This is a new technique introduced by Loom 0.11 to add methods into a specific existing class. More specifically, you can create an Interface, and then inject this interface into the class. As result the target class will acquire all the methods of the interface, as if it always had them. Interface injection is a compile time only feature, this means that a Mixin should also be used to implement the interface into the target class.

This is particularly useful for libraries, with this you can add new methods to existing classes and use them without the need of casting or reimplementing the interface every time.

Fabric API takes advantage of this technique. For example, TagBuilder implements FabricTagBuilder, BlockEntityType implements FabricBlockEntityType, so that you can directly use the instance methods in the Fabric API on the vanilla objects.

Let's explain better with an example:

The scope of this example is to add the following method into net.minecraft.fluid.FlowableFluid to get the sound of the bucket when emptied. This, normally, is not possible because net.minecraft.fluid.FlowableFluid does not have a similar method.

Optional<SoundEvent> getBucketEmptySound$myMod()

To add the method into the class, first of all you need to create an interface with it:

package net.fabricmc.example;
 
public interface BucketEmptySoundGetter {
	default Optional<SoundEvent> getBucketEmptySound$myMod() {
		return Optional.empty();
	}
}

The method body in the interface may not be used because it will be overridden by the mixin class. However, you must specify the method body, which means the method must be default. You can make it return null or throw UnsupportedOperationException, but you cannot make it abstract, such as OptionalSoundEvent getBucketEmptySound(), or exceptions will be thrown when compiling!

ℹ️ It's highly recommended to add a dollar-character or underscore character with the mod name as the prefix or suffix of the method name, in order to avoid method name conflict with other mods.

Now you need to implement this interface into net.minecraft.fluid.FlowableFluid with a mixin implementing the interface:

@Mixin(FlowableFluid.class)
abstract class MixinFlowableFluid implements BucketEmptySoundGetter {
	@Override
	public Optional<SoundEvent> getBucketEmptySound$myMod() {
	    //This is how to get the default sound, copied from BucketItem class.
	    return Optional.of(((FlowableFluid) (Object) this).isIn(FluidTags.LAVA) ? SoundEvents.ITEM_BUCKET_EMPTY_LAVA : SoundEvents.ITEM_BUCKET_EMPTY);
	}
}

Lastly you need to inject the interface into net.minecraft.fluid.FlowableFluid. The following snippet can be added to your fabric.mod.json file to add one or more interfaces to the net.minecraft.fluid.FlowableFluid class. Note that all class names here must use the “internal names” that use slashes instead of dots (path/to/my/Class).

fabric.mod.json

{
	"custom": {
		"loom:injected_interfaces": {
			"net/minecraft/class_3609": ["net/fabricmc/example/BucketEmptySoundGetter"]
		}
	}
}

Now you can use the new method:

Optional<SoundEvent> sound = mytestfluid.getBucketEmptySound$myMod();

You could also override this method in classes extending FlowableFluid to implement custom behaviours.

Sometimes, your interface injections may need to include the $ symbol, but the Groovy template processor may interpret this as a template variable if you are replacing variables (such as ${version}) in your fabric.mod.json. A workaround for this is to use the Unicode escape for $, which is \u0024.