Remapper extensions (#984)

* Remapper extension API

* Fix build

* More work

* Fixes, thanks Gradle.

* Build fix

* Cleanup

src/main/java/net/fabricmc/loom/LoomGradleExtension.java

@@ -45,6 +45,7 @@ import net.fabricmc.loom.configuration.providers.minecraft.mapped.IntermediaryMi
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.NamedMinecraftProvider;
 import net.fabricmc.loom.extension.LoomFiles;
 import net.fabricmc.loom.extension.MixinExtension;
+import net.fabricmc.loom.extension.RemapperExtensionHolder;
 import net.fabricmc.loom.util.download.DownloadBuilder;
 
 @ApiStatus.Internal
@@ -115,4 +116,6 @@ public interface LoomGradleExtension extends LoomGradleExtensionAPI {
 	boolean multiProjectOptimisation();
 
 	ListProperty<LibraryProcessorManager.LibraryProcessorFactory> getLibraryProcessors();
+
+	ListProperty<RemapperExtensionHolder> getRemapperExtensions();
 }

src/main/java/net/fabricmc/loom/api/LoomGradleExtensionAPI.java

@@ -44,6 +44,8 @@ import net.fabricmc.loom.api.decompilers.DecompilerOptions;
 import net.fabricmc.loom.api.mappings.intermediate.IntermediateMappingsProvider;
 import net.fabricmc.loom.api.mappings.layered.spec.LayeredMappingSpecBuilder;
 import net.fabricmc.loom.api.processor.MinecraftJarProcessor;
+import net.fabricmc.loom.api.remapping.RemapperExtension;
+import net.fabricmc.loom.api.remapping.RemapperParameters;
 import net.fabricmc.loom.configuration.ide.RunConfigSettings;
 import net.fabricmc.loom.configuration.processors.JarProcessor;
 import net.fabricmc.loom.configuration.providers.mappings.NoOpIntermediateMappingsProvider;
@@ -221,4 +223,6 @@ public interface LoomGradleExtensionAPI {
 	Property<Boolean> getRuntimeOnlyLog4j();
 
 	Property<Boolean> getSplitModDependencies();
+
+	<T extends RemapperParameters> void addRemapperExtension(Class<RemapperExtension<T>> remapperExtensionClass, Class<T> parametersClass, Action<T> parameterAction);
 }

src/main/java/net/fabricmc/loom/api/remapping/RemapperContext.java

@@ -0,0 +1,47 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.api.remapping;
+
+import org.objectweb.asm.commons.Remapper;
+
+/**
+ * Context for a {@link RemapperExtension}.
+ */
+public interface RemapperContext {
+	/**
+	 * @return The {@link Remapper} instance
+	 */
+	Remapper remapper();
+
+	/**
+	 * @return the source namespace
+	 */
+	String sourceNamespace();
+
+	/**
+	 * @return the target namespace
+	 */
+	String targetNamespace();
+}

src/main/java/net/fabricmc/loom/api/remapping/RemapperExtension.java

@@ -0,0 +1,53 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.api.remapping;
+
+import javax.inject.Inject;
+
+import org.gradle.api.Action;
+import org.objectweb.asm.ClassVisitor;
+
+/**
+ * A remapper extension can be used to add extra processing to the remapping process.
+ *
+ * <p>Implementations of RemapperExtension's must have the following:
+ * A single constructor annotated with {@link Inject}, and taking a single argument of the parameters.
+ * Or a single constructor annotated with {@link Inject} taking no arguments, when the extension does not have any parameters.
+ *
+ * <p>Use {@link net.fabricmc.loom.api.LoomGradleExtensionAPI#addRemapperExtension(Class, Class, Action)} to register a remapper extension.
+ *
+ * @param <T> Parameter type for the extension. Should be {@link RemapperParameters.None} if the action does not have parameters.
+ */
+public interface RemapperExtension<T extends RemapperParameters> {
+	/**
+	 * Return a {@link ClassVisitor} that will be used when remapping the given class.
+	 *
+	 * @param className The name of the class being remapped
+	 * @param remapperContext The remapper context
+	 * @param classVisitor The parent class visitor
+	 * @return A {@link ClassVisitor} that will be used when remapping the given class, or the given {@code classVisitor} if no extra processing is required for this class.
+	 */
+	ClassVisitor insertVisitor(String className, RemapperContext remapperContext, ClassVisitor classVisitor);
+}

src/main/java/net/fabricmc/loom/api/remapping/RemapperParameters.java

@@ -0,0 +1,42 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.api.remapping;
+
+import org.jetbrains.annotations.ApiStatus;
+
+/**
+ * Marker interface for parameter objects to {@link RemapperExtension}s.
+ *
+ * <p>Design based off of Gradle's {@link org.gradle.workers.WorkParameters}.
+ */
+public interface RemapperParameters {
+	final class None implements RemapperParameters {
+		@ApiStatus.Internal
+		public static None INSTANCE = new None();
+
+		private None() {
+		}
+	}
+}

src/main/java/net/fabricmc/loom/api/remapping/TinyRemapperExtension.java

@@ -0,0 +1,68 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.api.remapping;
+
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.Nullable;
+
+import net.fabricmc.tinyremapper.TinyRemapper;
+
+/**
+ * A remapper extension, that has direct access to the TinyRemapper APIs.
+ *
+ * <p>This API is not stable and may change without notice.
+ */
+@ApiStatus.Experimental
+public interface TinyRemapperExtension {
+	/**
+	 * See: {@link TinyRemapper.Builder#extraAnalyzeVisitor(TinyRemapper.AnalyzeVisitorProvider)}.
+	 *
+	 * @return A {@link TinyRemapper.AnalyzeVisitorProvider} or {@code null}.
+	 */
+	@Nullable
+	default TinyRemapper.AnalyzeVisitorProvider getAnalyzeVisitorProvider() {
+		return null;
+	}
+
+	/**
+	 * See: {@link TinyRemapper.Builder#extraPreApplyVisitor(TinyRemapper.ApplyVisitorProvider)}.
+	 *
+	 * @return A {@link TinyRemapper.ApplyVisitorProvider} or {@code null}.
+	 */
+	@Nullable
+	default TinyRemapper.ApplyVisitorProvider getPreApplyVisitor() {
+		return null;
+	}
+
+	/**
+	 * See: {@link TinyRemapper.Builder#extraPostApplyVisitor(TinyRemapper.ApplyVisitorProvider)}.
+	 *
+	 * @return A {@link TinyRemapper.ApplyVisitorProvider} or {@code null}.
+	 */
+	@Nullable
+	default TinyRemapper.ApplyVisitorProvider getPostApplyVisitor() {
+		return null;
+	}
+}

src/main/java/net/fabricmc/loom/configuration/mods/ModProcessor.java

@@ -51,6 +51,7 @@ import net.fabricmc.loom.api.RemapConfigurationSettings;
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
 import net.fabricmc.loom.configuration.mods.dependency.ModDependency;
 import net.fabricmc.loom.configuration.providers.mappings.MappingConfiguration;
+import net.fabricmc.loom.extension.RemapperExtensionHolder;
 import net.fabricmc.loom.util.Constants;
 import net.fabricmc.loom.util.Pair;
 import net.fabricmc.loom.util.TinyRemapperHelper;
@@ -157,6 +158,10 @@ public class ModProcessor {
 			builder.extension(new MixinExtension(remapMixins::contains));
 		}
 
+		for (RemapperExtensionHolder holder : extension.getRemapperExtensions().get()) {
+			holder.apply(builder, fromM, toM, project.getObjects());
+		}
+
 		final TinyRemapper remapper = builder.build();
 
 		for (Path minecraftJar : extension.getMinecraftJars(MappingsNamespace.INTERMEDIARY)) {

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionApiImpl.java

@@ -34,6 +34,7 @@ import org.gradle.api.Project;
 import org.gradle.api.artifacts.Dependency;
 import org.gradle.api.file.ConfigurableFileCollection;
 import org.gradle.api.file.RegularFileProperty;
+import org.gradle.api.model.ObjectFactory;
 import org.gradle.api.provider.ListProperty;
 import org.gradle.api.provider.Property;
 import org.gradle.api.provider.SetProperty;
@@ -50,6 +51,8 @@ import net.fabricmc.loom.api.decompilers.DecompilerOptions;
 import net.fabricmc.loom.api.mappings.intermediate.IntermediateMappingsProvider;
 import net.fabricmc.loom.api.mappings.layered.spec.LayeredMappingSpecBuilder;
 import net.fabricmc.loom.api.processor.MinecraftJarProcessor;
+import net.fabricmc.loom.api.remapping.RemapperExtension;
+import net.fabricmc.loom.api.remapping.RemapperParameters;
 import net.fabricmc.loom.configuration.RemapConfigurations;
 import net.fabricmc.loom.configuration.ide.RunConfigSettings;
 import net.fabricmc.loom.configuration.processors.JarProcessor;
@@ -91,6 +94,7 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 	private final NamedDomainObjectContainer<ModSettings> mods;
 	private final NamedDomainObjectList<RemapConfigurationSettings> remapConfigurations;
 	private final ListProperty<MinecraftJarProcessor<?>> minecraftJarProcessors;
+	protected final ListProperty<RemapperExtensionHolder> remapperExtensions;
 
 	// A common mistake with layered mappings is to call the wrong `officialMojangMappings` method, use this to keep track of when we are building a layered mapping spec.
 	protected final ThreadLocal<Boolean> layeredSpecBuilderScope = ThreadLocal.withInitial(() -> false);
@@ -150,6 +154,9 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		this.splitEnvironmentalSourceSet = project.getObjects().property(Boolean.class).convention(false);
 		this.splitEnvironmentalSourceSet.finalizeValueOnRead();
 
+		remapperExtensions = project.getObjects().listProperty(RemapperExtensionHolder.class);
+		remapperExtensions.finalizeValueOnRead();
+
 		// Enable dep iface injection by default
 		interfaceInjection(interfaceInjection -> {
 			interfaceInjection.getEnableDependencyInterfaceInjection().convention(true).finalizeValueOnRead();
@@ -382,6 +389,23 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		RemapConfigurations.setupForSourceSet(getProject(), sourceSet);
 	}
 
+	@Override
+	public <T extends RemapperParameters> void addRemapperExtension(Class<RemapperExtension<T>> remapperExtensionClass, Class<T> parametersClass, Action<T> parameterAction) {
+		final ObjectFactory objectFactory = getProject().getObjects();
+		final RemapperExtensionHolder holder;
+
+		if (parametersClass != RemapperParameters.None.class) {
+			T parameters = objectFactory.newInstance(parametersClass);
+			parameterAction.execute(parameters);
+			holder = objectFactory.newInstance(RemapperExtensionHolder.class, parameters);
+		} else {
+			holder = objectFactory.newInstance(RemapperExtensionHolder.class, RemapperParameters.None.INSTANCE);
+		}
+
+		holder.getRemapperExtensionClassName().set(remapperExtensionClass.getName());
+		remapperExtensions.add(holder);
+	}
+
 	// This is here to ensure that LoomGradleExtensionApiImpl compiles without any unimplemented methods
 	private final class EnsureCompile extends LoomGradleExtensionApiImpl {
 		private EnsureCompile() {

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionImpl.java

@@ -247,6 +247,11 @@ public class LoomGradleExtensionImpl extends LoomGradleExtensionApiImpl implemen
 		return libraryProcessorFactories;
 	}
 
+	@Override
+	public ListProperty<RemapperExtensionHolder> getRemapperExtensions() {
+		return remapperExtensions;
+	}
+
 	@Override
 	protected <T extends IntermediateMappingsProvider> void configureIntermediateMappingsProviderInternal(T provider) {
 		provider.getMinecraftVersion().set(getProject().provider(() -> getMinecraftProvider().minecraftVersion()));

src/main/java/net/fabricmc/loom/extension/RemapperExtensionHolder.java

@@ -0,0 +1,128 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.extension;
+
+import javax.inject.Inject;
+
+import org.gradle.api.model.ObjectFactory;
+import org.gradle.api.provider.Property;
+import org.gradle.api.tasks.Input;
+import org.gradle.api.tasks.Nested;
+import org.gradle.api.tasks.Optional;
+import org.jetbrains.annotations.Nullable;
+import org.objectweb.asm.ClassVisitor;
+import org.objectweb.asm.commons.Remapper;
+
+import net.fabricmc.loom.api.remapping.RemapperContext;
+import net.fabricmc.loom.api.remapping.RemapperExtension;
+import net.fabricmc.loom.api.remapping.RemapperParameters;
+import net.fabricmc.loom.api.remapping.TinyRemapperExtension;
+import net.fabricmc.tinyremapper.TinyRemapper;
+import net.fabricmc.tinyremapper.api.TrClass;
+
+public abstract class RemapperExtensionHolder {
+	// Null when RemapperParameters.None.class
+	private final RemapperParameters remapperParameters;
+
+	@Inject
+	public RemapperExtensionHolder(RemapperParameters remapperParameters) {
+		this.remapperParameters = remapperParameters;
+	}
+
+	@Input
+	public abstract Property<String> getRemapperExtensionClassName();
+
+	@Nested
+	@Optional
+	public RemapperParameters getRemapperParameters() {
+		return remapperParameters;
+	}
+
+	public void apply(TinyRemapper.Builder tinyRemapperBuilder, String sourceNamespace, String targetNamespace, ObjectFactory objectFactory) {
+		final RemapperExtension<?> remapperExtension = newInstance(objectFactory);
+
+		tinyRemapperBuilder.extraPostApplyVisitor(new RemapperExtensionImpl(remapperExtension, sourceNamespace, targetNamespace));
+
+		if (remapperExtension instanceof TinyRemapperExtension tinyRemapperExtension) {
+			final TinyRemapper.AnalyzeVisitorProvider analyzeVisitorProvider = tinyRemapperExtension.getAnalyzeVisitorProvider();
+			final TinyRemapper.ApplyVisitorProvider preApplyVisitorProvider = tinyRemapperExtension.getPreApplyVisitor();
+			final TinyRemapper.ApplyVisitorProvider postApplyVisitorProvider = tinyRemapperExtension.getPostApplyVisitor();
+
+			if (analyzeVisitorProvider != null) {
+				tinyRemapperBuilder.extraAnalyzeVisitor(analyzeVisitorProvider);
+			}
+
+			if (preApplyVisitorProvider != null) {
+				tinyRemapperBuilder.extraPreApplyVisitor(preApplyVisitorProvider);
+			}
+
+			if (postApplyVisitorProvider != null) {
+				tinyRemapperBuilder.extraPostApplyVisitor(postApplyVisitorProvider);
+			}
+		}
+	}
+
+	private RemapperExtension<?> newInstance(ObjectFactory objectFactory) {
+		try {
+			Class<? extends RemapperExtension> remapperExtensionClass = Class.forName(getRemapperExtensionClassName().get())
+					.asSubclass(RemapperExtension.class);
+
+			if (remapperParameters == RemapperParameters.None.INSTANCE) {
+				return objectFactory.newInstance(remapperExtensionClass);
+			}
+
+			return objectFactory.newInstance(remapperExtensionClass, remapperParameters);
+		} catch (Exception e) {
+			throw new RuntimeException("Failed to create remapper extension", e);
+		}
+	}
+
+	private static final class RemapperExtensionImpl implements TinyRemapper.ApplyVisitorProvider {
+		private final RemapperExtension<?> remapperExtension;
+		private final String sourceNamespace;
+		private final String targetNamespace;
+
+		@Nullable
+		private RemapperContext context;
+
+		private RemapperExtensionImpl(RemapperExtension<?> remapperExtension, String sourceNamespace, String targetNamespace) {
+			this.remapperExtension = remapperExtension;
+			this.sourceNamespace = sourceNamespace;
+			this.targetNamespace = targetNamespace;
+		}
+
+		@Override
+		public ClassVisitor insertApplyVisitor(TrClass cls, ClassVisitor next) {
+			if (context == null) {
+				context = new RemapperContextImpl(cls.getEnvironment().getRemapper(), sourceNamespace, targetNamespace);
+			}
+
+			return remapperExtension.insertVisitor(cls.getName(), context, next);
+		}
+	}
+
+	private record RemapperContextImpl(Remapper remapper, String sourceNamespace, String targetNamespace) implements RemapperContext {
+	}
+}

src/main/java/net/fabricmc/loom/task/service/TinyRemapperService.java

@@ -39,11 +39,13 @@ import java.util.StringJoiner;
 
 import org.gradle.api.Project;
 import org.gradle.api.invocation.Gradle;
+import org.gradle.api.model.ObjectFactory;
 import org.gradle.api.tasks.SourceSet;
 import org.jetbrains.annotations.Nullable;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.build.mixin.AnnotationProcessorInvoker;
+import net.fabricmc.loom.extension.RemapperExtensionHolder;
 import net.fabricmc.loom.task.AbstractRemapJarTask;
 import net.fabricmc.loom.util.gradle.GradleUtils;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
@@ -90,7 +92,7 @@ public class TinyRemapperService implements SharedService {
 				mappings.add(gradleMixinMappingProvider(serviceManager, project.getGradle(), extension.getMappingConfiguration().mappingsIdentifier, from, to));
 			}
 
-			return new TinyRemapperService(mappings, !legacyMixin, kotlinClasspathService, extension.getKnownIndyBsms().get());
+			return new TinyRemapperService(mappings, !legacyMixin, kotlinClasspathService, extension.getKnownIndyBsms().get(), extension.getRemapperExtensions().get(), from, to, project.getObjects());
 		});
 
 		service.readClasspath(remapJarTask.getClasspath().getFiles().stream().map(File::toPath).filter(Files::exists).toList());
@@ -129,7 +131,7 @@ public class TinyRemapperService implements SharedService {
 	// Set to true once remapping has started, once set no inputs can be read.
 	private boolean isRemapping = false;
 
-	public TinyRemapperService(List<IMappingProvider> mappings, boolean useMixinExtension, @Nullable KotlinClasspath kotlinClasspath, Set<String> knownIndyBsms) {
+	private TinyRemapperService(List<IMappingProvider> mappings, boolean useMixinExtension, @Nullable KotlinClasspath kotlinClasspath, Set<String> knownIndyBsms, List<RemapperExtensionHolder> remapperExtensions, String sourceNamespace, String targetNamespace, ObjectFactory objectFactory) {
 		TinyRemapper.Builder builder = TinyRemapper.newRemapper().withKnownIndyBsm(knownIndyBsms);
 
 		for (IMappingProvider provider : mappings) {
@@ -145,6 +147,10 @@ public class TinyRemapperService implements SharedService {
 			builder.extension(kotlinRemapperClassloader.getTinyRemapperExtension());
 		}
 
+		for (RemapperExtensionHolder holder : remapperExtensions) {
+			holder.apply(builder, sourceNamespace, targetNamespace, objectFactory);
+		}
+
 		tinyRemapper = builder.build();
 	}
 

src/test/groovy/net/fabricmc/loom/test/integration/SimpleProjectTest.groovy

@@ -42,6 +42,7 @@ class SimpleProjectTest extends Specification implements GradleProjectTestTrait
 	def "build and run (gradle #version)"() {
 		setup:
 		def gradle = gradleProject(project: "simple", version: version)
+		gradle.buildSrc("remapext") // apply the remap extension plugin
 
 		def server = ServerRunner.create(gradle.projectDir, "1.16.5")
 				.withMod(gradle.getOutputFile("fabric-example-mod-1.0.0.jar"))
@@ -60,6 +61,7 @@ class SimpleProjectTest extends Specification implements GradleProjectTestTrait
 
 		serverResult.successful()
 		serverResult.output.contains("Hello simple Fabric mod") // A check to ensure our mod init was actually called
+		serverResult.output.contains("Hello Loom!") // Check that the remapper extension worked
 		where:
 		version << STANDARD_TEST_VERSIONS
 	}

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/StringReplacementClassVisitor.groovy

@@ -0,0 +1,61 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration.buildSrc.remapext
+
+import org.objectweb.asm.ClassVisitor
+import org.objectweb.asm.MethodVisitor
+
+class StringReplacementClassVisitor extends ClassVisitor {
+	final Map<String, String> replacements
+
+	StringReplacementClassVisitor(int api, ClassVisitor classVisitor, Map<String, String> replacements) {
+		super(api, classVisitor)
+		this.replacements = replacements
+	}
+
+	@Override
+	MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
+		def methodVisitor = super.visitMethod(access, name, descriptor, signature, exceptions)
+		return new StringReplacementMethodVisitor(api, methodVisitor)
+	}
+
+	class StringReplacementMethodVisitor extends MethodVisitor {
+		StringReplacementMethodVisitor(int api, MethodVisitor methodVisitor) {
+			super(api, methodVisitor)
+		}
+
+		@Override
+		void visitLdcInsn(Object value) {
+			if (value instanceof String) {
+				String replacement = replacements.get(value)
+				if (replacement != null) {
+					value = replacement
+				}
+			}
+
+			super.visitLdcInsn(value)
+		}
+	}
+}

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/TestPlugin.groovy

@@ -0,0 +1,43 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration.buildSrc.remapext
+
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+
+import net.fabricmc.loom.LoomGradleExtension
+import net.fabricmc.loom.api.remapping.RemapperParameters
+
+class TestPlugin implements Plugin<Project> {
+	@Override
+	void apply(Project project) {
+		def extension = LoomGradleExtension.get(project)
+		extension.addRemapperExtension(TestRemapperExtension.class, TestRemapperExtension.Params.class) { TestRemapperExtension.Params p ->
+			p.replacements.put("Hello World!", "Hello Loom!")
+		}
+		extension.addRemapperExtension(TestTinyRemapperExtension.class, RemapperParameters.None.class) { RemapperParameters.None p ->
+		}
+	}
+}

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/TestRemapperExtension.groovy

@@ -0,0 +1,54 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration.buildSrc.remapext
+
+import javax.inject.Inject
+
+import org.gradle.api.provider.MapProperty
+import org.objectweb.asm.ClassVisitor
+
+import net.fabricmc.loom.api.remapping.RemapperContext
+import net.fabricmc.loom.api.remapping.RemapperExtension
+import net.fabricmc.loom.api.remapping.RemapperParameters
+import net.fabricmc.loom.util.Constants
+
+abstract class TestRemapperExtension implements RemapperExtension<Params> {
+	final Params parameters
+
+	@Inject
+	TestRemapperExtension(Params parameters) {
+		this.parameters = parameters
+	}
+
+	@Override
+	ClassVisitor insertVisitor(String className, RemapperContext remapperContext, ClassVisitor classVisitor) {
+		def replacements = parameters.replacements.get()
+		return new StringReplacementClassVisitor(Constants.ASM_VERSION, classVisitor, replacements)
+	}
+
+	interface Params extends RemapperParameters {
+		MapProperty<String, String> getReplacements()
+	}
+}

src/test/groovy/net/fabricmc/loom/test/integration/buildSrc/remapext/TestTinyRemapperExtension.groovy

@@ -0,0 +1,44 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2023 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration.buildSrc.remapext
+
+import org.objectweb.asm.ClassVisitor
+
+import net.fabricmc.loom.api.remapping.RemapperContext
+import net.fabricmc.loom.api.remapping.RemapperExtension
+import net.fabricmc.loom.api.remapping.RemapperParameters
+import net.fabricmc.loom.api.remapping.TinyRemapperExtension
+import net.fabricmc.tinyremapper.TinyRemapper
+
+class TestTinyRemapperExtension implements RemapperExtension<RemapperParameters.None>, TinyRemapperExtension {
+	@Override
+	ClassVisitor insertVisitor(String className, RemapperContext remapperContext, ClassVisitor classVisitor) {
+		return classVisitor
+	}
+
+	TinyRemapper.AnalyzeVisitorProvider analyzeVisitorProvider = null
+	TinyRemapper.ApplyVisitorProvider preApplyVisitor = null
+	TinyRemapper.ApplyVisitorProvider PostApplyVisitor = null
+}

src/test/resources/projects/simple/src/main/java/net/fabricmc/example/ExampleMod.java

@@ -1,9 +1,10 @@
 package net.fabricmc.example;
 
-import net.fabricmc.api.ModInitializer;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+import net.fabricmc.api.ModInitializer;
+
 public class ExampleMod implements ModInitializer {
 	public static final Logger LOGGER = LogManager.getLogger("modid");
 
@@ -13,5 +14,6 @@ public class ExampleMod implements ModInitializer {
 	@Override
 	public void onInitialize() {
 		LOGGER.info("Hello simple Fabric mod!");
+		LOGGER.info("Hello World!");
 	}
-}
+}