Even more work :)

src/main/java/net/fabricmc/loom/configuration/decompile/SingleJarDecompileConfiguration.java

@@ -58,7 +58,7 @@ public final void afterEvaluation() {
 			// Decompiler will be passed to the constructor of GenerateSourcesTask
 			project.getTasks().register(taskName, GenerateSourcesTask.class, options).configure(task -> {
 				task.getInputJarName().set(minecraftJar.getName());
-				task.getOutputJar().fileValue(GenerateSourcesTask.getMappedJarFileWithSuffix("-sources.jar", minecraftJar.getPath()));
+				task.getOutputJar().fileValue(GenerateSourcesTask.getJarFileWithSuffix("-sources.jar", minecraftJar.getPath()));
 
 				task.dependsOn(project.getTasks().named("validateAccessWidener"));
 				task.setDescription("Decompile minecraft using %s.".formatted(decompilerName));

src/main/java/net/fabricmc/loom/configuration/decompile/SplitDecompileConfiguration.java

@@ -55,7 +55,7 @@ public void afterEvaluation() {
 
 		final TaskProvider<Task> commonDecompileTask = createDecompileTasks("Common", task -> {
 			task.getInputJarName().set(commonJar.getName());
-			task.getOutputJar().fileValue(GenerateSourcesTask.getMappedJarFileWithSuffix("-sources.jar", commonJar.getPath()));
+			task.getOutputJar().fileValue(GenerateSourcesTask.getJarFileWithSuffix("-sources.jar", commonJar.getPath()));
 
 			if (mappingConfiguration.hasUnpickDefinitions()) {
 				File unpickJar = new File(extension.getMappingConfiguration().mappingsWorkingDir().toFile(), "minecraft-common-unpicked.jar");
@@ -65,7 +65,7 @@ public void afterEvaluation() {
 
 		final TaskProvider<Task> clientOnlyDecompileTask = createDecompileTasks("ClientOnly", task -> {
 			task.getInputJarName().set(clientOnlyJar.getName());
-			task.getOutputJar().fileValue(GenerateSourcesTask.getMappedJarFileWithSuffix("-sources.jar", clientOnlyJar.getPath()));
+			task.getOutputJar().fileValue(GenerateSourcesTask.getJarFileWithSuffix("-sources.jar", clientOnlyJar.getPath()));
 
 			if (mappingConfiguration.hasUnpickDefinitions()) {
 				File unpickJar = new File(extension.getMappingConfiguration().mappingsWorkingDir().toFile(), "minecraft-clientonly-unpicked.jar");

src/main/java/net/fabricmc/loom/decompilers/ClassLineNumbers.java

@@ -24,6 +24,8 @@
 
 package net.fabricmc.loom.decompilers;
 
+import org.jetbrains.annotations.Nullable;
+
 import static java.text.MessageFormat.format;
 
 import java.io.BufferedReader;
@@ -38,6 +40,14 @@
 import java.util.Objects;
 
 public record ClassLineNumbers(Map<String, ClassLineNumbers.Entry> lineMap) {
+	public ClassLineNumbers {
+		Objects.requireNonNull(lineMap, "lineMap");
+
+		if (lineMap.isEmpty()) {
+			throw new IllegalArgumentException("lineMap is empty");
+		}
+	}
+
 	public static ClassLineNumbers readMappings(Path lineMappingsPath) {
 		try (BufferedReader reader = Files.newBufferedReader(lineMappingsPath)) {
 			return readMappings(reader);
@@ -108,7 +118,14 @@ public void write(Writer writer) throws IOException {
 	/**
 	 * Merge two ClassLineNumbers together, throwing an exception if there are any duplicate class line mappings.
 	 */
-	public static ClassLineNumbers merge(ClassLineNumbers a, ClassLineNumbers b) {
+	@Nullable
+	public static ClassLineNumbers merge(@Nullable ClassLineNumbers a, @Nullable ClassLineNumbers b) {
+		if (a == null) {
+			return b;
+		} else if (b == null) {
+			return a;
+		}
+
 		var lineMap = new HashMap<>(a.lineMap());
 
 		for (Map.Entry<String, Entry> entry : b.lineMap().entrySet()) {

src/main/java/net/fabricmc/loom/decompilers/LineNumberRemapper.java

@@ -38,12 +38,15 @@
 import org.objectweb.asm.ClassWriter;
 import org.objectweb.asm.Label;
 import org.objectweb.asm.MethodVisitor;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import net.fabricmc.loom.util.Constants;
-import net.fabricmc.loom.util.IOStringConsumer;
 
 public record LineNumberRemapper(ClassLineNumbers lineNumbers) {
-	public void process(IOStringConsumer logger, Path input, Path output) throws IOException {
+	private static final Logger LOGGER = LoggerFactory.getLogger(LineNumberRemapper.class);
+
+	public void process(Path input, Path output) throws IOException {
 		Files.walkFileTree(input, new SimpleFileVisitor<>() {
 			@Override
 			public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
@@ -64,9 +67,7 @@ public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IO
 
 					String idx = rel.substring(0, rel.length() - 6);
 
-					if (logger != null) {
-						logger.accept("Remapping " + idx);
-					}
+					LOGGER.debug("Remapping line numbers for class: " + idx);
 
 					int dollarPos = idx.indexOf('$'); //This makes the assumption that only Java classes are to be remapped.
 

src/main/java/net/fabricmc/loom/decompilers/cache/CachedFileStoreImpl.java

@@ -35,11 +35,16 @@
 import java.util.Comparator;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Stream;
 
 import org.jetbrains.annotations.Nullable;
 
 public record CachedFileStoreImpl<T>(Path root, EntrySerializer<T> entrySerializer, CacheRules cacheRules) implements CachedFileStore<T> {
+	public CachedFileStoreImpl {
+		Objects.requireNonNull(root, "root");
+	}
+
 	@Override
 	public @Nullable T getEntry(String key) throws IOException {
 		Path path = resolve(key);
@@ -56,6 +61,7 @@ public record CachedFileStoreImpl<T>(Path root, EntrySerializer<T> entrySerializ
 	@Override
 	public void putEntry(String key, T data) throws IOException {
 		Path path = resolve(key);
+		Files.createDirectories(path.getParent());
 		entrySerializer.write(data, path);
 	}
 

src/main/java/net/fabricmc/loom/decompilers/cache/CachedJarProcessor.java

@@ -34,6 +34,7 @@
 import java.util.Map;
 import java.util.stream.Stream;
 
+import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -65,7 +66,7 @@ public WorkRequest prepareJob(Path inputJar) throws IOException {
 
 			for (ClassEntry entry : inputClasses) {
 				String outputFileName = entry.sourcesFileName();
-				String fullHash = baseHash + entry.hash(inputFs.getRoot());
+				String fullHash = baseHash + "/" + entry.hash(inputFs.getRoot());
 
 				final CachedData entryData = fileStore.getEntry(fullHash);
 
@@ -92,7 +93,7 @@ public WorkRequest prepareJob(Path inputJar) throws IOException {
 		Path outputJar = Files.createTempFile("loom-cache-output", ".jar");
 		Files.delete(outputJar);
 
-		final var lineNumbers = new ClassLineNumbers(Collections.unmodifiableMap(lineNumbersMap));
+		final ClassLineNumbers lineNumbers = lineNumbersMap.isEmpty() ? null : new ClassLineNumbers(Collections.unmodifiableMap(lineNumbersMap));
 
 		if (isIncomplete && !hasSomeExisting) {
 			// The cache contained nothing of use, fully process the input jar
@@ -149,7 +150,13 @@ public void completeJob(Path output, WorkJob workJob, ClassLineNumbers lineNumbe
 					final String className = fsPath.toString().substring(1, fsPath.toString().length() - ".java".length());
 					final String sources = Files.readString(fsPath);
 
-					final var cachedData = new CachedData(className, sources, lineNumbers.lineMap().get(className));
+					ClassLineNumbers.Entry lineMapEntry = null;
+
+					if (lineNumbers != null) {
+						lineMapEntry = lineNumbers.lineMap().get(className);
+					}
+
+					final var cachedData = new CachedData(className, sources, lineMapEntry);
 					fileStore.putEntry(hash, cachedData);
 
 					LOGGER.debug("Saving processed entry to cache: {}", fsPath);
@@ -191,11 +198,11 @@ public void completeJob(Path output, WorkJob workJob, ClassLineNumbers lineNumbe
 		}
 	}
 
-	public record WorkRequest(WorkJob job, ClassLineNumbers lineNumbers) {
+	public record WorkRequest(WorkJob job, @Nullable ClassLineNumbers lineNumbers) {
 	}
 
 	public sealed interface WorkJob permits CompletedWorkJob, WorkToDoJob {
-		default WorkRequest asRequest(ClassLineNumbers lineNumbers) {
+		default WorkRequest asRequest(@Nullable ClassLineNumbers lineNumbers) {
 			return new WorkRequest(this, lineNumbers);
 		}
 	}

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

@@ -50,4 +50,5 @@ static LoomFiles create(Settings settings) {
 	File getRemapClasspathFile();
 	File getGlobalMinecraftRepo();
 	File getLocalMinecraftRepo();
+	File getDecompileCache(String version);
 }

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

@@ -107,4 +107,9 @@ public File getGlobalMinecraftRepo() {
 	public File getLocalMinecraftRepo() {
 		return new File(getRootProjectPersistentCache(), "minecraftMaven");
 	}
+
+	@Override
+	public File getDecompileCache(String version) {
+		return new File(getUserCache(), "decompile/" + version + ".cache");
+	}
 }