Add error-prone rule for converting Stream to Iterator

README.md

@@ -222,6 +222,7 @@ Safe Logging can be found at [github.com/palantir/safe-logging](https://github.c
 - `InvocationTargetExceptionGetTargetException`: InvocationTargetException.getTargetException() predates the general-purpose exception chaining facility. The Throwable.getCause() method is now the preferred means of obtaining this information. [(source)](https://docs.oracle.com/en/java/javase/17/docs/api//java.base/java/lang/reflect/InvocationTargetException.html#getTargetException())
 - `PreferInputStreamTransferTo`: Prefer JDK `InputStream.transferTo(OutputStream)` over utility methods such as `com.google.common.io.ByteStreams.copy(InputStream, OutputStream)`, `org.apache.commons.io.IOUtils.copy(InputStream, OutputStream)`, `org.apache.commons.io.IOUtils.copyLong(InputStream, OutputStream)`.
 - `ConjureEndpointDeprecatedForRemoval`: Conjure endpoints marked with Deprecated and `forRemoval = true` should not be used as they are scheduled to be removed.
+- `StreamToIterator`: Converting from Stream to Iterator can result in collecting the entire stream in some cases. Prefer sticking with one paradigm or another unless absolutely necessary. (https://bugs.openjdk.org/browse/JDK-8267359)
 
 ### Programmatic Application
 

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

@@ -0,0 +1,81 @@
+/*
+ * (c) Copyright 2025 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.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.matchers.method.MethodMatchers;
+import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MemberReferenceTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.tools.javac.code.Symbol.MethodSymbol;
+import com.sun.tools.javac.code.Type;
+import java.util.stream.BaseStream;
+
+@AutoService(BugChecker.class)
+@BugPattern(
+        link = "https://github.com/palantir/gradle-baseline#baseline-error-prone-checks",
+        linkType = BugPattern.LinkType.CUSTOM,
+        severity = BugPattern.SeverityLevel.WARNING,
+        summary =
+                "Converting from Stream to Iterator can result in collecting the entire stream in some cases. Prefer sticking with one paradigm or another unless absolutely necessary. (https://bugs.openjdk.org/browse/JDK-8267359)")
+public final class StreamToIterator extends BugChecker
+        implements BugChecker.MethodInvocationTreeMatcher, BugChecker.MemberReferenceTreeMatcher {
+    private static final long serialVersionUID = 1L;
+
+    private static final Matcher<ExpressionTree> STREAM_TO_ITERATOR_INVOCATION = MethodMatchers.instanceMethod()
+            .onDescendantOf(BaseStream.class.getName())
+            .namedAnyOf("iterator", "spliterator")
+            .withNoParameters();
+
+    @Override
+    public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+        if (STREAM_TO_ITERATOR_INVOCATION.matches(tree, state)) {
+            return describeMatch(tree);
+        }
+        return Description.NO_MATCH;
+    }
+
+    @Override
+    public Description matchMemberReference(MemberReferenceTree tree, VisitorState state) {
+        if (tree.getMode() != MemberReferenceTree.ReferenceMode.INVOKE) {
+            return Description.NO_MATCH;
+        }
+
+        MethodSymbol method = ASTHelpers.getSymbol(tree);
+        if (!(method.getSimpleName().contentEquals("iterator")
+                || method.getSimpleName().contentEquals("spliterator"))) {
+            return Description.NO_MATCH;
+        }
+
+        if (!method.getParameters().isEmpty()) {
+            return Description.NO_MATCH;
+        }
+
+        Type baseStream = state.getTypeFromString(BaseStream.class.getName());
+        if (!state.getTypes().isSubtype(method.owner.type, state.getTypes().erasure(baseStream))) {
+            return Description.NO_MATCH;
+        }
+
+        return describeMatch(tree);
+    }
+}

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

@@ -0,0 +1,197 @@
+/*
+ * (c) Copyright 2025 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.errorprone.CompilationTestHelper;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public final class StreamToIteratorTest {
+
+    private CompilationTestHelper compilationHelper;
+
+    @BeforeEach
+    public void before() {
+        compilationHelper = CompilationTestHelper.newInstance(StreamToIterator.class, getClass());
+    }
+
+    @Test
+    public void testDirectCase() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        Stream.of(1, 2, 3).iterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testIndirectStream() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.List;",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        List.of(1, 2, 3).stream().iterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testStreamChain() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        Stream.of(1, 2, 3).map(x -> x + 1).flatMap(x -> Stream.of(x)).iterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testStreamAsParameter() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test(Stream<Object> stream) {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        stream.iterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testAsMethodReference() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test(Stream<Stream<Object>> stream) {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        stream.map(Stream::iterator);",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testIntStream() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.IntStream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        IntStream.range(1, 3).iterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testIntStreamReference() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.IntStream;",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        Stream.of(IntStream.range(1, 3)).map(IntStream::iterator);",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testSpliterator() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        Stream.of(1, 2, 3).spliterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testSpliteratorMethodReference() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test(Stream<Stream<Object>> stream) {",
+                        "        // BUG: Diagnostic contains: StreamToIterator",
+                        "        stream.map(Stream::spliterator);",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testNoFalsePositiveOnNonStream() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.List;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        List.of(1, 2, 3).iterator();",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testNoFalsePositiveOnNonStreamReference() {
+        compilationHelper
+                .addSourceLines(
+                        "Test.java",
+                        "import java.util.List;",
+                        "import java.util.stream.Stream;",
+                        "class Test {",
+                        "    private void test() {",
+                        "        Stream.of(List.of(1, 2, 3)).map(List::iterator);",
+                        "    }",
+                        "}")
+                .doTest();
+    }
+}

build.gradle

@@ -28,9 +28,9 @@ id 'org.inferred.processors' version '3.7.0'
 apply plugin: 'com.palantir.external-publish'
 apply plugin: 'com.palantir.failure-reports'
 apply plugin: 'com.palantir.jdks'
-apply plugin: 'com.palantir.consistent-versions'
 apply plugin: 'com.palantir.baseline'
 apply plugin: 'com.palantir.baseline-java-versions'
+apply plugin: 'com.palantir.consistent-versions'
 apply plugin: 'com.palantir.jdks.latest'
 
 allprojects {

changelog/@unreleased/pr-3058.v2.yml

@@ -0,0 +1,5 @@
+type: improvement
+improvement:
+  description: Add error-prone rule for converting Stream to Iterator
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/3058