VoidUsed: flag Void variables being _used_, where they can simply be replaced with a literal null.

Findings here (unknown commit) before I fixed the bug with assignments.

PiperOrigin-RevId: 627676095

Author: graememorgan

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

@@ -0,0 +1,76 @@
+/*
+ * 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.common.collect.Sets.immutableEnumSet;
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.matchers.Description.NO_MATCH;
+import static com.google.errorprone.suppliers.Suppliers.JAVA_LANG_VOID_TYPE;
+import static com.google.errorprone.util.ASTHelpers.getSymbol;
+import static com.google.errorprone.util.ASTHelpers.isSameType;
+import static javax.lang.model.element.ElementKind.FIELD;
+import static javax.lang.model.element.ElementKind.LOCAL_VARIABLE;
+import static javax.lang.model.element.ElementKind.PARAMETER;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.IdentifierTreeMatcher;
+import com.google.errorprone.bugpatterns.BugChecker.MemberSelectTreeMatcher;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.AssignmentTree;
+import com.sun.source.tree.IdentifierTree;
+import com.sun.source.tree.MemberSelectTree;
+import com.sun.source.tree.Tree;
+import javax.lang.model.element.ElementKind;
+
+/** A BugPattern; see the summary. */
+@BugPattern(
+    summary =
+        "Using a Void-typed variable is potentially confusing, and can be replaced with a literal"
+            + " `null`.",
+    severity = WARNING)
+public final class VoidUsed extends BugChecker
+    implements IdentifierTreeMatcher, MemberSelectTreeMatcher {
+  @Override
+  public Description matchIdentifier(IdentifierTree tree, VisitorState state) {
+    return handle(tree, state);
+  }
+
+  @Override
+  public Description matchMemberSelect(MemberSelectTree tree, VisitorState state) {
+    return handle(tree, state);
+  }
+
+  private Description handle(Tree tree, VisitorState state) {
+    var parent = state.getPath().getParentPath().getLeaf();
+    if (parent instanceof AssignmentTree && ((AssignmentTree) parent).getVariable().equals(tree)) {
+      return NO_MATCH;
+    }
+    var symbol = getSymbol(tree);
+    if (symbol == null
+        || !KINDS.contains(symbol.getKind())
+        || !isSameType(symbol.type, JAVA_LANG_VOID_TYPE.get(state), state)) {
+      return NO_MATCH;
+    }
+    return describeMatch(tree, SuggestedFix.replace(tree, "null"));
+  }
+
+  private static final ImmutableSet<ElementKind> KINDS =
+      immutableEnumSet(PARAMETER, LOCAL_VARIABLE, FIELD);
+}

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

@@ -434,6 +434,7 @@
 import com.google.errorprone.bugpatterns.VarTypeName;
 import com.google.errorprone.bugpatterns.VariableNameSameAsType;
 import com.google.errorprone.bugpatterns.Varifier;
+import com.google.errorprone.bugpatterns.VoidUsed;
 import com.google.errorprone.bugpatterns.WaitNotInLoop;
 import com.google.errorprone.bugpatterns.WildcardImport;
 import com.google.errorprone.bugpatterns.WithSignatureDiscouraged;
@@ -1094,6 +1095,7 @@ public static ScannerSupplier warningChecks() {
           UnusedVariable.class,
           UseBinds.class,
           VariableNameSameAsType.class,
+          VoidUsed.class,
           WaitNotInLoop.class,
           WakelockReleasedDangerously.class,
           WithSignatureDiscouraged.class

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

@@ -0,0 +1,68 @@
+/*
+ * 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 com.google.errorprone.CompilationTestHelper;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class VoidUsedTest {
+  private final CompilationTestHelper helper =
+      CompilationTestHelper.newInstance(VoidUsed.class, getClass());
+
+  @Test
+  public void positive() {
+    helper
+        .addSourceLines(
+            "Test.java", //
+            "class Test {",
+            "  void test(Void v) {",
+            "    // BUG: Diagnostic contains: null",
+            "    System.out.println(v);",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void notVoid_noFinding() {
+    helper
+        .addSourceLines(
+            "Test.java", //
+            "class Test {",
+            "  void test(Integer v) {",
+            "    System.out.println(v);",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void assignedTo_noFinding() {
+    helper
+        .addSourceLines(
+            "Test.java", //
+            "class Test {",
+            "  void test(Void v) {",
+            "    v = null;",
+            "  }",
+            "}")
+        .doTest();
+  }
+}