Replace Collectors.toUnmodifiableList() with Stream#toList()

baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/StreamToList.java

@@ -0,0 +1,75 @@
+/*
+ * (c) Copyright 2024 Palantir Technologies Inc. All rights reserved.
+ *
+ * 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.palantir.baseline.errorprone;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.matchers.ChildMultiMatcher.MatchType;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.matchers.Matchers;
+import com.google.errorprone.matchers.method.MethodMatchers;
+import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+@AutoService(BugChecker.class)
+@BugPattern(
+        link = "https://github.com/palantir/gradle-baseline#baseline-error-prone-checks",
+        linkType = BugPattern.LinkType.CUSTOM,
+        severity = BugPattern.SeverityLevel.WARNING,
+        summary = "`Stream.toList()` is more efficient than `Stream.collect(Collectors.toUnmodifiableList())`")
+public final class StreamToList extends BugChecker implements BugChecker.MethodInvocationTreeMatcher {
+    private static final long serialVersionUID = 1L;
+
+    private static final Matcher<ExpressionTree> STREAM_COLLECT = MethodMatchers.instanceMethod()
+            .onDescendantOf(Stream.class.getName())
+            .namedAnyOf("collect")
+            .withParameters(Collector.class.getName());
+
+    private static final Matcher<ExpressionTree> COLLECTORS_TO_UNMODIFIABLE_LIST = MethodMatchers.staticMethod()
+            .onDescendantOf(Collectors.class.getName())
+            .namedAnyOf("toUnmodifiableList")
+            .withNoParameters();
+
+    private static final Matcher<ExpressionTree> STREAM_COLLECT_TO_UNMODIFIABLE_LIST = Matchers.methodInvocation(
+            STREAM_COLLECT,
+            // Any of the three MatchTypes are reasonable in this case, given a single arg
+            MatchType.LAST,
+            COLLECTORS_TO_UNMODIFIABLE_LIST);
+
+    @Override
+    public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+        if (STREAM_COLLECT_TO_UNMODIFIABLE_LIST.matches(tree, state)) {
+            ExpressionTree receiver = ASTHelpers.getReceiver(tree.getMethodSelect());
+            if (receiver != null) {
+                return buildDescription(tree)
+                        .addFix(SuggestedFix.builder()
+                                .replace(state.getEndPosition(receiver), state.getEndPosition(tree), ".toList()")
+                                .build())
+                        .build();
+            }
+        }
+        return Description.NO_MATCH;
+    }
+}

baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/StreamToListTest.java

@@ -0,0 +1,63 @@
+/*
+ * (c) Copyright 2024 Palantir Technologies Inc. All rights reserved.
+ *
+ * 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.palantir.baseline.errorprone;
+
+import org.junit.jupiter.api.Test;
+
+class StreamToListTest {
+    @Test
+    public void test() {
+        fix().addInputLines(
+                        "Test.java",
+                        "import java.util.List;",
+                        "import java.util.stream.Collectors;",
+                        "import java.util.stream.Stream;",
+                        "public class Test {",
+                        "  List<String> f0(Stream<String> in) {",
+                        "    return in.toList();",
+                        "  }",
+                        // Collectors.toList() supports nulls & is mutable while Stream#toList() does not
+                        "  List<String> f1(Stream<String> in) {",
+                        "    return in.collect(Collectors.toList());",
+                        "  }",
+                        "  List<String> f2(Stream<String> in) {",
+                        "    return in.collect(Collectors.toUnmodifiableList());",
+                        "  }",
+                        "}")
+                .addOutputLines(
+                        "Test.java",
+                        "import java.util.List;",
+                        "import java.util.stream.Collectors;",
+                        "import java.util.stream.Stream;",
+                        "public class Test {",
+                        "  List<String> f0(Stream<String> in) {",
+                        "    return in.toList();",
+                        "  }",
+                        "  List<String> f1(Stream<String> in) {",
+                        "    return in.collect(Collectors.toList());",
+                        "  }",
+                        "  List<String> f2(Stream<String> in) {",
+                        "    return in.toList();",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    private RefactoringValidator fix() {
+        return RefactoringValidator.of(StreamToList.class, getClass());
+    }
+}

gradle-baseline-java/src/main/java/com/palantir/baseline/extensions/BaselineErrorProneExtension.java

@@ -73,6 +73,7 @@ public class BaselineErrorProneExtension {
             "Slf4jThrowable",
             "StreamOfEmpty",
             "StreamFlatMapOptional",
+            "StreamToList",
             "StrictUnusedVariable",
             "StringBuilderConstantParameters",
             "ThrowError",