Add PicnicConfigClass rule

.gitignore

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+target/
+.idea/
+*.iml

error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/PicnicConfigClassCheck.java

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+package tech.picnic.errorprone.bugpatterns;
+
+import static com.google.errorprone.matchers.Description.NO_MATCH;
+import static com.google.errorprone.util.ASTHelpers.getDeclaredSymbol;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.BugPattern.LinkType;
+import com.google.errorprone.BugPattern.ProvidesFix;
+import com.google.errorprone.BugPattern.SeverityLevel;
+import com.google.errorprone.BugPattern.StandardTags;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.bugpatterns.BugChecker.ClassTreeMatcher;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.AnnotationTree;
+import com.sun.source.tree.ClassTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol;
+import java.util.Optional;
+import javax.lang.model.element.Modifier;
+
+/** A {@link BugChecker} which flags `Configuration` classes that can be simplified. */
+@AutoService(BugChecker.class)
+@BugPattern(
+    name = "PicnicConfigClass",
+    summary = "Enforces rules pertaining to Picnic `Configuration` classes.",
+    linkType = LinkType.NONE,
+    severity = SeverityLevel.SUGGESTION,
+    tags = StandardTags.SIMPLIFICATION,
+    providesFix = ProvidesFix.REQUIRES_HUMAN_ATTENTION)
+public final class PicnicConfigClassCheck extends BugChecker implements ClassTreeMatcher {
+  private static final long serialVersionUID = 1L;
+  private static final String CONFIG_CLASS_SUFFIX = "Config";
+  private static final String CONFIG_ANNOTATION =
+      "org.springframework.context.annotation.Configuration";
+
+  @Override
+  public Description matchClass(ClassTree clazz, VisitorState state) {
+    if (!isConfigClass(clazz)) {
+      return NO_MATCH;
+    }
+
+    SuggestedFix.Builder fixBuilder = SuggestedFix.builder();
+    SuggestedFixes.addModifiers(clazz, state, Modifier.FINAL).ifPresent(fixBuilder::merge);
+    tryRemoveConfigurationAnnotation(clazz).ifPresent(fixBuilder::merge);
+    SuggestedFix fix = fixBuilder.build();
+
+    if (!fix.isEmpty()) {
+      String message =
+          String.format(
+              "Configuration class %s should be final and not be annotated with `@Configuration`",
+              clazz.getSimpleName());
+      return buildDescription(clazz).setMessage(message).addFix(fix).build();
+    }
+
+    return NO_MATCH;
+  }
+
+  /** If present removes the `@Configuration` annotation from the supplied class element. */
+  private static Optional<SuggestedFix> tryRemoveConfigurationAnnotation(ClassTree clazz) {
+    for (AnnotationTree annotation : clazz.getModifiers().getAnnotations()) {
+      Symbol annotationSymbol = getDeclaredSymbol(annotation);
+      if (annotationSymbol != null
+          && annotationSymbol.getQualifiedName().contentEquals(CONFIG_ANNOTATION)) {
+        return Optional.of(SuggestedFix.delete(annotation));
+        /* Defer cleanup of possibly unused import to another rule. */
+      }
+    }
+    return Optional.empty();
+  }
+
+  /**
+   * Determines whether a class is a `Configuration` class according to Picnic naming conventions.
+   */
+  private static boolean isConfigClass(ClassTree clazz) {
+    return clazz.getKind() == Tree.Kind.CLASS
+        && clazz.getSimpleName().toString().endsWith(CONFIG_CLASS_SUFFIX);
+  }
+}

error-prone-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/PicnicConfigClassCheckTest.java

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+package tech.picnic.errorprone.bugpatterns;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import org.testng.annotations.Test;
+
+public final class PicnicConfigClassCheckTest {
+  @Test
+  public void testNonFinalWithAnnotation() {
+    BugCheckerRefactoringTestHelper.newInstance(new PicnicConfigClassCheck(), getClass())
+        .addInputLines(
+            "TestConfig.java",
+            "import org.springframework.context.annotation.Configuration;",
+            "@Configuration",
+            "public class TestConfig {}")
+        .addOutputLines(
+            "TestConfig.java",
+            "import org.springframework.context.annotation.Configuration;",
+            "public final class TestConfig {}")
+        .doTest();
+  }
+
+  @Test
+  public void tesetFinalWithoutAnnotation() {
+    BugCheckerRefactoringTestHelper.newInstance(new PicnicConfigClassCheck(), getClass())
+        .addInputLines("TestConfig.java", "", "public final class TestConfig {}")
+        .addOutputLines("TestConfig.java", "", "public final class TestConfig {}")
+        .doTest();
+  }
+
+  @Test
+  public void testNonFinalWithoutAnnotation() {
+    BugCheckerRefactoringTestHelper.newInstance(new PicnicConfigClassCheck(), getClass())
+        .addInputLines("TestConfig.java", "public class TestConfig {}")
+        .addOutputLines("TestConfig.java", "public final class TestConfig {}")
+        .doTest();
+  }
+
+  @Test
+  public void testInnerClass() {
+    BugCheckerRefactoringTestHelper.newInstance(new PicnicConfigClassCheck(), getClass())
+        .addInputLines(
+            "TestClass.java",
+            "import org.springframework.context.annotation.Configuration;",
+            "public class TestClass {",
+            "  @Configuration",
+            "  static class InnerTestConfig {}",
+            "}")
+        .addOutputLines(
+            "TestClass.java",
+            "import org.springframework.context.annotation.Configuration;",
+            "public class TestClass {",
+            "  static final class InnerTestConfig {}",
+            "}")
+        .doTest();
+  }
+}