Add option to drop non root methods when using Mojang mappings. (#1258)

* Attempt to fix #1209

* More layered mappings tests

* Fix build

* Make opt in

* Rename

src/main/java/net/fabricmc/loom/api/mappings/layered/MappingContext.java

@@ -1,7 +1,7 @@
 /*
  * This file is part of fabric-loom, licensed under the MIT License (MIT).
  *
- * Copyright (c) 2016-2021 FabricMC
+ * Copyright (c) 2016-2025 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
@@ -46,6 +46,8 @@ public interface MappingContext {
 
 	Supplier<MemoryMappingTree> intermediaryTree();
 
+	boolean isUsingIntermediateMappings();
+
 	MinecraftProvider minecraftProvider();
 
 	default String minecraftVersion() {
@@ -62,4 +64,6 @@ public interface MappingContext {
 	DownloadBuilder download(String url);
 
 	boolean refreshDeps();
+
+	boolean hasProperty(String property);
 }

src/main/java/net/fabricmc/loom/configuration/providers/mappings/GradleMappingContext.java

@@ -1,7 +1,7 @@
 /*
  * This file is part of fabric-loom, licensed under the MIT License (MIT).
  *
- * Copyright (c) 2018-2021 FabricMC
+ * Copyright (c) 2018-2025 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
@@ -41,6 +41,7 @@ import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.api.mappings.layered.MappingContext;
 import net.fabricmc.loom.configuration.providers.minecraft.MinecraftProvider;
 import net.fabricmc.loom.util.download.DownloadBuilder;
+import net.fabricmc.loom.util.gradle.GradleUtils;
 import net.fabricmc.loom.util.service.ScopedServiceFactory;
 import net.fabricmc.mappingio.tree.MemoryMappingTree;
 
@@ -85,6 +86,11 @@ public class GradleMappingContext implements MappingContext {
 		};
 	}
 
+	@Override
+	public boolean isUsingIntermediateMappings() {
+		return !(extension.getIntermediateMappingsProvider() instanceof NoOpIntermediateMappingsProvider);
+	}
+
 	@Override
 	public MinecraftProvider minecraftProvider() {
 		return extension.getMinecraftProvider();
@@ -110,6 +116,11 @@ public class GradleMappingContext implements MappingContext {
 		return extension.refreshDeps();
 	}
 
+	@Override
+	public boolean hasProperty(String property) {
+		return GradleUtils.getBooleanProperty(project, property);
+	}
+
 	public Project getProject() {
 		return project;
 	}

src/main/java/net/fabricmc/loom/configuration/providers/mappings/mojmap/MojangMappingLayer.java

@@ -1,7 +1,7 @@
 /*
  * This file is part of fabric-loom, licensed under the MIT License (MIT).
  *
- * Copyright (c) 2021 FabricMC
+ * Copyright (c) 2021-2025 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
@@ -30,9 +30,11 @@ import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.List;
+import java.util.function.Supplier;
 import java.util.regex.Pattern;
 
 import org.gradle.api.logging.Logger;
+import org.jetbrains.annotations.Nullable;
 
 import net.fabricmc.loom.api.mappings.layered.MappingLayer;
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
@@ -41,21 +43,51 @@ import net.fabricmc.loom.configuration.providers.mappings.utils.DstNameFilterMap
 import net.fabricmc.mappingio.MappingVisitor;
 import net.fabricmc.mappingio.adapter.MappingSourceNsSwitch;
 import net.fabricmc.mappingio.format.proguard.ProGuardFileReader;
+import net.fabricmc.mappingio.tree.MemoryMappingTree;
 
-public record MojangMappingLayer(Path clientMappings, Path serverMappings, boolean nameSyntheticMembers,
-									Logger logger) implements MappingLayer {
+public record MojangMappingLayer(Path clientMappings, Path serverMappings, boolean nameSyntheticMembers, boolean dropNoneIntermediaryRoots,
+									@Nullable Supplier<MemoryMappingTree> intermediarySupplier, Logger logger) implements MappingLayer {
 	private static final Pattern SYNTHETIC_NAME_PATTERN = Pattern.compile("^(access|this|val\\$this|lambda\\$.*)\\$[0-9]+$");
 
 	@Override
 	public void visit(MappingVisitor mappingVisitor) throws IOException {
 		printMappingsLicense(clientMappings);
 
+		if (!dropNoneIntermediaryRoots) {
+			logger().debug("Not attempting to drop none intermediary roots");
+
+			readMappings(mappingVisitor);
+			return;
+		}
+
+		logger().info("Attempting to drop none intermediary roots");
+
+		if (intermediarySupplier == null) {
+			// Using no-op intermediary mappings
+			readMappings(mappingVisitor);
+			return;
+		}
+
+		// Create a mapping tree with src: official dst: named, intermediary
+		MemoryMappingTree mappingTree = new MemoryMappingTree();
+		intermediarySupplier.get().accept(mappingTree);
+		readMappings(mappingTree);
+
+		// The following code first switches the src namespace to intermediary dropping any entries that don't have an intermediary name
+		// This removes any none root methods before switching it back to official
+		var officialSwitch = new MappingSourceNsSwitch(mappingVisitor, getSourceNamespace().toString(), false);
+		var intermediarySwitch = new MappingSourceNsSwitch(officialSwitch, MappingsNamespace.INTERMEDIARY.toString(), true);
+		mappingTree.accept(intermediarySwitch);
+	}
+
+	private void readMappings(MappingVisitor mappingVisitor) throws IOException {
 		// Filter out field names matching the pattern
-		DstNameFilterMappingVisitor nameFilter = new DstNameFilterMappingVisitor(mappingVisitor, SYNTHETIC_NAME_PATTERN);
+		var nameFilter = new DstNameFilterMappingVisitor(mappingVisitor, SYNTHETIC_NAME_PATTERN);
 
 		// Make official the source namespace
-		MappingSourceNsSwitch nsSwitch = new MappingSourceNsSwitch(nameSyntheticMembers() ? mappingVisitor : nameFilter, MappingsNamespace.OFFICIAL.toString());
+		var nsSwitch = new MappingSourceNsSwitch(nameSyntheticMembers() ? mappingVisitor : nameFilter, MappingsNamespace.OFFICIAL.toString());
 
+		// Read both server and client mappings
 		try (BufferedReader clientBufferedReader = Files.newBufferedReader(clientMappings, StandardCharsets.UTF_8);
 				BufferedReader serverBufferedReader = Files.newBufferedReader(serverMappings, StandardCharsets.UTF_8)) {
 			ProGuardFileReader.read(clientBufferedReader, MappingsNamespace.NAMED.toString(), MappingsNamespace.OFFICIAL.toString(), nsSwitch);

src/main/java/net/fabricmc/loom/configuration/providers/mappings/mojmap/MojangMappingsSpec.java

@@ -1,7 +1,7 @@
 /*
  * This file is part of fabric-loom, licensed under the MIT License (MIT).
  *
- * Copyright (c) 2016-2021 FabricMC
+ * Copyright (c) 2016-2025 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
@@ -30,6 +30,7 @@ import java.nio.file.Path;
 import net.fabricmc.loom.api.mappings.layered.MappingContext;
 import net.fabricmc.loom.api.mappings.layered.spec.MappingsSpec;
 import net.fabricmc.loom.configuration.providers.minecraft.MinecraftVersionMeta;
+import net.fabricmc.loom.util.Constants;
 import net.fabricmc.loom.util.download.DownloadException;
 
 public record MojangMappingsSpec(boolean nameSyntheticMembers) implements MappingsSpec<MojangMappingLayer> {
@@ -66,6 +67,8 @@ public record MojangMappingsSpec(boolean nameSyntheticMembers) implements Mappin
 				clientMappings,
 				serverMappings,
 				nameSyntheticMembers(),
+				context.hasProperty(Constants.Properties.DROP_NON_INTERMEDIATE_ROOT_METHODS),
+				context.isUsingIntermediateMappings() ? context.intermediaryTree() : null,
 				context.getLogger()
 		);
 	}

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

@@ -153,6 +153,10 @@ public class Constants {
 		 * Skip the signature verification of the Minecraft jar after downloading it.
 		 */
 		public static final String DISABLE_MINECRAFT_VERIFICATION = "fabric.loom.disableMinecraftVerification";
+		/**
+		 * When using the MojangMappingLayer this will remove names for non root methods by using the intermediary mappings.
+		 */
+		public static final String DROP_NON_INTERMEDIATE_ROOT_METHODS = "fabric.loom.dropNonIntermediateRootMethods";
 	}
 
 	public static final class Manifest {