Fail hard with an error message when calling the wrong officialMojangMappings method.

Before this would have been a weird silent failure and resulted in bad mappings.

src/main/java/net/fabricmc/loom/api/LoomGradleExtensionAPI.java

@@ -70,9 +70,7 @@ public interface LoomGradleExtensionAPI {
 
 	ConfigurableFileCollection getLog4jConfigs();
 
-	default Dependency officialMojangMappings() {
-		return layered(LayeredMappingSpecBuilder::officialMojangMappings);
-	}
+	Dependency officialMojangMappings();
 
 	Dependency layered(Action<LayeredMappingSpecBuilder> action);
 

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

@@ -78,6 +78,9 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 	private final NamedDomainObjectContainer<RunConfigSettings> runConfigs;
 	private final NamedDomainObjectContainer<DecompilerOptions> decompilers;
 
+	// A common mistake with layered mappings is to call the wrong `officialMojangMappings` method, use this to keep track of when we are building a layered mapping spec.
+	protected final ThreadLocal<Boolean> layeredSpecBuilderScope = ThreadLocal.withInitial(() -> false);
+
 	protected LoomGradleExtensionApiImpl(Project project, LoomFiles directories) {
 		this.jarProcessors = project.getObjects().listProperty(JarProcessor.class)
 				.empty();
@@ -162,10 +165,23 @@ public abstract class LoomGradleExtensionApiImpl implements LoomGradleExtensionA
 		return jarProcessors;
 	}
 
+	@Override
+	public Dependency officialMojangMappings() {
+		if (layeredSpecBuilderScope.get()) {
+			throw new IllegalStateException("Use `officialMojangMappings()` when configuring layered mappings, not the extension method `loom.officialMojangMappings()`");
+		}
+
+		return layered(LayeredMappingSpecBuilder::officialMojangMappings);
+	}
+
 	@Override
 	public Dependency layered(Action<LayeredMappingSpecBuilder> action) {
 		LayeredMappingSpecBuilderImpl builder = new LayeredMappingSpecBuilderImpl();
+
+		layeredSpecBuilderScope.set(true);
 		action.execute(builder);
+		layeredSpecBuilderScope.set(false);
+
 		LayeredMappingSpec builtSpec = builder.build();
 		return new LayeredMappingsDependency(getProject(), new GradleMappingContext(getProject(), builtSpec.getVersion().replace("+", "_").replace(".", "_")), builtSpec, builtSpec.getVersion());
 	}

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

@@ -1,7 +1,7 @@
 /*
  * This file is part of fabric-loom, licensed under the MIT License (MIT).
  *
- * Copyright (c) 2018-2021 FabricMC
+ * Copyright (c) 2018-2022 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
@@ -74,4 +74,29 @@ class MojangMappingsProjectTest extends Specification implements GradleProjectTe
 		where:
 			version << STANDARD_TEST_VERSIONS
 	}
+
+	@Unroll
+	def "fail with wrong officialMojangMappings usage (gradle #version)"() {
+		setup:
+			def gradle = gradleProject(project: "minimalBase", version: version)
+
+			gradle.buildGradle << '''
+				dependencies {
+					minecraft "com.mojang:minecraft:1.18.2"
+					mappings loom.layered {
+						// This is the wrong method to call!
+						loom.officialMojangMappings()
+					}
+				}
+			'''
+
+		when:
+			def result = gradle.run(task: "build", expectFailure: true)
+
+		then:
+			result.output.contains("Use `officialMojangMappings()` when configuring layered mappings, not the extension method `loom.officialMojangMappings()`")
+
+		where:
+			version << STANDARD_TEST_VERSIONS
+	}
 }