomidi wrote above. This could be a good candidate for an external contribution, happy to help anyone who wants to pick this up.