Disallow using traditional :-style switches as switch expressions

PiperOrigin-RevId: 653064183

check_api/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -2808,6 +2808,31 @@ private static Method getCaseTreeGetExpressionsMethod() {
     }
   }
 
+  /**
+   * Returns true if the given {@link CaseTree} is in the form: {@code case <expression> ->
+   * <expression>}.
+   */
+  public static boolean isRuleKind(CaseTree caseTree) {
+    Enum<?> kind;
+    try {
+      kind = (Enum<?>) GET_CASE_KIND_METHOD.invoke(caseTree);
+    } catch (ReflectiveOperationException e) {
+      return false;
+    }
+    return kind.name().equals("RULE");
+  }
+
+  private static final Method GET_CASE_KIND_METHOD = getGetCaseKindMethod();
+
+  @Nullable
+  private static Method getGetCaseKindMethod() {
+    try {
+      return CaseTree.class.getMethod("getCaseKind");
+    } catch (NoSuchMethodException e) {
+      return null;
+    }
+  }
+
   /**
    * Retrieves a stream containing all case expressions, in order, for a given {@code CaseTree}.
    * This method acts as a facade to the {@code CaseTree.getExpressions()} API, falling back to

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

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2024 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 static com.google.errorprone.util.ASTHelpers.isRuleKind;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.CaseTree;
+import com.sun.source.tree.Tree;
+
+/** A {@link BugChecker}; see the associated {@link BugPattern} annotation for details. */
+@BugPattern(summary = "Prefer -> switches for switch expressions", severity = WARNING)
+public class TraditionalSwitchExpression extends BugChecker implements BugChecker.CaseTreeMatcher {
+
+  @Override
+  public Description matchCase(CaseTree tree, VisitorState state) {
+    if (isRuleKind(tree)) {
+      return NO_MATCH;
+    }
+    Tree parent = state.getPath().getParentPath().getLeaf();
+    if (!parent.getKind().name().equals("SWITCH_EXPRESSION")) {
+      return NO_MATCH;
+    }
+    return describeMatch(parent);
+  }
+}

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

@@ -379,6 +379,7 @@
 import com.google.errorprone.bugpatterns.ThrowsUncheckedException;
 import com.google.errorprone.bugpatterns.ToStringReturnsNull;
 import com.google.errorprone.bugpatterns.TooManyParameters;
+import com.google.errorprone.bugpatterns.TraditionalSwitchExpression;
 import com.google.errorprone.bugpatterns.TransientMisuse;
 import com.google.errorprone.bugpatterns.TreeToString;
 import com.google.errorprone.bugpatterns.TruthAssertExpected;
@@ -1075,6 +1076,7 @@ public static ScannerSupplier warningChecks() {
           ThreeLetterTimeZoneID.class,
           TimeUnitConversionChecker.class,
           ToStringReturnsNull.class,
+          TraditionalSwitchExpression.class,
           TruthAssertExpected.class,
           TruthConstantAsserts.class,
           TruthGetOrDefault.class,

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

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2024 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 org.junit.Assume.assumeTrue;
+
+import com.google.errorprone.CompilationTestHelper;
+import com.google.errorprone.util.RuntimeVersion;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class TraditionalSwitchExpressionTest {
+
+  private final CompilationTestHelper testHelper =
+      CompilationTestHelper.newInstance(TraditionalSwitchExpression.class, getClass());
+
+  @Test
+  public void positive() {
+    assumeTrue(RuntimeVersion.isAtLeast14());
+    testHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  int f(int i) {",
+            "    // BUG: Diagnostic contains: Prefer -> switches for switch expressions",
+            "    return switch (i) {",
+            "      default:",
+            "        yield -1;",
+            "    };",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void negativeStatement() {
+    assumeTrue(RuntimeVersion.isAtLeast14());
+    testHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  void f(int i) {",
+            "    switch (i) {",
+            "      default:",
+            "        return;",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void negativeArrowStatement() {
+    assumeTrue(RuntimeVersion.isAtLeast14());
+    testHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  void f(int i) {",
+            "    switch (i) {",
+            "      default -> System.err.println();",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void negativeArrow() {
+    assumeTrue(RuntimeVersion.isAtLeast14());
+    testHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  int f(int i) {",
+            "    return switch (i) {",
+            "      default -> -1;",
+            "    };",
+            "  }",
+            "}")
+        .doTest();
+  }
+}