Fix record signatures based on data provided by mappings. (#507)

Reads "extras/record_signatures.json" from mappings jar, just works with yarn.

Mojmap:

```
mappings loom.layered {
   officialMojangMappings()
   signatureFix("net.fabricmc:yarn:21w38a+build.9:v2")
}
```

17 files changed, 347 insertions, 46 deletions

diff --git a/src/main/java/net/fabricmc/loom/api/mappings/layered/spec/LayeredMappingSpecBuilder.java b/src/main/java/net/fabricmc/loom/api/mappings/layered/spec/LayeredMappingSpecBuilder.java
index 05180172..fd16f23c 100644
--- a/src/main/java/net/fabricmc/loom/api/mappings/layered/spec/LayeredMappingSpecBuilder.java
+++ b/src/main/java/net/fabricmc/loom/api/mappings/layered/spec/LayeredMappingSpecBuilder.java
@@ -47,4 +47,10 @@ public interface LayeredMappingSpecBuilder {
 	}
 
 	LayeredMappingSpecBuilder parchment(Object object, Action<ParchmentMappingsSpecBuilder> action);
+
+	/**
+	 * Add a signatureFix layer. Reads the @extras/record_signatures.json" file in a jar file such as yarn.
+	 */
+	@ApiStatus.Experimental
+	LayeredMappingSpecBuilder signatureFix(Object object);
 }
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingSpecBuilderImpl.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingSpecBuilderImpl.java
index 4f47cfbf..ffa0ab12 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingSpecBuilderImpl.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingSpecBuilderImpl.java
@@ -34,6 +34,7 @@ import net.fabricmc.loom.api.mappings.layered.spec.FileSpec;
 import net.fabricmc.loom.api.mappings.layered.spec.LayeredMappingSpecBuilder;
 import net.fabricmc.loom.api.mappings.layered.spec.MappingsSpec;
 import net.fabricmc.loom.api.mappings.layered.spec.ParchmentMappingsSpecBuilder;
+import net.fabricmc.loom.configuration.providers.mappings.extras.signatures.SignatureFixesSpec;
 import net.fabricmc.loom.configuration.providers.mappings.intermediary.IntermediaryMappingsSpec;
 import net.fabricmc.loom.configuration.providers.mappings.mojmap.MojangMappingsSpec;
 import net.fabricmc.loom.configuration.providers.mappings.parchment.ParchmentMappingsSpecBuilderImpl;
@@ -59,6 +60,11 @@ public class LayeredMappingSpecBuilderImpl implements LayeredMappingSpecBuilder
 		return addLayer(builder.build());
 	}
 
+	@Override
+	public LayeredMappingSpecBuilder signatureFix(Object object) {
+		return addLayer(new SignatureFixesSpec(FileSpec.create(object)));
+	}
+
 	public LayeredMappingSpec build() {
 		List<MappingsSpec<?>> builtLayers = new LinkedList<>();
 		// Intermediary is always the base layer
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsDependency.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsDependency.java
index e15f6239..90da9367 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsDependency.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsDependency.java
@@ -32,6 +32,8 @@ import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.Collections;
+import java.util.List;
+import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
 
@@ -44,6 +46,7 @@ import org.zeroturnaround.zip.ZipUtil;
 
 import net.fabricmc.loom.LoomGradlePlugin;
 import net.fabricmc.loom.api.mappings.layered.MappingContext;
+import net.fabricmc.loom.api.mappings.layered.MappingLayer;
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
 import net.fabricmc.mappingio.adapter.MappingDstNsReorder;
 import net.fabricmc.mappingio.adapter.MappingSourceNsSwitch;
@@ -72,29 +75,51 @@ public class LayeredMappingsDependency implements SelfResolvingDependency {
 		if (!Files.exists(mappingsFile) || LoomGradlePlugin.refreshDeps) {
 			try {
 				var processor = new LayeredMappingsProcessor(layeredMappingSpec);
-				MemoryMappingTree mappings = processor.getMappings(mappingContext);
+				List<MappingLayer> layers = processor.resolveLayers(mappingContext);
 
-				try (Writer writer = new StringWriter()) {
-					Tiny2Writer tiny2Writer = new Tiny2Writer(writer, false);
+				Files.deleteIfExists(mappingsFile);
 
-					MappingDstNsReorder nsReorder = new MappingDstNsReorder(tiny2Writer, Collections.singletonList(MappingsNamespace.NAMED.toString()));
-					MappingSourceNsSwitch nsSwitch = new MappingSourceNsSwitch(nsReorder, MappingsNamespace.INTERMEDIARY.toString(), true);
-					mappings.accept(nsSwitch);
-
-					Files.deleteIfExists(mappingsFile);
-
-					ZipUtil.pack(new ZipEntrySource[] {
-							new ByteSource("mappings/mappings.tiny", writer.toString().getBytes(StandardCharsets.UTF_8))
-					}, mappingsFile.toFile());
-				}
+				writeMapping(processor, layers, mappingsFile);
+				writeSignatureFixes(processor, layers, mappingsFile);
 			} catch (IOException e) {
-				throw new RuntimeException("Failed to resolve Mojang mappings", e);
+				throw new RuntimeException("Failed to resolve layered mappings", e);
 			}
 		}
 
 		return Collections.singleton(mappingsFile.toFile());
 	}
 
+	private void writeMapping(LayeredMappingsProcessor processor, List<MappingLayer> layers, Path mappingsFile) throws IOException {
+		MemoryMappingTree mappings = processor.getMappings(layers);
+
+		try (Writer writer = new StringWriter()) {
+			Tiny2Writer tiny2Writer = new Tiny2Writer(writer, false);
+
+			MappingDstNsReorder nsReorder = new MappingDstNsReorder(tiny2Writer, Collections.singletonList(MappingsNamespace.NAMED.toString()));
+			MappingSourceNsSwitch nsSwitch = new MappingSourceNsSwitch(nsReorder, MappingsNamespace.INTERMEDIARY.toString(), true);
+			mappings.accept(nsSwitch);
+
+			ZipUtil.pack(new ZipEntrySource[] {
+					new ByteSource("mappings/mappings.tiny", writer.toString().getBytes(StandardCharsets.UTF_8))
+			}, mappingsFile.toFile());
+		}
+	}
+
+	private void writeSignatureFixes(LayeredMappingsProcessor processor, List<MappingLayer> layers, Path mappingsFile) throws IOException {
+		Map<String, String> signatureFixes = processor.getSignatureFixes(layers);
+
+		if (signatureFixes == null) {
+			return;
+		}
+
+		byte[] data = LoomGradlePlugin.OBJECT_MAPPER.writeValueAsString(signatureFixes).getBytes(StandardCharsets.UTF_8);
+
+		ZipUtil.addEntry(
+				mappingsFile.toFile(),
+				new ByteSource("extras/record_signatures.json", data)
+		);
+	}
+
 	@Override
 	public Set<File> resolve(boolean transitive) {
 		return resolve();
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsProcessor.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsProcessor.java
index b14902c5..f2edfb2b 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsProcessor.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/LayeredMappingsProcessor.java
@@ -26,12 +26,19 @@ package net.fabricmc.loom.configuration.providers.mappings;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.LinkedList;
 import java.util.List;
+import java.util.Map;
+
+import org.jetbrains.annotations.Nullable;
 
 import net.fabricmc.loom.api.mappings.layered.MappingContext;
 import net.fabricmc.loom.api.mappings.layered.MappingLayer;
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
 import net.fabricmc.loom.api.mappings.layered.spec.MappingsSpec;
+import net.fabricmc.loom.configuration.providers.mappings.extras.signatures.SignatureFixesLayer;
 import net.fabricmc.mappingio.adapter.MappingSourceNsSwitch;
 import net.fabricmc.mappingio.tree.MemoryMappingTree;
 
@@ -42,9 +49,8 @@ public class LayeredMappingsProcessor {
 		this.layeredMappingSpec = spec;
 	}
 
-	public MemoryMappingTree getMappings(MappingContext context) throws IOException {
-		MemoryMappingTree mappingTree = new MemoryMappingTree();
-
+	public List<MappingLayer> resolveLayers(MappingContext context) {
+		List<MappingLayer> layers = new LinkedList<>();
 		List<Class<? extends MappingLayer>> visitedLayers = new ArrayList<>();
 
 		for (MappingsSpec<?> spec : layeredMappingSpec.layers()) {
@@ -56,8 +62,17 @@ public class LayeredMappingsProcessor {
 				}
 			}
 
+			layers.add(layer);
 			visitedLayers.add(layer.getClass());
+		}
+
+		return Collections.unmodifiableList(layers);
+	}
+
+	public MemoryMappingTree getMappings(List<MappingLayer> layers) throws IOException {
+		MemoryMappingTree mappingTree = new MemoryMappingTree();
 
+		for (MappingLayer layer : layers) {
 			// We have to rebuild a new tree to work on when a layer doesnt merge into layered
 			boolean rebuild = layer.getSourceNamespace() != MappingsNamespace.NAMED;
 			MemoryMappingTree workingTree;
@@ -90,4 +105,21 @@ public class LayeredMappingsProcessor {
 
 		return mappingTree;
 	}
+
+	@Nullable
+	public Map<String, String> getSignatureFixes(List<MappingLayer> layers) {
+		Map<String, String> signatureFixes = new HashMap<>();
+
+		for (MappingLayer layer : layers) {
+			if (layer instanceof SignatureFixesLayer signatureFixesLayer) {
+				signatureFixes.putAll(signatureFixesLayer.getSignatureFixes());
+			}
+		}
+
+		if (signatureFixes.isEmpty()) {
+			return null;
+		}
+
+		return Collections.unmodifiableMap(signatureFixes);
+	}
 }
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingsProviderImpl.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingsProviderImpl.java
index d2cc3d03..f099e3c1 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingsProviderImpl.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingsProviderImpl.java
@@ -27,6 +27,7 @@ package net.fabricmc.loom.configuration.providers.mappings;
 import java.io.BufferedReader;
 import java.io.File;
 import java.io.IOException;
+import java.io.Reader;
 import java.net.URL;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.FileSystem;
@@ -37,6 +38,7 @@ import java.nio.file.StandardCopyOption;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.Objects;
+import java.util.Map;
 import java.util.function.Consumer;
 import java.util.regex.Pattern;
 
@@ -45,6 +47,7 @@ import com.google.common.net.UrlEscapers;
 import com.google.gson.JsonObject;
 import org.apache.tools.ant.util.StringUtils;
 import org.gradle.api.Project;
+import org.jetbrains.annotations.Nullable;
 import org.zeroturnaround.zip.FileSource;
 import org.zeroturnaround.zip.ZipEntrySource;
 import org.zeroturnaround.zip.ZipUtil;
@@ -92,6 +95,7 @@ public class MappingsProviderImpl extends DependencyProvider implements Mappings
 	private boolean hasUnpickDefinitions;
 	private UnpickMetadata unpickMetadata;
 	private MemoryMappingTree mappingTree;
+	private Map<String, String> signatureFixes;
 
 	public MappingsProviderImpl(Project project) {
 		super(project);
@@ -120,7 +124,7 @@ public class MappingsProviderImpl extends DependencyProvider implements Mappings
 			storeMappings(getProject(), minecraftProvider, mappingsJar.toPath());
 		} else {
 			try (FileSystem fileSystem = FileSystems.newFileSystem(mappingsJar.toPath(), (ClassLoader) null)) {
-				extractUnpickDefinitions(fileSystem, unpickDefinitions);
+				extractExtras(fileSystem);
 			}
 		}
 
@@ -209,7 +213,7 @@ public class MappingsProviderImpl extends DependencyProvider implements Mappings
 
 		try (FileSystem fileSystem = FileSystems.newFileSystem(yarnJar, (ClassLoader) null)) {
 			extractMappings(fileSystem, baseTinyMappings);
-			extractUnpickDefinitions(fileSystem, unpickDefinitions);
+			extractExtras(fileSystem);
 		}
 
 		if (areMappingsV2(baseTinyMappings)) {
@@ -253,7 +257,12 @@ public class MappingsProviderImpl extends DependencyProvider implements Mappings
 		Files.copy(jar.getPath("mappings/mappings.tiny"), extractTo, StandardCopyOption.REPLACE_EXISTING);
 	}
 
-	private void extractUnpickDefinitions(FileSystem jar, Path extractTo) throws IOException {
+	private void extractExtras(FileSystem jar) throws IOException {
+		extractUnpickDefinitions(jar);
+		extractSignatureFixes(jar);
+	}
+
+	private void extractUnpickDefinitions(FileSystem jar) throws IOException {
 		Path unpickPath = jar.getPath("extras/definitions.unpick");
 		Path unpickMetadataPath = jar.getPath("extras/unpick.json");
 
@@ -261,12 +270,25 @@ public class MappingsProviderImpl extends DependencyProvider implements Mappings
 			return;
 		}
 
-		Files.copy(unpickPath, extractTo, StandardCopyOption.REPLACE_EXISTING);
+		Files.copy(unpickPath, unpickDefinitions, StandardCopyOption.REPLACE_EXISTING);
 
 		unpickMetadata = parseUnpickMetadata(unpickMetadataPath);
 		hasUnpickDefinitions = true;
 	}
 
+	private void extractSignatureFixes(FileSystem jar) throws IOException {
+		Path recordSignaturesJsonPath = jar.getPath("extras/record_signatures.json");
+
+		if (!Files.exists(recordSignaturesJsonPath)) {
+			return;
+		}
+
+		try (Reader reader = Files.newBufferedReader(recordSignaturesJsonPath, StandardCharsets.UTF_8)) {
+			//noinspection unchecked
+			signatureFixes = LoomGradlePlugin.OBJECT_MAPPER.readValue(reader, Map.class);
+		}
+	}
+
 	private UnpickMetadata parseUnpickMetadata(Path input) throws IOException {
 		JsonObject jsonObject = LoomGradlePlugin.GSON.fromJson(Files.readString(input), JsonObject.class);
 
@@ -461,6 +483,11 @@ public class MappingsProviderImpl extends DependencyProvider implements Mappings
 		return hasUnpickDefinitions;
 	}
 
+	@Nullable
+	public Map<String, String> getSignatureFixes() {
+		return signatureFixes;
+	}
+
 	@Override
 	public File intermediaryTinyFile() {
 		try {
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesLayer.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesLayer.java
new file mode 100644
index 00000000..845eed60
--- /dev/null
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesLayer.java
@@ -0,0 +1,34 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 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.configuration.providers.mappings.extras.signatures;
+
+import java.util.Map;
+
+import org.jetbrains.annotations.ApiStatus;
+
+@ApiStatus.Experimental
+public interface SignatureFixesLayer {
+	Map<String, String> getSignatureFixes();
+}
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesLayerImpl.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesLayerImpl.java
new file mode 100644
index 00000000..5f906384
--- /dev/null
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesLayerImpl.java
@@ -0,0 +1,64 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 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.configuration.providers.mappings.extras.signatures;
+
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.nio.file.Path;
+import java.util.Map;
+import java.util.Objects;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipFile;
+
+import org.jetbrains.annotations.ApiStatus;
+
+import net.fabricmc.loom.LoomGradlePlugin;
+import net.fabricmc.loom.api.mappings.layered.MappingLayer;
+import net.fabricmc.mappingio.MappingVisitor;
+
+@ApiStatus.Experimental
+public record SignatureFixesLayerImpl(Path mappingsFile) implements MappingLayer, SignatureFixesLayer {
+	private static final String SIGNATURE_FIXES_PATH = "extras/record_signatures.json";
+
+	@Override
+	public void visit(MappingVisitor mappingVisitor) throws IOException {
+		// Nothing to do here
+	}
+
+	@Override
+	public Map<String, String> getSignatureFixes() {
+		try (var zipFile = new ZipFile(mappingsFile().toFile())) {
+			ZipEntry zipFileEntry = zipFile.getEntry(SIGNATURE_FIXES_PATH);
+			Objects.requireNonNull(zipFileEntry, "Could not find %s in file".formatted(SIGNATURE_FIXES_PATH));
+
+			try (var reader = new InputStreamReader(zipFile.getInputStream(zipFileEntry))) {
+				//noinspection unchecked
+				return LoomGradlePlugin.OBJECT_MAPPER.readValue(reader, Map.class);
+			}
+		} catch (IOException e) {
+			throw new RuntimeException("Failed to extract signature fixes", e);
+		}
+	}
+}
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesSpec.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesSpec.java
new file mode 100644
index 00000000..1f4f1ae5
--- /dev/null
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/extras/signatures/SignatureFixesSpec.java
@@ -0,0 +1,39 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 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.configuration.providers.mappings.extras.signatures;
+
+import org.jetbrains.annotations.ApiStatus;
+
+import net.fabricmc.loom.api.mappings.layered.MappingContext;
+import net.fabricmc.loom.api.mappings.layered.spec.FileSpec;
+import net.fabricmc.loom.api.mappings.layered.spec.MappingsSpec;
+
+@ApiStatus.Experimental
+public record SignatureFixesSpec(FileSpec fileSpec) implements MappingsSpec<SignatureFixesLayerImpl> {
+	@Override
+	public SignatureFixesLayerImpl createLayer(MappingContext context) {
+		return new SignatureFixesLayerImpl(fileSpec.get(context));
+	}
+}
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftMappedProvider.java b/src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftMappedProvider.java
index 70f344d0..f78f9484 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftMappedProvider.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftMappedProvider.java
@@ -29,9 +29,15 @@ import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 
 import org.gradle.api.Project;
+import org.objectweb.asm.ClassVisitor;
+import org.objectweb.asm.commons.Remapper;
 
 import net.fabricmc.loom.api.mappings.layered.MappingsNamespace;
 import net.fabricmc.loom.configuration.DependencyProvider;
@@ -41,6 +47,7 @@ import net.fabricmc.loom.util.Constants;
 import net.fabricmc.loom.util.TinyRemapperHelper;
 import net.fabricmc.tinyremapper.OutputConsumerPath;
 import net.fabricmc.tinyremapper.TinyRemapper;
+import net.fabricmc.tinyremapper.api.TrClass;
 
 public class MinecraftMappedProvider extends DependencyProvider {
 	private File minecraftMappedJar;
@@ -101,13 +108,67 @@ public class MinecraftMappedProvider extends DependencyProvider {
 		Path outputIntermediary = minecraftIntermediaryJar.toPath();
 
 		for (String toM : Arrays.asList(MappingsNamespace.NAMED.toString(), MappingsNamespace.INTERMEDIARY.toString())) {
-			Path output = MappingsNamespace.NAMED.toString().equals(toM) ? outputMapped : outputIntermediary;
+			final boolean toNamed = MappingsNamespace.NAMED.toString().equals(toM);
+			final boolean toIntermediary = MappingsNamespace.INTERMEDIARY.toString().equals(toM);
+			final boolean fixSignatures = mappingsProvider.getSignatureFixes() != null;
+			final Path output = toNamed ? outputMapped : outputIntermediary;
 
 			getProject().getLogger().lifecycle(":remapping minecraft (TinyRemapper, " + fromM + " -> " + toM + ")");
 
 			Files.deleteIfExists(output);
 
-			TinyRemapper remapper = TinyRemapperHelper.getTinyRemapper(getProject(), fromM, toM, true);
+			// Bit ugly but whatever, the whole issue is a bit ugly :)
+			AtomicReference<Map<String, String>> remappedSignatures = new AtomicReference<>();
+			TinyRemapper remapper = TinyRemapperHelper.getTinyRemapper(getProject(), fromM, toM, true, (builder) -> {
+				if (!fixSignatures) {
+					return;
+				}
+
+				builder.extraPostApplyVisitor(new TinyRemapper.ApplyVisitorProvider() {
+					@Override
+					public ClassVisitor insertApplyVisitor(TrClass cls, ClassVisitor next) {
+						return new ClassVisitor(Constants.ASM_VERSION, next) {
+							@Override
+							public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
+								Map<String, String> signatureFixes = Objects.requireNonNull(remappedSignatures.get(), "Could not get remapped signatures");
+
+								if (signature == null) {
+									signature = signatureFixes.getOrDefault(name, null);
+
+									if (signature != null) {
+										getProject().getLogger().info("Replaced signature for {} with {}", name, signature);
+									}
+								}
+
+								super.visit(version, access, name, signature, superName, interfaces);
+							}
+						};
+					}
+				});
+			});
+
+			if (fixSignatures) {
+				if (toIntermediary) {
+					// No need to remap, as these are already intermediary
+					remappedSignatures.set(mappingsProvider.getSignatureFixes());
+				} else {
+					// Remap the sig fixes from intermediary to the target namespace
+					final Map<String, String> remapped = new HashMap<>();
+					final TinyRemapper sigTinyRemapper = TinyRemapperHelper.getTinyRemapper(getProject(), MappingsNamespace.INTERMEDIARY.toString(), toM);
+					final Remapper sigAsmRemapper = sigTinyRemapper.getRemapper();
+
+					// Remap the class names and the signatures using a new tiny remapper instance.
+					for (Map.Entry<String, String> entry : mappingsProvider.getSignatureFixes().entrySet()) {
+						remapped.put(
+								sigAsmRemapper.map(entry.getKey()),
+								sigAsmRemapper.mapSignature(entry.getValue(), false)
+						);
+					}
+
+					sigTinyRemapper.finish();
+					remappedSignatures.set(remapped);
+				}
+			}
 
 			try (OutputConsumerPath outputConsumer = new OutputConsumerPath.Builder(output).build()) {
 				outputConsumer.addNonClassFiles(input);
diff --git a/src/main/java/net/fabricmc/loom/util/TinyRemapperHelper.java b/src/main/java/net/fabricmc/loom/util/TinyRemapperHelper.java
index 7f593db5..c94c801a 100644
--- a/src/main/java/net/fabricmc/loom/util/TinyRemapperHelper.java
+++ b/src/main/java/net/fabricmc/loom/util/TinyRemapperHelper.java
@@ -28,6 +28,7 @@ import java.io.File;
 import java.io.IOException;
 import java.nio.file.Path;
 import java.util.Map;
+import java.util.function.Consumer;
 import java.util.regex.Pattern;
 
 import com.google.common.collect.ImmutableMap;
@@ -59,10 +60,10 @@ public final class TinyRemapperHelper {
 	}
 
 	public static TinyRemapper getTinyRemapper(Project project, String fromM, String toM) throws IOException {
-		return getTinyRemapper(project, fromM, toM, false);
+		return getTinyRemapper(project, fromM, toM, false, (builder) -> { });
 	}
 
-	public static TinyRemapper getTinyRemapper(Project project, String fromM, String toM, boolean fixRecords) throws IOException {
+	public static TinyRemapper getTinyRemapper(Project project, String fromM, String toM, boolean fixRecords, Consumer<TinyRemapper.Builder> builderConsumer) throws IOException {
 		LoomGradleExtension extension = LoomGradleExtension.get(project);
 		MemoryMappingTree mappingTree = extension.getMappingsProvider().getMappings();
 
@@ -72,7 +73,7 @@ public final class TinyRemapperHelper {
 
 		int intermediaryNsId = mappingTree.getNamespaceId(MappingsNamespace.INTERMEDIARY.toString());
 
-		return TinyRemapper.newRemapper()
+		TinyRemapper.Builder builder = TinyRemapper.newRemapper()
 				.withMappings(create(mappingTree, fromM, toM, true))
 				.withMappings(out -> JSR_TO_JETBRAINS.forEach(out::acceptClass))
 				.renameInvalidLocals(true)
@@ -84,8 +85,10 @@ public final class TinyRemapperHelper {
 					}
 
 					return next;
-				})
-				.build();
+				});
+
+		builderConsumer.accept(builder);
+		return builder.build();
 	}
 
 	public static Path[] getMinecraftDependencies(Project project) {
diff --git a/src/test/groovy/net/fabricmc/loom/test/LoomTestConstants.groovy b/src/test/groovy/net/fabricmc/loom/test/LoomTestConstants.groovy
index 57c0f2cc..e478d61b 100644
--- a/src/test/groovy/net/fabricmc/loom/test/LoomTestConstants.groovy
+++ b/src/test/groovy/net/fabricmc/loom/test/LoomTestConstants.groovy
@@ -26,7 +26,7 @@ package net.fabricmc.loom.test
 
 class LoomTestConstants {
     public final static String DEFAULT_GRADLE = "7.0.1"
-    public final static String PRE_RELEASE_GRADLE = "7.3-20210906222431+0000"
+    public final static String PRE_RELEASE_GRADLE = "7.4-20210926222420+0000"
 
     public final static String[] STANDARD_TEST_VERSIONS = [DEFAULT_GRADLE, PRE_RELEASE_GRADLE]
 }
diff --git a/src/test/groovy/net/fabricmc/loom/test/unit/layeredmappings/LayeredMappingsSpecification.groovy b/src/test/groovy/net/fabricmc/loom/test/unit/layeredmappings/LayeredMappingsSpecification.groovy
index 1fba03dc..f751f3ab 100644
--- a/src/test/groovy/net/fabricmc/loom/test/unit/layeredmappings/LayeredMappingsSpecification.groovy
+++ b/src/test/groovy/net/fabricmc/loom/test/unit/layeredmappings/LayeredMappingsSpecification.groovy
@@ -85,7 +85,7 @@ abstract class LayeredMappingsSpecification extends Specification implements Lay
     MemoryMappingTree getLayeredMappings(MappingsSpec<? extends MappingLayer>... specs) {
         LayeredMappingSpec spec = new LayeredMappingSpec(specs.toList())
         LayeredMappingsProcessor processor = new LayeredMappingsProcessor(spec)
-        return processor.getMappings(mappingContext)
+        return processor.getMappings(processor.resolveLayers(mappingContext))
     }
 
     String getTiny(MemoryMappingTree mappingTree) {
diff --git a/src/test/resources/projects/java16/build.gradle b/src/test/resources/projects/java16/build.gradle
index 7372d08b..45acf714 100644
--- a/src/test/resources/projects/java16/build.gradle
+++ b/src/test/resources/projects/java16/build.gradle
@@ -10,12 +10,16 @@ archivesBaseName = project.archives_base_name
 version = project.mod_version
 group = project.maven_group
 
+loom {
+	accessWidenerPath = file("src/main/resources/modid.accesswidener")
+}
+
 dependencies {
 	// To change the versions see the gradle.properties file
 	minecraft "com.mojang:minecraft:${project.minecraft_version}"
 	mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"
 	modImplementation "net.fabricmc:fabric-loader:${project.loader_version}"
-	//modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
+	modImplementation "net.fabricmc.fabric-api:fabric-api:${project.fabric_version}"
 }
 
 tasks.withType(JavaCompile).configureEach {
diff --git a/src/test/resources/projects/java16/gradle.properties b/src/test/resources/projects/java16/gradle.properties
index 5c21b1cb..941d7f52 100644
--- a/src/test/resources/projects/java16/gradle.properties
+++ b/src/test/resources/projects/java16/gradle.properties
@@ -1,9 +1,9 @@
 org.gradle.jvmargs=-Xmx1G
 
-minecraft_version=21w37a
-yarn_mappings=21w37a+build.5
+minecraft_version=21w38a
+yarn_mappings=21w38a+build.9
 loader_version=0.11.7
-fabric_version=0.37.1+1.17
+fabric_version=0.40.4+1.18
 
 mod_version = 1.0.0
 maven_group = com.example
diff --git a/src/test/resources/projects/java16/src/main/java/net/fabricmc/example/mixin/ExampleMixin.java b/src/test/resources/projects/java16/src/main/java/net/fabricmc/example/mixin/ExampleMixin.java
index 83ee1a89..60002d28 100644
--- a/src/test/resources/projects/java16/src/main/java/net/fabricmc/example/mixin/ExampleMixin.java
+++ b/src/test/resources/projects/java16/src/main/java/net/fabricmc/example/mixin/ExampleMixin.java
@@ -1,15 +1,11 @@
 package net.fabricmc.example.mixin;
 
-import net.minecraft.client.gui.screen.TitleScreen;
+import net.minecraft.world.chunk.PalettedContainer;
 import org.spongepowered.asm.mixin.Mixin;
-import org.spongepowered.asm.mixin.injection.At;
-import org.spongepowered.asm.mixin.injection.Inject;
-import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+import org.spongepowered.asm.mixin.Shadow;
 
-@Mixin(TitleScreen.class)
-public class ExampleMixin {
-	@Inject(at = @At("HEAD"), method = "init()V")
-	private void init(CallbackInfo info) {
-		System.out.println("This line is printed by an example mod mixin!");
-	}
-}
+@Mixin(PalettedContainer.class)
+public abstract class ExampleMixin<T> {
+	@Shadow
+	private volatile PalettedContainer.Data<T> data;
+}
\ No newline at end of file
diff --git a/src/test/resources/projects/java16/src/main/resources/fabric.mod.json b/src/test/resources/projects/java16/src/main/resources/fabric.mod.json
index 9c6ccfa9..27a8bdf7 100644
--- a/src/test/resources/projects/java16/src/main/resources/fabric.mod.json
+++ b/src/test/resources/projects/java16/src/main/resources/fabric.mod.json
@@ -32,5 +32,6 @@
   },
   "suggests": {
     "another-mod": "*"
-  }
+  },
+  "accessWidener" : "modid.accesswidener"
 }
diff --git a/src/test/resources/projects/java16/src/main/resources/modid.accesswidener b/src/test/resources/projects/java16/src/main/resources/modid.accesswidener
new file mode 100644
index 00000000..0f681e38
--- /dev/null
+++ b/src/test/resources/projects/java16/src/main/resources/modid.accesswidener
@@ -0,0 +1,3 @@
+accessWidener	v1	named
+
+accessible    class    net/minecraft/world/chunk/PalettedContainer$Data