feat: Add diff_test_failure_message attribute to write_source_file*

[jameskuszmaul-brt]

This makes it so that developers can add a custom error message to the diff test failure messages when using write_source_file and write_source_files.

[jameskuszmaul-brt]

Note: I would be happy to get either more automated tests added or change the name of the attribute; however, I didn't see any preexisting tests for these failure messages and so I didn't want to go around refactoring/messing with things without knowing where to look, and if you all don't consider it necessary to add automated tests for this then I didn't want to spend the time figuring it out.

[kormide]

IMO appending to the end seems like a very specific use case. A more generally useful solution would be to replace the whole message while supporting templating for the target and suggested target to be inserted.

[jameskuszmaul-brt]

@kormide Do you think such templating should just go off of the target name or off of the whole pre-provided string?

Also, is there any standard pattern for what syntax you'd use for this sort of templating? I'd probably default to just implenting templating for the two target names and make it so that if you wanted to reproduce the existing error message you would do something like:

If you want to update all the files, do bazel run %all-targets%; if you want to update just this target, run %target%.

Suggestions on names & syntax welcome; I'm not inclined to implement any larger a feature set than described here.

[kormide]

@kormide Do you think such templating should just go off of the target name or off of the whole pre-provided string?

Also, is there any standard pattern for what syntax you'd use for this sort of templating? I'd probably default to just implenting templating for the two target names and make it so that if you wanted to reproduce the existing error message you would do something like:

If you want to update all the files, do bazel run %all-targets%; if you want to update just this target, run %target%.

Suggestions on names & syntax welcome; I'm not inclined to implement any larger a feature set than described here.

I think the {{VAR_NAME}} syntax would be suitable since we use that elsewhere in expand_template. We could have three variables: TARGET, SUGGESTED_UPDATE_TARGET, and ORIGINAL_MESSAGE, the latter containing the full message. Then it would be trivial for you to append something after that.

[jameskuszmaul-brt]

I think the {{VAR_NAME}} syntax would be suitable since we use that elsewhere in expand_template. We could have three variables: TARGET, SUGGESTED_UPDATE_TARGET, and ORIGINAL_MESSAGE, the latter containing the full message. Then it would be trivial for you to append something after that.

Sounds good; I'm not sure if I'll implement ORIGINAL_MESSAGE or not (depends on how much complexity it seems like it adds).