ExpicitArrayForVarargs: flag unnecessary explicit construction of an array to provide varargs.

This is definitely one of those "hard to be sure overloads won't cause pain" checks. Flume: http://unknown commit

PiperOrigin-RevId: 808102926

Author: graememorgan

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

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2025 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.getStartPosition;
+import static com.google.errorprone.util.ASTHelpers.getSymbol;
+import static com.google.errorprone.util.ASTHelpers.getType;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.MethodInvocationTreeMatcher;
+import com.google.errorprone.bugpatterns.BugChecker.NewClassTreeMatcher;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.source.tree.NewArrayTree;
+import com.sun.source.tree.NewClassTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol.MethodSymbol;
+import com.sun.tools.javac.code.Type.ArrayType;
+import java.util.List;
+
+/** A {@link BugChecker}; see the summary. */
+@BugPattern(summary = "Avoid explicit array creation for varargs", severity = WARNING)
+public final class ExplicitArrayForVarargs extends BugChecker
+    implements MethodInvocationTreeMatcher, NewClassTreeMatcher {
+  @Override
+  public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+    return handle(tree, getSymbol(tree), tree.getArguments(), state);
+  }
+
+  @Override
+  public Description matchNewClass(NewClassTree tree, VisitorState state) {
+    return handle(tree, getSymbol(tree), tree.getArguments(), state);
+  }
+
+  private Description handle(
+      Tree tree, MethodSymbol symbol, List<? extends ExpressionTree> args, VisitorState state) {
+    if (!symbol.isVarArgs()) {
+      return NO_MATCH;
+    }
+    if (args.isEmpty()) {
+      return NO_MATCH;
+    }
+    // The last argument isn't substituting for varargs if it isn't in place of the varargs
+    // parameter.
+    if (args.size() != symbol.getParameters().size()) {
+      return NO_MATCH;
+    }
+    if (!(args.get(args.size() - 1) instanceof NewArrayTree newArrayTree)) {
+      return NO_MATCH;
+    }
+    // Bail out if we're constructing a multidimensional array.
+    if (((ArrayType) getType(newArrayTree)).getComponentType() instanceof ArrayType) {
+      return NO_MATCH;
+    }
+    var initializers = newArrayTree.getInitializers();
+    var fix =
+        initializers.isEmpty()
+            ? SuggestedFixes.removeElement(newArrayTree, args, state)
+            : SuggestedFix.builder()
+                .replace(getStartPosition(newArrayTree), getStartPosition(initializers.get(0)), "")
+                .replace(
+                    state.getEndPosition(
+                        newArrayTree.getInitializers().get(initializers.size() - 1)),
+                    state.getEndPosition(newArrayTree),
+                    "")
+                .build();
+    return describeMatch(tree, fix);
+  }
+}

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

@@ -143,6 +143,7 @@
 import com.google.errorprone.bugpatterns.ErroneousThreadPoolConstructorChecker;
 import com.google.errorprone.bugpatterns.ExpectedExceptionChecker;
 import com.google.errorprone.bugpatterns.ExpensiveLenientFormatString;
+import com.google.errorprone.bugpatterns.ExplicitArrayForVarargs;
 import com.google.errorprone.bugpatterns.ExtendingJUnitAssert;
 import com.google.errorprone.bugpatterns.ExtendsAutoValue;
 import com.google.errorprone.bugpatterns.FallThrough;
@@ -1212,6 +1213,7 @@ public static ScannerSupplier warningChecks() {
           EqualsBrokenForNull.class,
           EqualsMissingNullable.class,
           ExpectedExceptionChecker.class,
+          ExplicitArrayForVarargs.class,
           ExtendsAutoValue.class,
           FieldCanBeFinal.class,
           FieldCanBeLocal.class,

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

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2025 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 ExplicitArrayForVarargsTest {
+  private final CompilationTestHelper helper =
+      CompilationTestHelper.newInstance(ExplicitArrayForVarargs.class, getClass());
+
+  @Test
+  public void positive() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              void test() {
+                // BUG: Diagnostic contains: String.format("foo %s", "foo")
+                String a = String.format("foo %s", new Object[] {"foo"});
+                // BUG: Diagnostic contains: String.format("foo %s")
+                String b = String.format("foo %s", new Object[] {});
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void multidimensionalArray() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            import java.util.Arrays;
+            import java.util.List;
+
+            class Test {
+              List<Object[]> test() {
+                return Arrays.asList(new Object[][] {{1, 2}, {3, 4}});
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void notFinalArgument() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            import java.util.Arrays;
+            import java.util.List;
+
+            class Test {
+              List<Object[]> test() {
+                return Arrays.asList(new Object[] {1, 2}, new Object[] {3, 4});
+              }
+            }
+            """)
+        .doTest();
+  }
+}