Getting Emacs' compile-mode to play nice with Gradle

So, I am still not free of Java, but I started writing it in GNU Emacs recently. It works surprisingly well with Eglot, JDTLS, java-ts-mode (sadly, CC Mode Java support is too far behind and I haven’t been able to allocate time to updating it), and a few hacks I had to make to deal with some oddities of JDTLS.

I’ve been using compilation-mode through project-compile to build my Java code and look for errors, but when building something with Gradle 8.12.1 (and possibly other versions), Gradle outputs diagnostics twice: once when they actually occur, and again later as a recap, but this time with two spaces prefixed on each line of the diagnostic.

GNU Emacs supports parsing these diagnostics out of the box (which came as a surprise to me), but these double diagnostics keep confusing it, by telling it that an error happened in some file whose path starts with a double space, and by making it seem like there’s twice the number of errors there are in reality.

This can be hacked around using compilation-transform-file-match-alist like so:

(add-to-list 'compilation-transform-file-match-alist '("\\`  " nil))

The above tells compile-mode to ignore filenames starting with a double space (which is the prefix Gradle is using in the recap).

Sadly, I don’t see a way to make this apply only to Gradle. Perhaps updating compilation-error-regexp-alist-alist entries for Gradle not to match filenames starting with is wiser, but I haven’t tried that yet (for a lack of time).