Add an initial check to simplify pattern matching instanceofs.

This only handles the simplest case of,

```
if (x instanceof Foo) {
  var foo = (Foo) x;
  ...
}
```

While we should also be catching:

```
if (!(x instanceof Foo)) {
  ...
}
...
var foo = (Foo) x;
```

That's certainly a bit harder to do, though!

PiperOrigin-RevId: 627010635

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

@@ -40,6 +40,11 @@ public static boolean supportsEffectivelyFinal(Context context) {
     return sourceIsAtLeast(context, 8);
   }
 
+  /** Returns whether the compiler supports pattern-matching instanceofs. */
+  public static boolean supportsPatternMatchingInstanceof(Context context) {
+    return sourceIsAtLeast(context, 21);
+  }
+
   private static boolean sourceIsAtLeast(Context context, int version) {
     Source lowerBound = Source.lookup(Integer.toString(version));
     return lowerBound != null && Source.instance(context).compareTo(lowerBound) >= 0;

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

@@ -0,0 +1,121 @@
+/*
+ * 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.getSymbol;
+import static com.google.errorprone.util.ASTHelpers.getType;
+import static com.google.errorprone.util.ASTHelpers.isSameType;
+import static com.google.errorprone.util.SourceVersion.supportsPatternMatchingInstanceof;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.IfTreeMatcher;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.BinaryTree;
+import com.sun.source.tree.BlockTree;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.IfTree;
+import com.sun.source.tree.InstanceOfTree;
+import com.sun.source.tree.ParenthesizedTree;
+import com.sun.source.tree.Tree.Kind;
+import com.sun.source.tree.TypeCastTree;
+import com.sun.source.tree.VariableTree;
+import com.sun.source.util.SimpleTreeVisitor;
+import com.sun.tools.javac.code.Symbol.VarSymbol;
+
+/** A BugPattern; see the summary. */
+@BugPattern(
+    severity = WARNING,
+    summary = "This code can be simplified to use a pattern-matching instanceof.")
+public final class PatternMatchingInstanceof extends BugChecker implements IfTreeMatcher {
+  @Override
+  public Description matchIf(IfTree tree, VisitorState state) {
+    if (!supportsPatternMatchingInstanceof(state.context)) {
+      return NO_MATCH;
+    }
+    ImmutableSet<InstanceOfTree> instanceofChecks = scanForInstanceOf(tree.getCondition());
+    if (instanceofChecks.isEmpty()) {
+      return NO_MATCH;
+    }
+    var body = tree.getThenStatement();
+    if (!(body instanceof BlockTree)) {
+      return NO_MATCH;
+    }
+    var block = (BlockTree) body;
+    if (block.getStatements().isEmpty()) {
+      return NO_MATCH;
+    }
+    var firstStatement = block.getStatements().get(0);
+    if (!(firstStatement instanceof VariableTree)) {
+      return NO_MATCH;
+    }
+    var variableTree = (VariableTree) firstStatement;
+    if (!(variableTree.getInitializer() instanceof TypeCastTree)) {
+      return NO_MATCH;
+    }
+    var typeCast = (TypeCastTree) variableTree.getInitializer();
+    var matchingInstanceof =
+        instanceofChecks.stream()
+            .filter(
+                i ->
+                    isSameType(getType(i.getType()), getType(typeCast.getType()), state)
+                        && getSymbol(i.getExpression()) instanceof VarSymbol
+                        && getSymbol(i.getExpression()).equals(getSymbol(typeCast.getExpression())))
+            .findFirst()
+            .orElse(null);
+    if (matchingInstanceof == null) {
+      return NO_MATCH;
+    }
+    return describeMatch(
+        firstStatement,
+        SuggestedFix.builder()
+            .delete(variableTree)
+            .postfixWith(matchingInstanceof, " " + variableTree.getName().toString())
+            .build());
+  }
+
+  private ImmutableSet<InstanceOfTree> scanForInstanceOf(ExpressionTree condition) {
+    ImmutableSet.Builder<InstanceOfTree> instanceOfs = ImmutableSet.builder();
+    new SimpleTreeVisitor<Void, Void>() {
+      @Override
+      public Void visitParenthesized(ParenthesizedTree tree, Void unused) {
+        return visit(tree.getExpression(), null);
+      }
+
+      @Override
+      public Void visitBinary(BinaryTree tree, Void unused) {
+        if (tree.getKind() != Kind.CONDITIONAL_AND) {
+          return null;
+        }
+        visit(tree.getLeftOperand(), null);
+        visit(tree.getRightOperand(), null);
+        return null;
+      }
+
+      @Override
+      public Void visitInstanceOf(InstanceOfTree tree, Void unused) {
+        instanceOfs.add(tree);
+        return null;
+      }
+    }.visit(condition, null);
+    return instanceOfs.build();
+  }
+}

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

@@ -300,6 +300,7 @@
 import com.google.errorprone.bugpatterns.ParameterComment;
 import com.google.errorprone.bugpatterns.ParameterName;
 import com.google.errorprone.bugpatterns.ParametersButNotParameterized;
+import com.google.errorprone.bugpatterns.PatternMatchingInstanceof;
 import com.google.errorprone.bugpatterns.PreconditionsCheckNotNullRepeated;
 import com.google.errorprone.bugpatterns.PreconditionsInvalidPlaceholder;
 import com.google.errorprone.bugpatterns.PreferredInterfaceType;
@@ -1028,6 +1029,7 @@ public static ScannerSupplier warningChecks() {
           OverridesGuiceInjectableMethod.class,
           OverridingMethodInconsistentArgumentNamesChecker.class,
           ParameterName.class,
+          PatternMatchingInstanceof.class,
           PreconditionsCheckNotNullRepeated.class,
           PrimitiveAtomicReference.class,
           ProtectedMembersInFinalClass.class,

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

@@ -0,0 +1,151 @@
+/*
+ * 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.truth.TruthJUnit.assume;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import com.google.errorprone.util.RuntimeVersion;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class PatternMatchingInstanceofTest {
+  private final BugCheckerRefactoringTestHelper helper =
+      BugCheckerRefactoringTestHelper.newInstance(PatternMatchingInstanceof.class, getClass());
+
+  @Test
+  public void positive() {
+    assume().that(RuntimeVersion.isAtLeast21()).isTrue();
+    helper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o instanceof Test) {",
+            "      Test test = (Test) o;",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .addOutputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o instanceof Test test) {",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void notDefinitelyChecked_noFinding() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o instanceof Test || o.hashCode() > 0) {",
+            "      Test test = (Test) o;",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void moreChecksInIf_stillMatches() {
+    assume().that(RuntimeVersion.isAtLeast21()).isTrue();
+    helper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o instanceof Test && o.hashCode() != 1) {",
+            "      Test test = (Test) o;",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .addOutputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o instanceof Test test && o.hashCode() != 1) {",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void differentTypeToCheck_noFinding() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o instanceof Test) {",
+            "      Integer test = (Integer) o;",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void noInstanceofAtAll_noFinding() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object o) {",
+            "    if (o.hashCode() > 0) {",
+            "      Integer test = (Integer) o;",
+            "      test(test);",
+            "    }",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+
+  @Test
+  public void differentVariable() {
+    helper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  void test(Object x, Object y) {",
+            "    if (x instanceof Test) {",
+            "      Test test = (Test) y;",
+            "      test(test, null);",
+            "    }",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
+}