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 {

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

@@ -1,7 +1,7 @@
 /*
  * This file is part of fabric-loom, licensed under the MIT License (MIT).
  *
- * Copyright (c) 2018-2022 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
@@ -28,11 +28,73 @@ import spock.lang.Specification
 import spock.lang.Unroll
 
 import net.fabricmc.loom.test.util.GradleProjectTestTrait
+import net.fabricmc.loom.util.Constants
 
-import static net.fabricmc.loom.test.LoomTestConstants.*
+import static net.fabricmc.loom.test.LoomTestConstants.DEFAULT_GRADLE
+import static net.fabricmc.loom.test.LoomTestConstants.STANDARD_TEST_VERSIONS
 import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
 
-class MojangMappingsProjectTest extends Specification implements GradleProjectTestTrait {
+class LayeredMappingsTest extends Specification implements GradleProjectTestTrait {
+	def "build #layer"() {
+		setup:
+		def gradle = gradleProject(project: "minimalBase", version: DEFAULT_GRADLE)
+		gradle.buildGradle << """
+            repositories {
+                maven {
+                    name = 'ParchmentMC'
+                    url = 'https://maven.parchmentmc.org'
+                }
+            }
+            dependencies {
+                minecraft "com.mojang:minecraft:1.21.4"
+                mappings loom.layered {
+                    $layer
+                }
+            }
+        """
+
+		if (layer.contains("// Drop none roots")) {
+			new File(gradle.projectDir, "gradle.properties").text = "${Constants.Properties.DROP_NON_INTERMEDIATE_ROOT_METHODS}=true"
+		}
+
+		when:
+		def result = gradle.run(task: "build")
+
+		then:
+		result.task(":build").outcome == SUCCESS
+
+		where:
+		layer << [
+			// Only mojang mappings
+			"""
+                officialMojangMappings()
+            """,
+			// Yarn on top of Mojmap
+			"""
+				// Drop none roots
+                officialMojangMappings()
+                mappings("net.fabricmc:yarn:1.21.4+build.8:v2")
+            """,
+			// Mojmap on top of yarn
+			"""
+                mappings("net.fabricmc:yarn:1.21.4+build.8:v2")
+                officialMojangMappings()
+            """,
+			//  Mojmap with parchment
+			"""
+                officialMojangMappings()
+                parchment("org.parchmentmc.data:parchment-1.21.4:2025.01.19@zip")
+            """,
+			// Yarn on top of Mojmap with parchment
+			"""
+				// Drop none roots
+                officialMojangMappings()
+                parchment("org.parchmentmc.data:parchment-1.21.4:2025.01.19@zip")
+                mappings("net.fabricmc:yarn:1.21.4+build.8:v2")
+            """,
+		]
+	}
+
 	@Unroll
 	def "build (gradle #version)"() {
 		setup:
@@ -146,4 +208,19 @@ class MojangMappingsProjectTest extends Specification implements GradleProjectTe
 		where:
 		version << STANDARD_TEST_VERSIONS
 	}
+
+	@Unroll
+	def "parchment #version"() {
+		setup:
+		def gradle = gradleProject(project: "parchment", version: version)
+
+		when:
+		def result = gradle.run(task: "build")
+
+		then:
+		result.task(":build").outcome == SUCCESS
+
+		where:
+		version << STANDARD_TEST_VERSIONS
+	}
 }

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

@@ -1,50 +0,0 @@
-/*
- * This file is part of fabric-loom, licensed under the MIT License (MIT).
- *
- * Copyright (c) 2018-2021 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
-
-import spock.lang.Specification
-import spock.lang.Unroll
-
-import net.fabricmc.loom.test.util.GradleProjectTestTrait
-
-import static net.fabricmc.loom.test.LoomTestConstants.STANDARD_TEST_VERSIONS
-import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
-
-class ParchmentTest extends Specification implements GradleProjectTestTrait {
-	@Unroll
-	def "parchment #version"() {
-		setup:
-		def gradle = gradleProject(project: "parchment", version: version)
-
-		when:
-		def result = gradle.run(task: "build")
-
-		then:
-		result.task(":build").outcome == SUCCESS
-
-		where:
-		version << STANDARD_TEST_VERSIONS
-	}
-}

src/test/groovy/net/fabricmc/loom/test/unit/layeredmappings/LayeredMappingsSpecification.groovy

@@ -48,6 +48,7 @@ import net.fabricmc.loom.configuration.providers.mappings.utils.AddConstructorMa
 import net.fabricmc.loom.configuration.providers.minecraft.MinecraftProvider
 import net.fabricmc.loom.test.LoomTestConstants
 import net.fabricmc.loom.test.unit.LoomMocks
+import net.fabricmc.loom.util.Constants
 import net.fabricmc.loom.util.download.Download
 import net.fabricmc.loom.util.download.DownloadBuilder
 import net.fabricmc.mappingio.MappingReader
@@ -60,7 +61,6 @@ abstract class LayeredMappingsSpecification extends Specification implements Lay
 	Logger mockLogger = Mock(Logger)
 	MinecraftProvider mockMinecraftProvider = Mock(MinecraftProvider)
 	String intermediaryUrl
-	MappingContext mappingContext = new TestMappingContext()
 
 	File tempDir = new File(LoomTestConstants.TEST_DIR, "layered/${getClass().name}")
 
@@ -92,18 +92,25 @@ abstract class LayeredMappingsSpecification extends Specification implements Lay
 
 	MemoryMappingTree getSingleMapping(MappingsSpec<? extends MappingLayer> spec) {
 		MemoryMappingTree mappingTree = new MemoryMappingTree()
-		spec.createLayer(mappingContext).visit(mappingTree)
+		spec.createLayer(new TestMappingContext([spec])).visit(mappingTree)
 		return mappingTree
 	}
 
 	MemoryMappingTree getLayeredMappings(MappingsSpec<? extends MappingLayer>... specs) {
 		LayeredMappingsProcessor processor = createLayeredMappingsProcessor(specs)
-		return processor.getMappings(processor.resolveLayers(mappingContext))
+		return processor.getMappings(processor.resolveLayers(new TestMappingContext(specs.toList())))
+	}
+
+	MemoryMappingTree getLayeredMappingsDropNoneIntermediaryRoots(MappingsSpec<? extends MappingLayer>... specs) {
+		LayeredMappingsProcessor processor = createLayeredMappingsProcessor(specs)
+		return processor.getMappings(processor.resolveLayers(new TestMappingContext(specs.toList(), [
+			Constants.Properties.DROP_NON_INTERMEDIATE_ROOT_METHODS
+		])))
 	}
 
 	UnpickLayer.UnpickData getUnpickData(MappingsSpec<? extends MappingLayer>... specs) {
 		LayeredMappingsProcessor processor = createLayeredMappingsProcessor(specs)
-		return processor.getUnpickData(processor.resolveLayers(mappingContext))
+		return processor.getUnpickData(processor.resolveLayers(new TestMappingContext(specs.toList())))
 	}
 
 	private static LayeredMappingsProcessor createLayeredMappingsProcessor(MappingsSpec<? extends MappingLayer>... specs) {
@@ -134,6 +141,14 @@ abstract class LayeredMappingsSpecification extends Specification implements Lay
 	}
 
 	class TestMappingContext implements MappingContext {
+		private final List<MappingsSpec<? extends MappingLayer>> specs
+		private final List<String> enabledProperties
+
+		TestMappingContext(List<MappingsSpec<? extends MappingLayer>> specs, List<String> enabledProperties = []) {
+			this.specs = specs
+			this.enabledProperties = enabledProperties
+		}
+
 		@Override
 		Path resolveDependency(Dependency dependency) {
 			throw new UnsupportedOperationException("TODO")
@@ -165,6 +180,11 @@ abstract class LayeredMappingsSpecification extends Specification implements Lay
 			}
 		}
 
+		@Override
+		boolean isUsingIntermediateMappings() {
+			return !specs.any { it instanceof NoIntermediateMappingsSpec }
+		}
+
 		@Override
 		MinecraftProvider minecraftProvider() {
 			return mockMinecraftProvider
@@ -189,6 +209,11 @@ abstract class LayeredMappingsSpecification extends Specification implements Lay
 		boolean refreshDeps() {
 			return false
 		}
+
+		@Override
+		boolean hasProperty(String property) {
+			return enabledProperties.contains(property)
+		}
 	}
 
 	@EqualsAndHashCode

src/test/groovy/net/fabricmc/loom/test/unit/layeredmappings/MojangMappingLayerTest.groovy

@@ -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
@@ -70,6 +70,48 @@ class MojangMappingLayerTest extends LayeredMappingsSpecification {
 		!tiny.contains('this$0')
 	}
 
+	def "Read mojang mappings with synthetic field names drop roots" () {
+		setup:
+		intermediaryUrl = INTERMEDIARY_1_17_URL
+		mockMinecraftProvider.getVersionInfo() >> VERSION_META_1_17
+		mockMinecraftProvider.minecraftVersion() >> "1.17"
+		when:
+		def mappings = getLayeredMappingsDropNoneIntermediaryRoots(
+				new IntermediaryMappingsSpec(),
+				buildMojangMappingsSpec(true)
+				)
+		def tiny = getTiny(mappings)
+		then:
+		mappings.srcNamespace == "named"
+		mappings.dstNamespaces == ["intermediary", "official"]
+		mappings.classes.size() == 6107
+		mappings.classes[0].srcName.hashCode() == 1869546970 // MojMap name, just check the hash
+		mappings.classes[0].getDstName(0) == "net/minecraft/class_2354"
+		mappings.classes[0].methods[0].args.size() == 0 // No Args
+		tiny.contains('this$0')
+	}
+
+	def "Read mojang mappings without synthetic field names drop roots" () {
+		setup:
+		intermediaryUrl = INTERMEDIARY_1_17_URL
+		mockMinecraftProvider.getVersionInfo() >> VERSION_META_1_17
+		mockMinecraftProvider.minecraftVersion() >> "1.17"
+		when:
+		def mappings = getLayeredMappingsDropNoneIntermediaryRoots(
+				new IntermediaryMappingsSpec(),
+				buildMojangMappingsSpec(false)
+				)
+		def tiny = getTiny(mappings)
+		then:
+		mappings.srcNamespace == "named"
+		mappings.dstNamespaces == ["intermediary", "official"]
+		mappings.classes.size() == 6107
+		mappings.classes[0].srcName.hashCode() == 1869546970 // MojMap name, just check the hash
+		mappings.classes[0].getDstName(0) == "net/minecraft/class_2354"
+		mappings.classes[0].methods[0].args.size() == 0 // No Args
+		!tiny.contains('this$0')
+	}
+
 	def "Read mojang mappings with no intermediary" () {
 		setup:
 		intermediaryUrl = INTERMEDIARY_1_17_URL