DuplicateAssertion: detect duplicated assertion lines where the argument to assertThat is pure.

This just looks for successive assertion statements with the same source code, and uses our friend `ConstantExpressions` to check whether the argument to `assertThat` seems pure at heart.

Flume: unknown commit
PiperOrigin-RevId: 821773490

Author: graememorgan

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

@@ -0,0 +1,96 @@
+/*
+ * 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.common.collect.Iterables.getOnlyElement;
+import static com.google.errorprone.matchers.Description.NO_MATCH;
+import static com.google.errorprone.util.ASTHelpers.getReceiver;
+import static com.google.errorprone.util.ASTHelpers.getSymbol;
+
+import com.google.common.collect.ImmutableSetMultimap;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.BugPattern.SeverityLevel;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker.BlockTreeMatcher;
+import com.google.errorprone.bugpatterns.threadsafety.ConstantExpressions;
+import com.google.errorprone.bugpatterns.threadsafety.ConstantExpressions.ConstantExpression;
+import com.google.errorprone.matchers.Description;
+import com.sun.source.tree.BlockTree;
+import com.sun.source.tree.ExpressionStatementTree;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import javax.inject.Inject;
+
+/** A BugPattern; see the {@code summary}. */
+@BugPattern(summary = "This assertion is duplicate.", severity = SeverityLevel.WARNING)
+public final class DuplicateAssertion extends BugChecker implements BlockTreeMatcher {
+  private final ConstantExpressions constantExpressions;
+
+  @Inject
+  DuplicateAssertion(ConstantExpressions constantExpressions) {
+    this.constantExpressions = constantExpressions;
+  }
+
+  @Override
+  public Description matchBlock(BlockTree tree, VisitorState state) {
+    var assertionLines = extractAssertionLines(tree, state);
+
+    for (var entry : assertionLines.entries()) {
+      var source = entry.getKey();
+      var line = entry.getValue();
+      if (assertionLines.containsEntry(source, line - 1)) {
+        state.reportMatch(
+            buildDescription(tree.getStatements().get(line))
+                .setMessage("This assertion is duplicated on the line above. Is that a mistake?")
+                .build());
+      }
+    }
+
+    return NO_MATCH;
+  }
+
+  private ImmutableSetMultimap<Assertion, Integer> extractAssertionLines(
+      BlockTree tree, VisitorState state) {
+    ImmutableSetMultimap.Builder<Assertion, Integer> lines = ImmutableSetMultimap.builder();
+    for (int i = 0; i < tree.getStatements().size(); i++) {
+      int finalI = i;
+      var statement = tree.getStatements().get(i);
+      if (!(statement instanceof ExpressionStatementTree est)) {
+        continue;
+      }
+      if (!(est.getExpression() instanceof MethodInvocationTree mit)) {
+        continue;
+      }
+      for (ExpressionTree receiver = getReceiver(mit);
+          receiver != null;
+          receiver = getReceiver(receiver)) {
+        var symbol = getSymbol(receiver);
+        if ((receiver instanceof MethodInvocationTree method)
+            && symbol.getSimpleName().contentEquals("assertThat")
+            && method.getArguments().size() == 1) {
+          constantExpressions
+              .constantExpression(getOnlyElement(method.getArguments()), state)
+              .ifPresent(
+                  ce -> lines.put(new Assertion(state.getSourceForNode(statement), ce), finalI));
+        }
+      }
+    }
+    return lines.build();
+  }
+
+  private record Assertion(String line, ConstantExpression assertee) {}
+}

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

@@ -122,6 +122,7 @@
 import com.google.errorprone.bugpatterns.DoNotMockAutoValue;
 import com.google.errorprone.bugpatterns.DoNotMockChecker;
 import com.google.errorprone.bugpatterns.DoubleBraceInitialization;
+import com.google.errorprone.bugpatterns.DuplicateAssertion;
 import com.google.errorprone.bugpatterns.DuplicateBranches;
 import com.google.errorprone.bugpatterns.DuplicateDateFormatField;
 import com.google.errorprone.bugpatterns.DuplicateMapKeys;
@@ -940,6 +941,7 @@ public static ScannerSupplier warningChecks() {
           DoNotClaimAnnotations.class,
           DoNotMockAutoValue.class,
           DoubleCheckedLocking.class,
+          DuplicateAssertion.class,
           DuplicateBranches.class,
           DuplicateDateFormatField.class,
           EffectivelyPrivate.class,

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

@@ -0,0 +1,108 @@
+/*
+ * 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 DuplicateAssertionTest {
+  public final CompilationTestHelper helper =
+      CompilationTestHelper.newInstance(DuplicateAssertion.class, getClass());
+
+  @Test
+  public void positive() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            import static com.google.common.truth.Truth.assertThat;
+
+            import java.util.List;
+
+            class Test {
+              public void test(List<Integer> list) {
+                assertThat(list).contains(1);
+                // BUG: Diagnostic contains:
+                assertThat(list).contains(1);
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void negative_notDuplicated() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            import static com.google.common.truth.Truth.assertThat;
+
+            import java.util.List;
+
+            class Test {
+              public void test(List<Integer> list) {
+                assertThat(list).contains(1);
+                assertThat(list).contains(2);
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void negative_notAssertion() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              public void test() {
+                int x = 1;
+                x = 2;
+                x = 2;
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void negative_impure() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            import java.util.Iterator;
+            import static com.google.common.truth.Truth.assertThat;
+
+            import java.util.List;
+
+            class Test {
+              public void test(List<Integer> list) {
+                Iterator<Integer> it = list.iterator();
+                assertThat(it.next()).isEqualTo(1);
+                assertThat(it.next()).isEqualTo(1);
+              }
+            }
+            """)
+        .doTest();
+  }
+}