Injected Interface `FabricRecipeExporter` isn't implemented via mixin.

[RubixDev]

#4703 introduced the FabricRecipeExporter interface to the data generation API, which is injected into the RecipeExporter interface. The contributing guidelines state, that alongside every injected interface should come a mixin which implements it.

• Interface injection requires both a fabric.mod.json custom value to make it visible in Minecraft source code, and a mixin to actually implement the interface at runtime.

This isn't the case for FabricRecipeExporter though. I believe this wasn't done, because vanilla Minecraft only uses the RecipeExporter during its data generation and Fabric API also only constructs instances with the new interface, also only during data generation. So this never lead to any issues during gameplay.

In my case, I'm using the various RecipeJsonBuilders during normal game execution though, which then broke when invoking offerTo because a Fabric API mixin assumed the existence of the added method. My fix for that is currently to just have this implementation mixin in my mod, as you can see here. I believe this should just be included in Fabric API itself, though, unless I'm missing something.

The affected branches are 1.21.1, 1.21.5, 1.21.6, 1.21.7, 1.21.8, 1.21.9, 1.21.10, and 1.21.11 as far as I can tell.

If the fix is really just to add such a mixin, I'd be happy to open a PR.

[Juuxel]

Correct, the fix is just to add a mixin that adds the injected interface, which is a very easy fix. I somehow missed that in the original PR.

[Liam-Broome]

Created a PR for this.