Mixin VS Xray

And one day maybe you'll need that to inject code into specific location, but it's to do advanced things: https://github.com/SpongePowered/Mixin/wiki

SpongePowered isn't a Mod, they're the group that maintain the Framework for Mixins. The next mod in the stacktrace is cpw.mods.modlauncher

That source was probably outdated. Forge didn't natively allow you to do that. You could do Java voodoo to allow a library that implemented(? Not sure if it implemented the feature or simply allowed it to be used in a specific way) Mixins, those allow you to modify the game's bytecode. Modern Forge has that library natively. Source: First paragraph of https://github.com/SpongePowered/Mixin/wiki/Mixins-on-Minecraft-Forge

These days Mojang provides obfuscation maps, so you can work with proper class and method names (though no parameter or local variable names). There's also been a lot of effort been put into the tooling. Nowadays there exists Gradle plugins that will download the game jar, decompile it and deobfuscate it using the official mappings. You develop against the deobfuscated code, then the plugin will turn the unobfuscated names back into their obfuscated versions when you compile. There's also been technology developed that lets you easily modify specific parts of a method in the game, so you can e.g. insert calls to your own functions at runtime.[1] This saves from you having to modify the game jar itself.

[1]: https://github.com/SpongePowered/Mixin

quilt (and fabric) take mojang's jar and preprocess it similarly, but the rest of the modifications to the code are done at runtime using a ridiculously powerful instrumenting classloader. the game is not decompiled at all, other than giving you something to look at in the IDE.

If you can't use the API, I suggest you to use Mixins or really asking yourself if your idea should worth the instability/effort of creating a coremod.

see spongepowered's mixin wiki

I'm using this xray mod along with this brightness mod. Both mods have brightness-changing options and when I enable full-bright on the xray mod it allows the brightness mod to work, otherwise it'll stay at 100%. However that's the case only when I'm not in xray mode. When I go into xray mode every block face that has no light is almost pitch black and really hard to distinguish. I tried a lot of options between both mods, but nothing changes that. I'm also using sodium and lithium so that could conflict too. I'd like to know a solution so I can see every face as if I have full-bright. Version is 1.19.2

Sometimes you can't use the Forge API or you need to change a really specific things in the code, I've used this feature to add a hook for my Xray mod a while ago in Forge 1.14-15 before switching to Mixins (here is the transformer file). it's extremely unstable once someone else try to touch the same code section.