Discourage using labels on non-loop statements

PiperOrigin-RevId: 498879570

core/src/main/java/com/google/errorprone/bugpatterns/LabelledBreakTarget.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2023 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.matchers.Description.NO_MATCH;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.LabeledStatementTreeMatcher;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.LabeledStatementTree;
+
+/** A BugPattern; see the summary. */
+@BugPattern(summary = "Labels should only be used on loops.", severity = WARNING)
+public class LabelledBreakTarget extends BugChecker implements LabeledStatementTreeMatcher {
+  @Override
+  public Description matchLabeledStatement(LabeledStatementTree tree, VisitorState state) {
+    switch (tree.getStatement().getKind()) {
+      case DO_WHILE_LOOP:
+      case ENHANCED_FOR_LOOP:
+      case FOR_LOOP:
+      case WHILE_LOOP:
+        return NO_MATCH;
+      default:
+        break;
+    }
+    return describeMatch(tree);
+  }
+}

core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java

@@ -197,6 +197,7 @@
 import com.google.errorprone.bugpatterns.JavaLangClash;
 import com.google.errorprone.bugpatterns.JavaUtilDateChecker;
 import com.google.errorprone.bugpatterns.JdkObsolete;
+import com.google.errorprone.bugpatterns.LabelledBreakTarget;
 import com.google.errorprone.bugpatterns.LambdaFunctionalInterface;
 import com.google.errorprone.bugpatterns.LenientFormatStringValidation;
 import com.google.errorprone.bugpatterns.LiteByteStringUtf8;
@@ -909,6 +910,7 @@ public static ScannerSupplier errorChecks() {
           JodaPlusMinusLong.class,
           JodaTimeConverterManager.class,
           JodaWithDurationAddedLong.class,
+          LabelledBreakTarget.class,
           LiteEnumValueOf.class,
           LiteProtoToString.class,
           LockNotBeforeTry.class,

core/src/test/java/com/google/errorprone/bugpatterns/LabelledBreakTargetTest.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2023 The Error Prone Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.errorprone.bugpatterns;
+
+import com.google.errorprone.CompilationTestHelper;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** {@link LabelledBreakTarget}Test */
+@RunWith(JUnit4.class)
+public class LabelledBreakTargetTest {
+  private final CompilationTestHelper compilationHelper =
+      CompilationTestHelper.newInstance(LabelledBreakTarget.class, getClass());
+
+  @Test
+  public void positive() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  void f() {",
+            "    // BUG: Diagnostic contains:",
+            "    FOO:",
+            "    if (true) {",
+            "      break FOO;",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void negative() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  void f(Iterable<?> xs) {",
+            "    ONE:",
+            "    while (true) {",
+            "      break ONE;",
+            "    }",
+            "    TWO:",
+            "    do {",
+            "      break TWO;",
+            "    } while (true);",
+            "    THREE:",
+            "    for (var x : xs) {",
+            "      break THREE;",
+            "    }",
+            "    FOUR:",
+            "    for (int i = 0; i < 10; i++) {",
+            "      break FOUR;",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
+}

docs/bugpattern/LabelledBreakTarget.md

@@ -0,0 +1,2 @@
+Labels should only be used on loops. For other statements, consider refactoring
+to express the control flow without labelled breaks.