Support using TinyV2 mappings (#132)

build.gradle

@@ -39,15 +39,17 @@ dependencies {
 	implementation ('com.google.guava:guava:28.0-jre')
 
 	// game handling utils
-	implementation ('net.fabricmc:stitch:0.2.1.60') {
+	implementation ('net.fabricmc:stitch:0.3.0.66') {
 		exclude module: 'enigma'
 	}
 
 	// tinyfile management
-	implementation ('net.fabricmc:tiny-remapper:0.1.0.38') {
+	implementation ('net.fabricmc:tiny-remapper:0.2.0.52') {
 		transitive = false
 	}
-	implementation ('net.fabricmc:fabric-mixin-compile-extensions:0.1.0.+')
+	implementation ('net.fabricmc:fabric-mixin-compile-extensions:0.2.0.3'){
+		exclude group :"net.fabricmc"
+	}
 
 	// decompilers
 	implementation ('net.fabricmc:procyon-fabric-compilertools:0.5.35.+')
@@ -132,3 +134,8 @@ publishing {
 		}
 	}
 }
+configurations.all {
+    resolutionStrategy {
+        force 'org.codehaus.groovy:groovy-all:2.4.12'
+    }
+}

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

@@ -114,6 +114,7 @@ public class AbstractPlugin implements Plugin<Project> {
 		includeConfig.setTransitive(false); // Dont get transitive deps
 
 		project.getConfigurations().maybeCreate(Constants.MAPPINGS);
+		project.getConfigurations().maybeCreate(Constants.MAPPINGS_FINAL);
 
 		for (RemappedConfigurationEntry entry : Constants.MOD_COMPILE_ENTRIES) {
 			Configuration compileModsConfig = project.getConfigurations().maybeCreate(entry.getSourceConfiguration());
@@ -136,8 +137,8 @@ public class AbstractPlugin implements Plugin<Project> {
 		extendsFrom(Constants.MINECRAFT_NAMED, Constants.MINECRAFT_DEPENDENCIES);
 		extendsFrom(Constants.MINECRAFT_INTERMEDIARY, Constants.MINECRAFT_DEPENDENCIES);
 
-		extendsFrom("compile", Constants.MAPPINGS);
+		extendsFrom("compile", Constants.MAPPINGS_FINAL);
-		extendsFrom("annotationProcessor", Constants.MAPPINGS);
+		extendsFrom("annotationProcessor", Constants.MAPPINGS_FINAL);
 
 		configureIDEs();
 		configureCompile();
@@ -156,8 +157,8 @@ public class AbstractPlugin implements Plugin<Project> {
 						project.getLogger().lifecycle(":setting java compiler args");
 
 						try {
-							javaCompileTask.getOptions().getCompilerArgs().add("-AinMapFileNamedIntermediary=" + extension.getMappingsProvider().MAPPINGS_TINY.getCanonicalPath());
+							javaCompileTask.getOptions().getCompilerArgs().add("-AinMapFileNamedIntermediary=" + extension.getMappingsProvider().tinyMappings.getCanonicalPath());
-							javaCompileTask.getOptions().getCompilerArgs().add("-AoutMapFileNamedIntermediary=" + extension.getMappingsProvider().MAPPINGS_MIXIN_EXPORT.getCanonicalPath());
+							javaCompileTask.getOptions().getCompilerArgs().add("-AoutMapFileNamedIntermediary=" + extension.getMappingsProvider().mappingsMixinExport.getCanonicalPath());
 							javaCompileTask.getOptions().getCompilerArgs().add("-AoutRefMapFile=" + new File(javaCompileTask.getDestinationDir(), extension.getRefmapName()).getCanonicalPath());
 							javaCompileTask.getOptions().getCompilerArgs().add("-AdefaultObfuscationEnv=named:intermediary");
 						} catch (IOException e) {
@@ -183,8 +184,8 @@ public class AbstractPlugin implements Plugin<Project> {
 				project.getLogger().warn(":configuring scala compilation processing");
 
 				try {
-					task.getOptions().getCompilerArgs().add("-AinMapFileNamedIntermediary=" + extension.getMappingsProvider().MAPPINGS_TINY.getCanonicalPath());
+					task.getOptions().getCompilerArgs().add("-AinMapFileNamedIntermediary=" + extension.getMappingsProvider().tinyMappings.getCanonicalPath());
-					task.getOptions().getCompilerArgs().add("-AoutMapFileNamedIntermediary=" + extension.getMappingsProvider().MAPPINGS_MIXIN_EXPORT.getCanonicalPath());
+					task.getOptions().getCompilerArgs().add("-AoutMapFileNamedIntermediary=" + extension.getMappingsProvider().mappingsMixinExport.getCanonicalPath());
 					task.getOptions().getCompilerArgs().add("-AoutRefMapFile=" + new File(task.getDestinationDir(), extension.getRefmapName()).getCanonicalPath());
 					task.getOptions().getCompilerArgs().add("-AdefaultObfuscationEnv=named:intermediary");
 				} catch (IOException e) {

src/main/java/net/fabricmc/loom/providers/MappingsCache.java

@@ -24,8 +24,8 @@
 
 package net.fabricmc.loom.providers;
 
+import java.io.BufferedReader;
 import java.io.IOException;
-import java.io.InputStream;
 import java.lang.ref.SoftReference;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -33,32 +33,32 @@ import java.util.HashMap;
 import java.util.Map;
 
 import net.fabricmc.loom.util.StaticPathWatcher;
-import net.fabricmc.mappings.Mappings;
+import net.fabricmc.mapping.tree.TinyMappingFactory;
+import net.fabricmc.mapping.tree.TinyTree;
 
 public final class MappingsCache {
 	public static final MappingsCache INSTANCE = new MappingsCache();
 
-	private final Map<Path, SoftReference<Mappings>> mappingsCache = new HashMap<>();
+	private final Map<Path, SoftReference<TinyTree>> mappingsCache = new HashMap<>();
 
-	public Mappings get(Path mappingsPath) {
+	//TODO: loom doesn't actually use new mappings when the mappings change until the gradle daemons are stopped
+	public TinyTree get(Path mappingsPath) throws IOException {
 		mappingsPath = mappingsPath.toAbsolutePath();
 
 		if (StaticPathWatcher.INSTANCE.hasFileChanged(mappingsPath)) {
 			mappingsCache.remove(mappingsPath);
 		}
 
-		SoftReference<Mappings> ref = mappingsCache.get(mappingsPath);
+		SoftReference<TinyTree> ref = mappingsCache.get(mappingsPath);
 
 		if (ref != null && ref.get() != null) {
 			return ref.get();
 		} else {
-			try (InputStream stream = Files.newInputStream(mappingsPath)) {
+			try (BufferedReader reader = Files.newBufferedReader(mappingsPath)) {
-				Mappings mappings = net.fabricmc.mappings.MappingsProvider.readTinyMappings(stream, false);
+				TinyTree mappings = TinyMappingFactory.loadWithDetection(reader);
 				ref = new SoftReference<>(mappings);
 				mappingsCache.put(mappingsPath, ref);
 				return mappings;
-			} catch (IOException e) {
-				throw new RuntimeException(e);
 			}
 		}
 	}

src/main/java/net/fabricmc/loom/providers/MappingsProvider.java

@@ -24,25 +24,38 @@
 
 package net.fabricmc.loom.providers;
 
+import java.io.BufferedReader;
 import java.io.File;
 import java.io.IOException;
+import java.net.URL;
 import java.nio.file.FileSystem;
 import java.nio.file.FileSystems;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.StandardCopyOption;
 import java.util.function.Consumer;
 
+import com.google.common.net.UrlEscapers;
+import org.apache.commons.io.FileUtils;
+import org.apache.tools.ant.util.StringUtils;
 import org.gradle.api.Project;
+import org.zeroturnaround.zip.FileSource;
+import org.zeroturnaround.zip.ZipEntrySource;
+import org.zeroturnaround.zip.ZipUtil;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.util.Constants;
 import net.fabricmc.loom.util.DependencyProvider;
+import net.fabricmc.loom.util.DownloadUtil;
 import net.fabricmc.loom.util.Version;
-import net.fabricmc.mappings.Mappings;
+import net.fabricmc.mapping.reader.v2.TinyV2Factory;
+import net.fabricmc.mapping.tree.TinyTree;
+import net.fabricmc.stitch.Command;
 import net.fabricmc.stitch.commands.CommandProposeFieldNames;
+import net.fabricmc.stitch.commands.tinyv2.CommandMergeTinyV2;
+import net.fabricmc.stitch.commands.tinyv2.CommandReorderTinyV2;
 
-//TODO fix local mappings
-//TODO possibly use maven for mappings, can fix above at the same time
 public class MappingsProvider extends DependencyProvider {
 	public MinecraftMappedProvider mappedProvider;
 
@@ -50,13 +63,21 @@ public class MappingsProvider extends DependencyProvider {
 	public String minecraftVersion;
 	public String mappingsVersion;
 
-	public File MAPPINGS_DIR;
+	private Path mappingsDir;
-	public File MAPPINGS_TINY_BASE;
+	private Path mappingsStepsDir;
-	public File MAPPINGS_TINY;
+	// The mappings that gradle gives us
-	public File MAPPINGS_MIXIN_EXPORT;
+	private Path baseTinyMappings;
+	// The mappings we use in practice
+	public File tinyMappings;
+	public File tinyMappingsJar;
+	public File mappingsMixinExport;
 
-	public Mappings getMappings() throws IOException {
+	public void clean() throws IOException {
-		return MappingsCache.INSTANCE.get(MAPPINGS_TINY.toPath());
+		FileUtils.deleteDirectory(mappingsDir.toFile());
+	}
+
+	public TinyTree getMappings() throws IOException {
+		return MappingsCache.INSTANCE.get(tinyMappings.toPath());
 	}
 
 	@Override
@@ -66,50 +87,166 @@ public class MappingsProvider extends DependencyProvider {
 		project.getLogger().lifecycle(":setting up mappings (" + dependency.getDependency().getName() + " " + dependency.getResolvedVersion() + ")");
 
 		String version = dependency.getResolvedVersion();
-		File mappingsJar = dependency.resolveFile().orElseThrow(() -> new RuntimeException("Could not find dependency " + dependency));
+		File mappingsJar = dependency.resolveFile().orElseThrow(() -> new RuntimeException("Could not find yarn mappings: " + dependency));
 
-		this.mappingsName = dependency.getDependency().getGroup() + "." + dependency.getDependency().getName();
+		this.mappingsName = StringUtils.removeSuffix(dependency.getDependency().getGroup() + "." + dependency.getDependency().getName(), "-unmerged");
+
+		boolean isV2 = doesJarContainV2Mappings(mappingsJar.toPath());
 
 		Version mappingsVersion = new Version(version);
 		this.minecraftVersion = mappingsVersion.getMinecraftVersion();
-		this.mappingsVersion = mappingsVersion.getMappingsVersion();
+		this.mappingsVersion = mappingsVersion.getMappingsVersion() + (isV2 ? "-v2" : "");
 
 		initFiles(project);
 
-		if (!MAPPINGS_DIR.exists()) {
+		Files.createDirectories(mappingsDir);
-			MAPPINGS_DIR.mkdir();
+		Files.createDirectories(mappingsStepsDir);
+
+		String[] depStringSplit = dependency.getDepString().split(":");
+		String jarClassifier = "final";
+
+		if (depStringSplit.length >= 4) {
+			jarClassifier = jarClassifier + depStringSplit[3];
 		}
 
-		if (!MAPPINGS_TINY_BASE.exists() || !MAPPINGS_TINY.exists()) {
+		tinyMappings = mappingsDir.resolve(StringUtils.removeSuffix(mappingsJar.getName(), ".jar") + ".tiny").toFile();
-			if (!MAPPINGS_TINY_BASE.exists()) {
+		tinyMappingsJar = new File(extension.getUserCache(), mappingsJar.getName().replace(".jar", "-" + jarClassifier + ".jar"));
-				project.getLogger().lifecycle(":extracting " + mappingsJar.getName());
 
-				try (FileSystem fileSystem = FileSystems.newFileSystem(mappingsJar.toPath(), null)) {
+		if (!tinyMappings.exists()) {
-					Path fileToExtract = fileSystem.getPath("mappings/mappings.tiny");
+			storeMappings(project, minecraftProvider, mappingsJar.toPath());
-					Files.copy(fileToExtract, MAPPINGS_TINY_BASE.toPath());
-				}
-			}
-
-			if (MAPPINGS_TINY.exists()) {
-				MAPPINGS_TINY.delete();
-			}
-
-			project.getLogger().lifecycle(":populating field names");
-			new CommandProposeFieldNames().run(new String[]{minecraftProvider.MINECRAFT_MERGED_JAR.getAbsolutePath(), MAPPINGS_TINY_BASE.getAbsolutePath(), MAPPINGS_TINY.getAbsolutePath()});
 		}
 
+		if (!tinyMappingsJar.exists()) {
+			ZipUtil.pack(new ZipEntrySource[] {new FileSource("mappings/mappings.tiny", tinyMappings)}, tinyMappingsJar);
+		}
+
+		addDependency(tinyMappingsJar, project, Constants.MAPPINGS_FINAL);
+
 		mappedProvider = new MinecraftMappedProvider();
 		mappedProvider.initFiles(project, minecraftProvider, this);
 		mappedProvider.provide(dependency, project, extension, postPopulationScheduler);
 	}
 
-	public void initFiles(Project project) {
+	private void storeMappings(Project project, MinecraftProvider minecraftProvider, Path yarnJar) throws IOException {
-		LoomGradleExtension extension = project.getExtensions().getByType(LoomGradleExtension.class);
+		project.getLogger().lifecycle(":extracting " + yarnJar.getFileName());
-		MAPPINGS_DIR = new File(extension.getUserCache(), "mappings");
 
-		MAPPINGS_TINY_BASE = new File(MAPPINGS_DIR, mappingsName + "-tiny-" + minecraftVersion + "-" + mappingsVersion + "-base");
+		try (FileSystem fileSystem = FileSystems.newFileSystem(yarnJar, null)) {
-		MAPPINGS_TINY = new File(MAPPINGS_DIR, mappingsName + "-tiny-" + minecraftVersion + "-" + mappingsVersion);
+			extractMappings(fileSystem, baseTinyMappings);
-		MAPPINGS_MIXIN_EXPORT = new File(extension.getProjectBuildCache(), "mixin-map-" + minecraftVersion + "-" + mappingsVersion + ".tiny");
+		}
+
+		if (baseMappingsAreV2()) {
+			// These are unmerged v2 mappings
+
+			// Download and extract intermediary
+			String encodedMinecraftVersion = UrlEscapers.urlFragmentEscaper().escape(minecraftVersion);
+			String intermediaryArtifactUrl = "https://maven.fabricmc.net/net/fabricmc/intermediary/" + encodedMinecraftVersion + "/intermediary-" + encodedMinecraftVersion + "-v2.jar";
+			Path intermediaryJar = mappingsStepsDir.resolve("v2-intermediary-" + minecraftVersion + ".jar");
+			DownloadUtil.downloadIfChanged(new URL(intermediaryArtifactUrl), intermediaryJar.toFile(), project.getLogger());
+
+			mergeAndSaveMappings(project, intermediaryJar, yarnJar);
+		} else {
+			// These are merged v1 mappings
+			if (tinyMappings.exists()) {
+				tinyMappings.delete();
+			}
+
+			project.getLogger().lifecycle(":populating field names");
+			suggestFieldNames(minecraftProvider, baseTinyMappings, tinyMappings.toPath());
+		}
+	}
+
+	private boolean baseMappingsAreV2() throws IOException {
+		try (BufferedReader reader = Files.newBufferedReader(baseTinyMappings)) {
+			TinyV2Factory.readMetadata(reader);
+			return true;
+		} catch (IllegalArgumentException e) {
+			// TODO: just check the mappings version when Parser supports V1 in readMetadata()
+			return false;
+		}
+	}
+
+	private boolean doesJarContainV2Mappings(Path path) throws IOException {
+		try (FileSystem fs = FileSystems.newFileSystem(path, null)) {
+			try (BufferedReader reader = Files.newBufferedReader(fs.getPath("mappings", "mappings.tiny"))) {
+				TinyV2Factory.readMetadata(reader);
+				return true;
+			} catch (IllegalArgumentException e) {
+				return false;
+			}
+		}
+	}
+
+	public static void extractMappings(FileSystem jar, Path extractTo) throws IOException {
+		Files.copy(jar.getPath("mappings/mappings.tiny"), extractTo, StandardCopyOption.REPLACE_EXISTING);
+	}
+
+	private void mergeAndSaveMappings(Project project, Path unmergedIntermediaryJar, Path unmergedYarnJar) throws IOException {
+		Path unmergedIntermediary = Paths.get(mappingsStepsDir.toString(), "unmerged-intermediary.tiny");
+		project.getLogger().info(":extracting " + unmergedIntermediaryJar.getFileName());
+
+		try (FileSystem unmergedIntermediaryFs = FileSystems.newFileSystem(unmergedIntermediaryJar, null)) {
+			extractMappings(unmergedIntermediaryFs, unmergedIntermediary);
+		}
+
+		Path unmergedYarn = Paths.get(mappingsStepsDir.toString(), "unmerged-yarn.tiny");
+		project.getLogger().info(":extracting " + unmergedYarnJar.getFileName());
+
+		try (FileSystem unmergedYarnJarFs = FileSystems.newFileSystem(unmergedYarnJar, null)) {
+			extractMappings(unmergedYarnJarFs, unmergedYarn);
+		}
+
+		Path invertedIntermediary = Paths.get(mappingsStepsDir.toString(), "inverted-intermediary.tiny");
+		reorderMappings(unmergedIntermediary, invertedIntermediary, "intermediary", "official");
+		Path unorderedMergedMappings = Paths.get(mappingsStepsDir.toString(), "unordered-merged.tiny");
+		project.getLogger().info(":merging");
+		mergeMappings(invertedIntermediary, unmergedYarn, unorderedMergedMappings);
+		reorderMappings(unorderedMergedMappings, tinyMappings.toPath(), "official", "intermediary", "named");
+	}
+
+	private void reorderMappings(Path oldMappings, Path newMappings, String... newOrder) {
+		Command command = new CommandReorderTinyV2();
+		String[] args = new String[2 + newOrder.length];
+		args[0] = oldMappings.toAbsolutePath().toString();
+		args[1] = newMappings.toAbsolutePath().toString();
+		System.arraycopy(newOrder, 0, args, 2, newOrder.length);
+		runCommand(command, args);
+	}
+
+	private void mergeMappings(Path intermediaryMappings, Path yarnMappings, Path newMergedMappings) {
+		try {
+			Command command = new CommandMergeTinyV2();
+			runCommand(command, intermediaryMappings.toAbsolutePath().toString(),
+							yarnMappings.toAbsolutePath().toString(),
+							newMergedMappings.toAbsolutePath().toString(),
+							"intermediary", "official");
+		} catch (Exception e) {
+			throw new RuntimeException("Could not merge mappings from " + intermediaryMappings.toString()
+							+ " with mappings from " + yarnMappings, e);
+		}
+	}
+
+	private void suggestFieldNames(MinecraftProvider minecraftProvider, Path oldMappings, Path newMappings) {
+		Command command = new CommandProposeFieldNames();
+		runCommand(command, minecraftProvider.MINECRAFT_MERGED_JAR.getAbsolutePath(),
+						oldMappings.toAbsolutePath().toString(),
+						newMappings.toAbsolutePath().toString());
+	}
+
+	private void runCommand(Command command, String... args) {
+		try {
+			command.run(args);
+		} catch (Exception e) {
+			throw new RuntimeException(e);
+		}
+	}
+
+	private void initFiles(Project project) {
+		LoomGradleExtension extension = project.getExtensions().getByType(LoomGradleExtension.class);
+		mappingsDir = extension.getUserCache().toPath().resolve("mappings");
+		mappingsStepsDir = mappingsDir.resolve("steps");
+
+		baseTinyMappings = mappingsDir.resolve(mappingsName + "-tiny-" + minecraftVersion + "-" + mappingsVersion + "-base");
+		mappingsMixinExport = new File(extension.getProjectBuildCache(), "mixin-map-" + minecraftVersion + "-" + mappingsVersion + ".tiny");
 	}
 
 	@Override

src/main/java/net/fabricmc/loom/providers/MinecraftMappedProvider.java

@@ -39,11 +39,14 @@ public class MinecraftMappedProvider extends DependencyProvider {
 	public File MINECRAFT_MAPPED_JAR;
 	public File MINECRAFT_INTERMEDIARY_JAR;
 
+	public MinecraftMappedProvider() {
+	}
+
 	private MinecraftProvider minecraftProvider;
 
 	@Override
 	public void provide(DependencyInfo dependency, Project project, LoomGradleExtension extension, Consumer<Runnable> postPopulationScheduler) throws Exception {
-		if (!extension.getMappingsProvider().MAPPINGS_TINY.exists()) {
+		if (!extension.getMappingsProvider().tinyMappings.exists()) {
 			throw new RuntimeException("mappings file not found");
 		}
 
@@ -67,17 +70,28 @@ public class MinecraftMappedProvider extends DependencyProvider {
 			throw new RuntimeException("mapped jar not found");
 		}
 
-		String version = minecraftProvider.minecraftVersion + "-mapped-" + extension.getMappingsProvider().mappingsName + "-" + extension.getMappingsProvider().mappingsVersion;
+		MappingsProvider mappingsProvider = extension.getMappingsProvider();
-		project.getDependencies().add(Constants.MINECRAFT_NAMED, project.getDependencies().module("net.minecraft:minecraft:" + version));
+		project.getDependencies().add(Constants.MINECRAFT_NAMED,
-		version = minecraftProvider.minecraftVersion + "-intermediary-" + extension.getMappingsProvider().mappingsName;
+						project.getDependencies().module("net.minecraft:minecraft:" + getNamedJarVersionString(mappingsProvider.mappingsName, mappingsProvider.mappingsVersion)));
-		project.getDependencies().add(Constants.MINECRAFT_INTERMEDIARY, project.getDependencies().module("net.minecraft:minecraft:" + version));
+		project.getDependencies().add(Constants.MINECRAFT_INTERMEDIARY,
+						project.getDependencies().module("net.minecraft:minecraft:" + getIntermediaryJarVersionString(mappingsProvider.mappingsName, mappingsProvider.mappingsVersion)));
 	}
 
 	public void initFiles(Project project, MinecraftProvider minecraftProvider, MappingsProvider mappingsProvider) {
 		LoomGradleExtension extension = project.getExtensions().getByType(LoomGradleExtension.class);
 		this.minecraftProvider = minecraftProvider;
-		MINECRAFT_INTERMEDIARY_JAR = new File(extension.getUserCache(), "minecraft-" + minecraftProvider.minecraftVersion + "-intermediary-" + mappingsProvider.mappingsName + ".jar");
+		MINECRAFT_INTERMEDIARY_JAR = new File(extension.getUserCache(),
-		MINECRAFT_MAPPED_JAR = new File(extension.getUserCache(), "minecraft-" + minecraftProvider.minecraftVersion + "-mapped-" + mappingsProvider.mappingsName + "-" + mappingsProvider.mappingsVersion + ".jar");
+						"minecraft-" + getIntermediaryJarVersionString(mappingsProvider.mappingsName, mappingsProvider.mappingsVersion) + ".jar");
+		MINECRAFT_MAPPED_JAR = new File(extension.getUserCache(),
+						"minecraft-" + getNamedJarVersionString(mappingsProvider.mappingsName, mappingsProvider.mappingsVersion) + ".jar");
+	}
+
+	private String getNamedJarVersionString(String mappingsName, String mappingsVersion) {
+		return minecraftProvider.minecraftVersion + "-mapped-" + mappingsName + "-" + mappingsVersion;
+	}
+
+	private String getIntermediaryJarVersionString(String mappingsName, String mappingsVersion) {
+		return minecraftProvider.minecraftVersion + "-intermediary-" + mappingsName + "-" + mappingsVersion;
 	}
 
 	public Collection<File> getMapperPaths() {

src/main/java/net/fabricmc/loom/task/CleanLoomMappings.java

@@ -36,14 +36,12 @@ import net.fabricmc.loom.util.DeletingFileVisitor;
 public class CleanLoomMappings extends AbstractLoomTask {
 	@TaskAction
 	public void run() {
-		Project project = this.getProject();
-		LoomGradleExtension extension = project.getExtensions().getByType(LoomGradleExtension.class);
-		extension.getMappingsProvider().MAPPINGS_TINY.delete();
-		extension.getMappingsProvider().MAPPINGS_TINY_BASE.delete();
-		extension.getMinecraftMappedProvider().getIntermediaryJar().delete();
-		extension.getMinecraftMappedProvider().getMappedJar().delete();
-
 		try {
+			Project project = this.getProject();
+			LoomGradleExtension extension = project.getExtensions().getByType(LoomGradleExtension.class);
+			extension.getMappingsProvider().clean();
+			extension.getMinecraftMappedProvider().getIntermediaryJar().delete();
+			extension.getMinecraftMappedProvider().getMappedJar().delete();
 			Files.walkFileTree(extension.getRootProjectBuildCache().toPath(), new DeletingFileVisitor());
 		} catch (IOException e) {
 			throw new RuntimeException(e);

src/main/java/net/fabricmc/loom/task/MigrateMappingsTask.java

@@ -24,27 +24,41 @@
 
 package net.fabricmc.loom.task;
 
+import java.io.BufferedReader;
 import java.io.File;
-import java.io.FileInputStream;
 import java.io.IOException;
+import java.net.URL;
+import java.nio.file.FileSystem;
+import java.nio.file.FileSystems;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.function.BiFunction;
 
+import com.google.common.net.UrlEscapers;
+import org.apache.commons.io.FileUtils;
 import org.cadixdev.lorenz.MappingSet;
 import org.cadixdev.lorenz.io.MappingsReader;
+import org.cadixdev.lorenz.model.ClassMapping;
+import org.cadixdev.lorenz.model.Mapping;
 import org.cadixdev.mercury.Mercury;
 import org.cadixdev.mercury.remapper.MercuryRemapper;
 import org.gradle.api.Project;
 import org.gradle.api.tasks.TaskAction;
 
 import net.fabricmc.loom.LoomGradleExtension;
-import net.fabricmc.loom.util.Constants;
+import net.fabricmc.loom.providers.MappingsProvider;
-import net.fabricmc.loom.util.Version;
+import net.fabricmc.loom.providers.MinecraftMappedProvider;
-import net.fabricmc.mappings.ClassEntry;
+import net.fabricmc.loom.util.SourceRemapper;
-import net.fabricmc.mappings.EntryTriple;
+import net.fabricmc.mapping.tree.ClassDef;
-import net.fabricmc.mappings.FieldEntry;
+import net.fabricmc.mapping.tree.Descriptored;
-import net.fabricmc.mappings.Mappings;
+import net.fabricmc.mapping.tree.FieldDef;
-import net.fabricmc.mappings.MethodEntry;
+import net.fabricmc.mapping.tree.MethodDef;
+import net.fabricmc.mapping.tree.TinyMappingFactory;
+import net.fabricmc.mapping.tree.TinyTree;
 
 public class MigrateMappingsTask extends AbstractLoomTask {
 	@TaskAction
@@ -55,75 +69,95 @@ public class MigrateMappingsTask extends AbstractLoomTask {
 
 		project.getLogger().lifecycle(":loading mappings");
 
-		File mappingsFile = null;
+		String inputDir = (String) properties.get("inputDir");
+		if (inputDir == null) inputDir = "src/main/java";
+		String outputDir = (String) properties.get("outputDir");
+		if (outputDir == null) outputDir = "remappedSrc";
+		String officialMappingsVersion = (String) properties.get("targetMappings");
+		String localMappingsPath = (String) properties.get("targetLocalMappings");
 
-		if (properties.containsKey("targetMappingsFile")) {
+		if (officialMappingsVersion != null && localMappingsPath != null) {
-			mappingsFile = new File((String) properties.get("targetMappingsFile"));
+			throw new IllegalArgumentException("targetMappings and targetLocalMappings are mutually exclusive;"
-		} else if (properties.containsKey("targetMappingsArtifact")) {
+							+ " you either specify an official yarn mappings version with targetMappings, "
-			String[] artifactName = ((String) properties.get("targetMappingsArtifact")).split(":");
+							+ "or a local one with targetLocalMappings.");
-
-			if (artifactName.length != 3) {
-				throw new RuntimeException("Invalid artifact name: " + properties.get("targetMappingsArtifact"));
-			}
-
-			String mappingsName = artifactName[0] + "." + artifactName[1];
-
-			Version v = new Version(artifactName[2]);
-			String minecraftVersion = v.getMinecraftVersion();
-			String mappingsVersion = v.getMappingsVersion();
-
-			mappingsFile = new File(extension.getMappingsProvider().MAPPINGS_DIR, mappingsName + "-tiny-" + minecraftVersion + "-" + mappingsVersion);
 		}
 
-		if (mappingsFile == null || !mappingsFile.exists()) {
+		if (officialMappingsVersion == null && localMappingsPath == null) {
-			throw new RuntimeException("Could not find mappings file: " + (mappingsFile != null ? mappingsFile : "null"));
+			throw new IllegalArgumentException("You must specify a new mappings version with -PtargetMappings (or local mappings with -PtargetLocalMappings).");
 		}
 
-		if (!properties.containsKey("inputDir") || !properties.containsKey("outputDir")) {
+		boolean useLocalMappings = localMappingsPath != null;
-			throw new RuntimeException("Must specify input and output dir!");
+
+		if (useLocalMappings && !Files.exists(Paths.get(localMappingsPath))) {
+			throw new IllegalArgumentException("Can't find local mappings in specified location: " + localMappingsPath);
 		}
 
-		File inputDir = new File((String) properties.get("inputDir"));
+		String targetMappingsName = useLocalMappings ? localMappingsPath : officialMappingsVersion;
-		File outputDir = new File((String) properties.get("outputDir"));
 
-		if (!inputDir.exists() || !inputDir.isDirectory()) {
+		Path inputDirPath = Paths.get(System.getProperty("user.dir"), inputDir);
-			throw new RuntimeException("Could not find input directory: " + inputDir);
+		Path outputDirPath = Paths.get(System.getProperty("user.dir"), outputDir);
+
+		if (!Files.exists(inputDirPath) || !Files.isDirectory(inputDirPath)) {
+			throw new IllegalArgumentException("Could not find input directory: " + inputDirPath.toAbsolutePath());
 		}
 
-		if (!outputDir.exists()) {
+		Files.createDirectories(outputDirPath);
-			if (!outputDir.mkdirs()) {
+
-				throw new RuntimeException("Could not create output directory:" + outputDir);
+		MappingsProvider mappingsProvider = extension.getMappingsProvider();
+
+		try {
+			TinyTree currentMappings = mappingsProvider.getMappings();
+			TinyTree targetMappings = getMappings(project, targetMappingsName, useLocalMappings);
+			migrateMappings(project, extension.getMinecraftMappedProvider(), inputDirPath, outputDirPath, currentMappings, targetMappings, extension);
+			project.getLogger().lifecycle(":remapped project written to " + outputDirPath.toAbsolutePath());
+		} catch (IOException e) {
+			throw new IllegalArgumentException("Could not find mappings for version " + officialMappingsVersion, e);
+		}
+	}
+
+	private TinyTree getMappings(Project project, String mappingsVersionOrPath, boolean useLocalMappings) throws IOException {
+		Path migrateMappingsDir = Files.createTempDirectory("migrate");
+		Path localMappingsOfVersion = migrateMappingsDir.resolve(mappingsVersionOrPath + ".tiny");
+
+		if (!Files.exists(localMappingsOfVersion)) {
+			if (useLocalMappings) {
+				Files.copy(Paths.get(mappingsVersionOrPath), localMappingsOfVersion);
+			} else {
+				String versionRelativePath = UrlEscapers.urlFragmentEscaper().escape(mappingsVersionOrPath);
+				String artifactUrl = "https://maven.fabricmc.net/net/fabricmc/yarn/" + versionRelativePath + "/yarn-" + versionRelativePath + ".jar";
+				File mappingsJar = File.createTempFile("migrateMappingsJar", ".jar");
+				project.getLogger().lifecycle(":downloading new mappings from " + artifactUrl);
+				FileUtils.copyURLToFile(new URL(artifactUrl), mappingsJar);
+
+				try (FileSystem jar = FileSystems.newFileSystem(mappingsJar.toPath(), null)) {
+					if (!Files.exists(migrateMappingsDir)) Files.createDirectory(migrateMappingsDir);
+					MappingsProvider.extractMappings(jar, localMappingsOfVersion);
+				}
 			}
 		}
 
-		Mappings sourceMappings = extension.getMappingsProvider().getMappings();
+		try (BufferedReader reader = Files.newBufferedReader(localMappingsOfVersion)) {
-		Mappings targetMappings;
+			return TinyMappingFactory.loadWithDetection(reader);
-
-		try (FileInputStream stream = new FileInputStream(mappingsFile)) {
-			targetMappings = net.fabricmc.mappings.MappingsProvider.readTinyMappings(stream, false);
 		}
+	}
 
+	private static void migrateMappings(Project project, MinecraftMappedProvider minecraftMappedProvider,
+										Path inputDir, Path outputDir, TinyTree currentMappings, TinyTree targetMappings, LoomGradleExtension extension
+	) throws IOException {
 		project.getLogger().lifecycle(":joining mappings");
-		MappingSet mappingSet = new MappingsJoiner(sourceMappings, targetMappings, "intermediary", "named").read();
+		MappingSet mappingSet = new MappingsJoiner(currentMappings, targetMappings,
+						"intermediary", "named").read();
 
 		project.getLogger().lifecycle(":remapping");
-		Mercury mercury = new Mercury();
+		Mercury mercury = SourceRemapper.createMercuryWithClassPath(project, false);
 
-		for (File file : project.getConfigurations().getByName(Constants.MINECRAFT_DEPENDENCIES).getFiles()) {
+		mercury.getClassPath().add(minecraftMappedProvider.MINECRAFT_MAPPED_JAR.toPath());
-			mercury.getClassPath().add(file.toPath());
+		mercury.getClassPath().add(minecraftMappedProvider.MINECRAFT_INTERMEDIARY_JAR.toPath());
-		}
-
-		for (File file : project.getConfigurations().getByName("compileClasspath").getFiles()) {
-			mercury.getClassPath().add(file.toPath());
-		}
-
-		mercury.getClassPath().add(extension.getMinecraftMappedProvider().MINECRAFT_MAPPED_JAR.toPath());
-		mercury.getClassPath().add(extension.getMinecraftMappedProvider().MINECRAFT_INTERMEDIARY_JAR.toPath());
 
 		mercury.getProcessors().add(MercuryRemapper.create(mappingSet));
 
 		try {
-			mercury.rewrite(inputDir.toPath(), outputDir.toPath());
+			mercury.rewrite(inputDir, outputDir);
 		} catch (Exception e) {
 			project.getLogger().warn("Could not remap fully!", e);
 		}
@@ -133,10 +167,19 @@ public class MigrateMappingsTask extends AbstractLoomTask {
 	}
 
 	public static class MappingsJoiner extends MappingsReader {
-		private final Mappings sourceMappings, targetMappings;
+		private final TinyTree sourceMappings, targetMappings;
 		private final String fromNamespace, toNamespace;
 
-		public MappingsJoiner(Mappings sourceMappings, Mappings targetMappings, String fromNamespace, String toNamespace) {
+		/**
+		 * Say A is the source mappings and B is the target mappings.
+		 * It does not map from intermediary to named but rather maps from named-A to named-B, by matching intermediary names.
+		 * It goes through all of the intermediary names of A, and for every such intermediary name, call it I,
+		 * matches the named mapping of I in A, with the named mapping of I in B.
+		 * As you might imagine, this requires intermediary mappings to be stable across all versions.
+		 * Since we only use intermediary names (and not descriptors) to match, and intermediary names are unique,
+		 * this will migrate methods that have had their signature changed too.
+		 */
+		public MappingsJoiner(TinyTree sourceMappings, TinyTree targetMappings, String fromNamespace, String toNamespace) {
 			this.sourceMappings = sourceMappings;
 			this.targetMappings = targetMappings;
 			this.fromNamespace = fromNamespace;
@@ -144,40 +187,51 @@ public class MigrateMappingsTask extends AbstractLoomTask {
 		}
 
 		@Override
-		public MappingSet read(MappingSet mappings) throws IOException {
+		public MappingSet read(MappingSet mappings) {
-			Map<String, ClassEntry> targetClasses = new HashMap<>();
+			Map<String, ClassDef> targetClasses = new HashMap<>();
-			Map<EntryTriple, FieldEntry> targetFields = new HashMap<>();
+			Map<String, FieldDef> targetFields = new HashMap<>();
-			Map<EntryTriple, MethodEntry> targetMethods = new HashMap<>();
+			Map<String, MethodDef> targetMethods = new HashMap<>();
 
-			targetMappings.getClassEntries().forEach((c) -> targetClasses.put(c.get(fromNamespace), c));
+			for (ClassDef newClass : targetMappings.getClasses()) {
-			targetMappings.getFieldEntries().forEach((c) -> targetFields.put(c.get(fromNamespace), c));
+				targetClasses.put(newClass.getName(fromNamespace), newClass);
-			targetMappings.getMethodEntries().forEach((c) -> targetMethods.put(c.get(fromNamespace), c));
 
-			for (ClassEntry entry : sourceMappings.getClassEntries()) {
+				for (FieldDef field : newClass.getFields()) {
-				String from = entry.get(toNamespace);
+					targetFields.put(field.getName(fromNamespace), field);
-				String to = targetClasses.getOrDefault(entry.get(fromNamespace), entry).get(toNamespace);
+				}
 
-				mappings.getOrCreateClassMapping(from).setDeobfuscatedName(to);
+				for (MethodDef method : newClass.getMethods()) {
+					targetMethods.put(method.getName(fromNamespace), method);
+				}
 			}
 
-			for (FieldEntry entry : sourceMappings.getFieldEntries()) {
+			for (ClassDef oldClass : sourceMappings.getClasses()) {
-				EntryTriple fromEntry = entry.get(toNamespace);
+				String namedMappingOfSourceMapping = oldClass.getName(toNamespace);
-				EntryTriple toEntry = targetFields.getOrDefault(entry.get(fromNamespace), entry).get(toNamespace);
+				String namedMappingOfTargetMapping = targetClasses.getOrDefault(oldClass.getName(fromNamespace), oldClass).getName(toNamespace);
 
-				mappings.getOrCreateClassMapping(fromEntry.getOwner()).getOrCreateFieldMapping(fromEntry.getName(), fromEntry.getDesc()).setDeobfuscatedName(toEntry.getName());
+				ClassMapping classMapping = mappings.getOrCreateClassMapping(namedMappingOfSourceMapping).setDeobfuscatedName(namedMappingOfTargetMapping);
-			}
 
-			for (MethodEntry entry : sourceMappings.getMethodEntries()) {
+				mapMembers(oldClass.getFields(), targetFields, classMapping::getOrCreateFieldMapping);
-				EntryTriple fromEntry = entry.get(toNamespace);
+				mapMembers(oldClass.getMethods(), targetMethods, classMapping::getOrCreateMethodMapping);
-				EntryTriple toEntry = targetMethods.getOrDefault(entry.get(fromNamespace), entry).get(toNamespace);
-
-				mappings.getOrCreateClassMapping(fromEntry.getOwner()).getOrCreateMethodMapping(fromEntry.getName(), fromEntry.getDesc()).setDeobfuscatedName(toEntry.getName());
 			}
 
 			return mappings;
 		}
 
+		private <T extends Descriptored> void mapMembers(Collection<T> oldMembers, Map<String, T> newMembers,
+														BiFunction<String, String, Mapping> mapper) {
+			for (T oldMember : oldMembers) {
+				String oldName = oldMember.getName(toNamespace);
+				String oldDescriptor = oldMember.getDescriptor(toNamespace);
+				// We only use the intermediary name (and not the descriptor) because every method has a unique intermediary name
+				String newName = newMembers.getOrDefault(oldMember.getName(fromNamespace), oldMember).getName(toNamespace);
+
+				mapper.apply(oldName, oldDescriptor).setDeobfuscatedName(newName);
+			}
+		}
+
 		@Override
-		public void close() throws IOException { }
+		public void close() {
+		}
 	}
 }
+

src/main/java/net/fabricmc/loom/task/RemapJarTask.java

@@ -76,16 +76,16 @@ public class RemapJarTask extends Jar {
 		String toM = "intermediary";
 
 		Set<File> classpathFiles = new LinkedHashSet<>(
-				project.getConfigurations().getByName("compileClasspath").getFiles()
+						project.getConfigurations().getByName("compileClasspath").getFiles()
 		);
 		Path[] classpath = classpathFiles.stream().map(File::toPath).filter((p) -> !input.equals(p) && Files.exists(p)).toArray(Path[]::new);
 
-		File mixinMapFile = mappingsProvider.MAPPINGS_MIXIN_EXPORT;
+		File mixinMapFile = mappingsProvider.mappingsMixinExport;
 		Path mixinMapPath = mixinMapFile.toPath();
 
 		TinyRemapper.Builder remapperBuilder = TinyRemapper.newRemapper();
 
-		remapperBuilder = remapperBuilder.withMappings(TinyRemapperMappingsHelper.create(mappingsProvider.getMappings(), fromM, toM));
+		remapperBuilder = remapperBuilder.withMappings(TinyRemapperMappingsHelper.create(mappingsProvider.getMappings(), fromM, toM, false));
 
 		if (mixinMapFile.exists()) {
 			remapperBuilder = remapperBuilder.withMappings(TinyUtils.createTinyMappingProvider(mixinMapPath, fromM, toM));

src/main/java/net/fabricmc/loom/util/Constants.java

@@ -50,4 +50,5 @@ public class Constants {
 	public static final String MINECRAFT_NAMED = "minecraftNamed";
 	public static final String MINECRAFT_LINEMAPPED = "minecraftLinemapped";
 	public static final String MAPPINGS = "mappings";
+	public static final String MAPPINGS_FINAL = "mappingsFinal";
 }

src/main/java/net/fabricmc/loom/util/MapJarsTiny.java

@@ -56,7 +56,11 @@ public class MapJarsTiny {
 
 			project.getLogger().lifecycle(":remapping minecraft (TinyRemapper, " + fromM + " -> " + toM + ")");
 
-			TinyRemapper remapper = TinyRemapper.newRemapper().withMappings(TinyRemapperMappingsHelper.create(mappingsProvider.getMappings(), fromM, toM)).renameInvalidLocals(true).rebuildSourceFilenames(true).build();
+			TinyRemapper remapper = TinyRemapper.newRemapper()
+							.withMappings(TinyRemapperMappingsHelper.create(mappingsProvider.getMappings(), fromM, toM, true))
+							.renameInvalidLocals(true)
+							.rebuildSourceFilenames(true)
+							.build();
 
 			try (OutputConsumerPath outputConsumer = new OutputConsumerPath.Builder(output).build()) {
 				outputConsumer.addNonClassFiles(input);
@@ -64,7 +68,7 @@ public class MapJarsTiny {
 				remapper.readInputs(input);
 				remapper.apply(outputConsumer);
 			} catch (Exception e) {
-				throw new RuntimeException("Failed to remap JAR", e);
+				throw new RuntimeException("Failed to remap JAR " + input + " with mappings from " + mappingsProvider.tinyMappings, e);
 			} finally {
 				remapper.finish();
 			}

src/main/java/net/fabricmc/loom/util/ModProcessor.java

@@ -41,13 +41,12 @@ import com.google.gson.Gson;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonObject;
 import org.apache.commons.io.IOUtils;
+import org.gradle.api.Project;
+import org.gradle.api.artifacts.Configuration;
 import org.zeroturnaround.zip.ZipUtil;
 import org.zeroturnaround.zip.commons.FileUtils;
 import org.zeroturnaround.zip.transform.StringZipEntryTransformer;
 import org.zeroturnaround.zip.transform.ZipEntryTransformerEntry;
-import org.gradle.api.Project;
-import org.gradle.api.artifacts.Configuration;
-import org.gradle.internal.impldep.aQute.lib.strings.Strings;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.providers.MappingsProvider;
@@ -110,7 +109,7 @@ public class ModProcessor {
 		JarEntry entry = parentJar.getJarEntry(fileName);
 
 		if (entry == null) {
-			throw new RuntimeException(Strings.format("%s was not found in %s", fileName, parentJar.getName()));
+			throw new RuntimeException(String.format("%s was not found in %s", fileName, parentJar.getName()));
 		}
 
 		File nestedFile = new File(extension.getNestedModCache(), fileName.substring(fileName.lastIndexOf("/")));
@@ -133,7 +132,7 @@ public class ModProcessor {
 
 	private static void stripNestedJars(File file) {
 		//Strip out all contained jar info as we dont want loader to try and load the jars contained in dev.
-		ZipUtil.transformEntries(file, new ZipEntryTransformerEntry[]{(new ZipEntryTransformerEntry("fabric.mod.json", new StringZipEntryTransformer() {
+		ZipUtil.transformEntries(file, new ZipEntryTransformerEntry[] {(new ZipEntryTransformerEntry("fabric.mod.json", new StringZipEntryTransformer() {
 			@Override
 			protected String transform(ZipEntry zipEntry, String input) throws IOException {
 				JsonObject json = GSON.fromJson(input, JsonObject.class);
@@ -151,8 +150,6 @@ public class ModProcessor {
 		MinecraftMappedProvider mappedProvider = extension.getMinecraftMappedProvider();
 		MappingsProvider mappingsProvider = extension.getMappingsProvider();
 
-		File mappingsFile = mappingsProvider.MAPPINGS_TINY;
-		Path mappings = mappingsFile.toPath();
 		Path inputPath = input.getAbsoluteFile().toPath();
 		Path mc = mappedProvider.MINECRAFT_INTERMEDIARY_JAR.toPath();
 		Path[] mcDeps = mappedProvider.getMapperPaths().stream().map(File::toPath).toArray(Path[]::new);
@@ -170,7 +167,9 @@ public class ModProcessor {
 
 		project.getLogger().lifecycle(":remapping " + input.getName() + " (TinyRemapper, " + fromM + " -> " + toM + ")");
 
-		TinyRemapper remapper = TinyRemapper.newRemapper().withMappings(TinyRemapperMappingsHelper.create(mappingsProvider.getMappings(), fromM, toM)).build();
+		TinyRemapper remapper = TinyRemapper.newRemapper()
+						.withMappings(TinyRemapperMappingsHelper.create(mappingsProvider.getMappings(), fromM, toM, false))
+						.build();
 
 		try (OutputConsumerPath outputConsumer = new OutputConsumerPath.Builder(Paths.get(output.getAbsolutePath())).build()) {
 			outputConsumer.addNonClassFiles(inputPath);

src/main/java/net/fabricmc/loom/util/SourceRemapper.java

@@ -31,18 +31,18 @@ import java.nio.file.Path;
 
 import org.cadixdev.lorenz.MappingSet;
 import org.cadixdev.lorenz.io.MappingsReader;
+import org.cadixdev.lorenz.model.ClassMapping;
 import org.cadixdev.mercury.Mercury;
 import org.cadixdev.mercury.remapper.MercuryRemapper;
-import org.zeroturnaround.zip.ZipUtil;
 import org.gradle.api.Project;
+import org.zeroturnaround.zip.ZipUtil;
 
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.providers.MappingsProvider;
-import net.fabricmc.mappings.ClassEntry;
+import net.fabricmc.mapping.tree.ClassDef;
-import net.fabricmc.mappings.EntryTriple;
+import net.fabricmc.mapping.tree.FieldDef;
-import net.fabricmc.mappings.FieldEntry;
+import net.fabricmc.mapping.tree.MethodDef;
-import net.fabricmc.mappings.Mappings;
+import net.fabricmc.mapping.tree.TinyTree;
-import net.fabricmc.mappings.MethodEntry;
 import net.fabricmc.stitch.util.StitchUtil;
 
 public class SourceRemapper {
@@ -58,7 +58,7 @@ public class SourceRemapper {
 
 		MappingSet mappings = extension.getOrCreateSrcMappingCache(toNamed ? 1 : 0, () -> {
 			try {
-				Mappings m = mappingsProvider.getMappings();
+				TinyTree m = mappingsProvider.getMappings();
 				project.getLogger().lifecycle(":loading " + (toNamed ? "intermediary -> named" : "named -> intermediary") + " source mappings");
 				return new TinyReader(m, toNamed ? "intermediary" : "named", toNamed ? "named" : "intermediary").read();
 			} catch (Exception e) {
@@ -69,17 +69,7 @@ public class SourceRemapper {
 		project.getLogger().info(":remapping source jar");
 
 		Mercury mercury = extension.getOrCreateSrcMercuryCache(toNamed ? 1 : 0, () -> {
-			Mercury m = new Mercury();
+			Mercury m = createMercuryWithClassPath(project, toNamed);
-
-			for (File file : project.getConfigurations().getByName(Constants.MINECRAFT_DEPENDENCIES).getFiles()) {
-				m.getClassPath().add(file.toPath());
-			}
-
-			if (!toNamed) {
-				for (File file : project.getConfigurations().getByName("compileClasspath").getFiles()) {
-					m.getClassPath().add(file.toPath());
-				}
-			}
 
 			for (Path file : extension.getUnmappedMods()) {
 				if (Files.isRegularFile(file)) {
@@ -159,6 +149,22 @@ public class SourceRemapper {
 		});
 	}
 
+	public static Mercury createMercuryWithClassPath(Project project, boolean toNamed) {
+		Mercury m = new Mercury();
+
+		for (File file : project.getConfigurations().getByName(Constants.MINECRAFT_DEPENDENCIES).getFiles()) {
+			m.getClassPath().add(file.toPath());
+		}
+
+		if (!toNamed) {
+			for (File file : project.getConfigurations().getByName("compileClasspath").getFiles()) {
+				m.getClassPath().add(file.toPath());
+			}
+		}
+
+		return m;
+	}
+
 	private static boolean isJavaFile(Path path) {
 		String name = path.getFileName().toString();
 		// ".java" is not a valid java file
@@ -166,39 +172,36 @@ public class SourceRemapper {
 	}
 
 	public static class TinyReader extends MappingsReader {
-		private final Mappings m;
+		private final TinyTree mappings;
 		private final String from, to;
 
-		public TinyReader(Mappings m, String from, String to) {
+		public TinyReader(TinyTree m, String from, String to) {
-			this.m = m;
+			this.mappings = m;
 			this.from = from;
 			this.to = to;
 		}
 
 		@Override
 		public MappingSet read(final MappingSet mappings) {
-			for (ClassEntry entry : m.getClassEntries()) {
+			for (ClassDef classDef : this.mappings.getClasses()) {
-				mappings.getOrCreateClassMapping(entry.get(from)).setDeobfuscatedName(entry.get(to));
+				ClassMapping classMapping = mappings.getOrCreateClassMapping(classDef.getName(from))
-			}
+								.setDeobfuscatedName(classDef.getName(to));
 
-			for (FieldEntry entry : m.getFieldEntries()) {
+				for (FieldDef field : classDef.getFields()) {
-				EntryTriple fromEntry = entry.get(from);
+					classMapping.getOrCreateFieldMapping(field.getName(from), field.getDescriptor(from))
-				EntryTriple toEntry = entry.get(to);
+									.setDeobfuscatedName(field.getName(to));
+				}
 
-				mappings.getOrCreateClassMapping(fromEntry.getOwner()).getOrCreateFieldMapping(fromEntry.getName(), fromEntry.getDesc()).setDeobfuscatedName(toEntry.getName());
+				for (MethodDef method : classDef.getMethods()) {
-			}
+					classMapping.getOrCreateMethodMapping(method.getName(from), method.getDescriptor(from))
-
+									.setDeobfuscatedName(method.getName(to));
-			for (MethodEntry entry : m.getMethodEntries()) {
+				}
-				EntryTriple fromEntry = entry.get(from);
-				EntryTriple toEntry = entry.get(to);
-
-				mappings.getOrCreateClassMapping(fromEntry.getOwner()).getOrCreateMethodMapping(fromEntry.getName(), fromEntry.getDesc()).setDeobfuscatedName(toEntry.getName());
 			}
 
 			return mappings;
 		}
 
 		@Override
-		public void close() throws IOException { }
+		public void close() { }
 	}
 }

src/main/java/net/fabricmc/loom/util/TinyRemapperMappingsHelper.java

@@ -24,31 +24,47 @@
 
 package net.fabricmc.loom.util;
 
-import net.fabricmc.mappings.ClassEntry;
+import net.fabricmc.mapping.tree.ClassDef;
-import net.fabricmc.mappings.EntryTriple;
+import net.fabricmc.mapping.tree.FieldDef;
-import net.fabricmc.mappings.FieldEntry;
+import net.fabricmc.mapping.tree.LocalVariableDef;
-import net.fabricmc.mappings.Mappings;
+import net.fabricmc.mapping.tree.MethodDef;
-import net.fabricmc.mappings.MethodEntry;
+import net.fabricmc.mapping.tree.ParameterDef;
+import net.fabricmc.mapping.tree.TinyTree;
 import net.fabricmc.tinyremapper.IMappingProvider;
-import net.fabricmc.tinyremapper.MemberInstance;
 
 public class TinyRemapperMappingsHelper {
 	private TinyRemapperMappingsHelper() { }
 
-	public static IMappingProvider create(Mappings mappings, String from, String to) {
+	private static IMappingProvider.Member memberOf(String className, String memberName, String descriptor) {
-		return (classMap, fieldMap, methodMap) -> {
+		return new IMappingProvider.Member(className, memberName, descriptor);
-			for (ClassEntry entry : mappings.getClassEntries()) {
+	}
-				classMap.put(entry.get(from), entry.get(to));
-			}
 
-			for (FieldEntry entry : mappings.getFieldEntries()) {
+	public static IMappingProvider create(TinyTree mappings, String from, String to, boolean remapLocalVariables) {
-				EntryTriple fromTriple = entry.get(from);
+		return (acceptor) -> {
-				fieldMap.put(fromTriple.getOwner() + "/" + MemberInstance.getFieldId(fromTriple.getName(), fromTriple.getDesc()), entry.get(to).getName());
+			for (ClassDef classDef : mappings.getClasses()) {
-			}
+				String className = classDef.getName(from);
+				acceptor.acceptClass(className, classDef.getName(to));
 
-			for (MethodEntry entry : mappings.getMethodEntries()) {
+				for (FieldDef field : classDef.getFields()) {
-				EntryTriple fromTriple = entry.get(from);
+					acceptor.acceptField(memberOf(className, field.getName(from), field.getDescriptor(from)), field.getName(to));
-				methodMap.put(fromTriple.getOwner() + "/" + MemberInstance.getMethodId(fromTriple.getName(), fromTriple.getDesc()), entry.get(to).getName());
+				}
+
+				for (MethodDef method : classDef.getMethods()) {
+					IMappingProvider.Member methodIdentifier = memberOf(className, method.getName(from), method.getDescriptor(from));
+					acceptor.acceptMethod(methodIdentifier, method.getName(to));
+
+					if (remapLocalVariables) {
+						for (ParameterDef parameter : method.getParameters()) {
+							acceptor.acceptMethodArg(methodIdentifier, parameter.getLocalVariableIndex(), parameter.getName(to));
+						}
+
+						for (LocalVariableDef localVariable : method.getLocalVariables()) {
+							acceptor.acceptMethodVar(methodIdentifier, localVariable.getLocalVariableIndex(),
+											localVariable.getLocalVariableStartOffset(), localVariable.getLocalVariableTableIndex(),
+											localVariable.getName(to));
+						}
+					}
+				}
 			}
 		};
 	}

src/test/groovy/net/fabricmc/loom/BuildUtils.groovy

@@ -16,6 +16,7 @@ archivesBaseName = project.archives_base_name
 version = project.mod_version
 group = project.maven_group
 
+
 minecraft {
 }
 

src/test/groovy/net/fabricmc/loom/EmptyBuildFunctionalTest.groovy

@@ -35,7 +35,7 @@ class EmptyBuildFunctionalTest extends Specification {
 		when:
 		def result = GradleRunner.create()
 				.withProjectDir(testProjectDir.root)
-				.withArguments('build')
+				.withArguments('build',"--stacktrace")
 				.withPluginClasspath()
 				.withGradleVersion("4.9")
 				.build()

src/test/groovy/net/fabricmc/loom/SimpleBuildFunctionalTest.groovy

@@ -45,9 +45,10 @@ class SimpleBuildFunctionalTest extends Specification {
 		when:
 		def result = GradleRunner.create()
 				.withProjectDir(testProjectDir.root)
-				.withArguments('build')
+				.withArguments('build',"--stacktrace")
 				.withPluginClasspath()
 				.withGradleVersion("4.9")
+				.withDebug(true)
 				.build()
 
 		then:
@@ -55,6 +56,7 @@ class SimpleBuildFunctionalTest extends Specification {
 
 		where:
 		mcVersion | yarnVersion       | loaderVersion     | fabricVersion
+		'19w45a'  | '19w45a+build.2:v2'   | '0.6.2+build.166' | '0.4.9+build.258-1.15'
 		'1.14'    | '1.14+build.21'   | '0.4.8+build.155' | '0.3.0+build.184'
 		'1.14.1'  | '1.14.1+build.10' | '0.4.8+build.155' | '0.3.0+build.184'
 		'1.14.2'  | '1.14.2+build.7'  | '0.4.8+build.155' | '0.3.0+build.184'