Don't validate mixin names with more than one target.

Add tests.

build.gradle

@@ -122,6 +122,10 @@ dependencies {
 
 	compileOnly 'org.jetbrains:annotations:23.0.0'
 	testCompileOnly 'org.jetbrains:annotations:23.0.0'
+
+	testCompileOnly ('net.fabricmc:sponge-mixin:0.11.4+mixin.0.8.5') {
+		transitive = false
+	}
 }
 
 jar {

src/main/java/net/fabricmc/loom/task/ValidateMixinNameTask.java

@@ -48,6 +48,7 @@ import org.gradle.workers.WorkParameters;
 import org.gradle.workers.WorkQueue;
 import org.gradle.workers.WorkerExecutor;
 import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.VisibleForTesting;
 import org.objectweb.asm.AnnotationVisitor;
 import org.objectweb.asm.ClassReader;
 import org.objectweb.asm.ClassVisitor;
@@ -115,7 +116,7 @@ public abstract class ValidateMixinNameTask extends SourceTask {
 				}
 
 				final String mixinClassName = toSimpleName(mixin.className);
-				final String expectedMixinClassName = toSimpleName(mixin.target.getInternalName()).replace("$", "") + (mixin.accessor ? "Accessor" : "Mixin");
+				final String expectedMixinClassName = mixin.expectedClassName();
 
 				if (expectedMixinClassName.startsWith("class_")) {
 					// Don't enforce intermediary named mixins.
@@ -140,36 +141,48 @@ public abstract class ValidateMixinNameTask extends SourceTask {
 
 			throw new GradleException("Mixin name validation failed: " + errors.stream().collect(Collectors.joining(System.lineSeparator())));
 		}
+	}
 
-		private static String toSimpleName(String internalName) {
-			return internalName.substring(internalName.lastIndexOf("/") + 1);
-		}
+	private static String toSimpleName(String internalName) {
+		return internalName.substring(internalName.lastIndexOf("/") + 1);
+	}
 
-		@Nullable
-		private Mixin getMixin(File file) {
-			try (InputStream is = new FileInputStream(file)) {
-				ClassReader reader = new ClassReader(is);
-
-				var classVisitor = new MixinTargetClassVisitor();
-				reader.accept(classVisitor, ClassReader.SKIP_CODE);
-
-				if (classVisitor.mixinTarget != null) {
-					return new Mixin(classVisitor.className, classVisitor.mixinTarget, classVisitor.accessor);
-				}
-			} catch (IOException e) {
-				throw new UncheckedIOException("Failed to read input file: " + file, e);
-			}
-
-			return null;
+	@VisibleForTesting
+	public record Mixin(String className, Type target, boolean accessor) {
+		public String expectedClassName() {
+			return toSimpleName(target.getInternalName()).replace("$", "") + (accessor ? "Accessor" : "Mixin");
 		}
 	}
 
-	private record Mixin(String className, Type target, boolean accessor) { }
+	@Nullable
+	private static Mixin getMixin(File file) {
+		try (InputStream is = new FileInputStream(file)) {
+			return getMixin(is);
+		} catch (IOException e) {
+			throw new UncheckedIOException("Failed to read input file: " + file, e);
+		}
+	}
+
+	@Nullable
+	@VisibleForTesting
+	public static Mixin getMixin(InputStream is) throws IOException {
+		final ClassReader reader = new ClassReader(is);
+
+		var classVisitor = new MixinTargetClassVisitor();
+		reader.accept(classVisitor, ClassReader.SKIP_CODE);
+
+		if (classVisitor.mixinTarget != null && classVisitor.targets == 1) {
+			return new Mixin(classVisitor.className, classVisitor.mixinTarget, classVisitor.accessor);
+		}
+
+		return null;
+	}
 
 	private static class MixinTargetClassVisitor extends ClassVisitor {
 		Type mixinTarget;
 		String className;
 		boolean accessor;
+		int targets = 0;
 
 		boolean isInterface;
 
@@ -220,6 +233,7 @@ public abstract class ValidateMixinNameTask extends SourceTask {
 						@Override
 						public void visit(String name, Object value) {
 							mixinTarget = Objects.requireNonNull((Type) value);
+							targets++;
 
 							super.visit(name, value);
 						}

src/test/groovy/net/fabricmc/loom/test/unit/ValidateMixinNameTest.groovy

@@ -0,0 +1,102 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 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
+ * 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.unit
+
+import net.fabricmc.loom.task.ValidateMixinNameTask
+import org.spongepowered.asm.mixin.Mixin
+import org.spongepowered.asm.mixin.gen.Accessor
+import spock.lang.Specification
+
+class ValidateMixinNameTest extends Specification {
+	def "TestMixin"() {
+		when:
+			def mixin = getMixin(TestMixin.class)
+		then:
+			mixin.className() == "net/fabricmc/loom/test/unit/TestMixin"
+			mixin.target().internalName == "net/fabricmc/loom/test/unit/Test"
+			mixin.expectedClassName() == "TestMixin"
+			!mixin.accessor()
+	}
+
+	def "TestInnerMixin"() {
+		when:
+			def mixin = getMixin(TestInnerMixin.class)
+		then:
+			mixin.className() == "net/fabricmc/loom/test/unit/TestInnerMixin"
+			mixin.target().internalName == "net/fabricmc/loom/test/unit/Test\$Inner"
+			mixin.expectedClassName() == "TestInnerMixin"
+			!mixin.accessor()
+	}
+
+	def "TestAccessor"() {
+		when:
+			def mixin = getMixin(TestAccessor.class)
+		then:
+			mixin.className() == "net/fabricmc/loom/test/unit/TestAccessor"
+			mixin.target().internalName == "net/fabricmc/loom/test/unit/Test"
+			mixin.expectedClassName() == "TestAccessor"
+			mixin.accessor()
+	}
+
+	def "TestManyTargetsMixin"() {
+		when:
+			def mixin = getMixin(TestManyTargetsMixin.class)
+		then:
+			mixin == null
+	}
+
+	static ValidateMixinNameTask.Mixin getMixin(Class<?> clazz) {
+		return getInput(clazz).withCloseable {
+			return ValidateMixinNameTask.getMixin(it)
+		}
+	}
+
+	static InputStream getInput(Class<?> clazz) {
+		return clazz.classLoader.getResourceAsStream(clazz.name.replace('.', '/') + ".class")
+	}
+}
+
+@Mixin(Test.class)
+class TestMixin {
+}
+
+@Mixin(Test.Inner.class)
+class TestInnerMixin {
+}
+
+@Mixin(Test.class)
+interface TestAccessor {
+	@Accessor
+	Object getNothing();
+}
+
+@Mixin([Test.class, Test.Inner.class])
+class TestManyTargetsMixin {
+}
+
+class Test {
+	class Inner {
+	}
+}