Overview
Once upon a time, the most popular way to mod was by changing the class files in the minecraft jar and distributing those. But there was a problem.
If more than 1 mod touched the same class, then it was extremely difficult to fix because of overwriting. Hence a mod api was created and agreed upon so that mods could add their stuff without changing base classes and causing hard incompatibilities. This was all well and good until mods wanted to change the properties of vanilla objects.
It could be done using a coremod, however, it was so difficult that most people either didn't bother or wrote coremods so fragile that if another mod touched it would cause errors.
So in 1.12, registry replacement was invented as a way of allowing mods to alter vanilla blocks without coremods. However, it has several issues.
1) Only one mod per pack can override an object at a time, last one takes priority. Furthermore, there is no way to choose which mod "wins" since mod loading is nondeterministic and can change at any time.
2) Unlike coremods which usually only cause issues when touching the same method, registry replacements of the same object are always incompatible regardless of what they change.
3) The only way to be compatible with other mods that registry replaces is to write specific compats for every mod and those mods need to add support in turn or everything breaks down. This is often infeasible in the case of ARR/closed source mods where implementing code can be problematic. In either case, automatic hard incompatibility is a very bad thing.
4) These objects are cached everywhere in vanilla code including the fields of some objects, leading to issues in replacing some of them. (Cough DefaultBiomeFeatures).
So, in an effort to help identify such problematic mods, I created a tool that logs registry replacement of vanilla objects to track down incompatibilities. It will log any registry replacement that happens and what class it was replaced with. Example snippit below.
.Such mods should seek to find a more mod friendly way to alter blocks such as coremods or mixins as outright replacement is too destructive to be used on vanilla objects.
