[java] UnusedAssignment false positive in record compact constructor

Fixes #4603

docs/pages/release_notes.md

@@ -275,7 +275,8 @@ The rules have been moved into categories with PMD 6.
   * [#3751](https://github.com/pmd/pmd/issues/3751): \[java] Rename some node types
   * [#4628](https://github.com/pmd/pmd/pull/4628):   \[java] Support loading classes from java runtime images
   * [#4753](https://github.com/pmd/pmd/issues/4753): \[java] PMD crashes while using generics and wildcards
-* java-bestpractives
+* java-bestpractices
+  * [#4603](https://github.com/pmd/pmd/issues/4603): \[java] UnusedAssignment false positive in record compact constructor
   * [#4625](https://github.com/pmd/pmd/issues/4625): \[java] UnusedPrivateMethod false positive: Autoboxing into Number
 * java-codestyle
   * [#2847](https://github.com/pmd/pmd/issues/2847): \[java] New Rule: Use Explicit Types
@@ -1406,6 +1407,7 @@ Language specific fixes:
     * [#4516](https://github.com/pmd/pmd/issues/4516): \[java] UnusedLocalVariable: false-negative with try-with-resources
     * [#4517](https://github.com/pmd/pmd/issues/4517): \[java] UnusedLocalVariable: false-negative with compound assignments
     * [#4518](https://github.com/pmd/pmd/issues/4518): \[java] UnusedLocalVariable: false-positive with multiple for-loop indices
+    * [#4603](https://github.com/pmd/pmd/issues/4603): \[java] UnusedAssignment false positive in record compact constructor
     * [#4625](https://github.com/pmd/pmd/issues/4625): \[java] UnusedPrivateMethod false positive: Autoboxing into Number
     * [#4634](https://github.com/pmd/pmd/issues/4634): \[java] JUnit4TestShouldUseTestAnnotation false positive with TestNG
 * java-codestyle

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/internal/DataflowPass.java

@@ -54,6 +54,7 @@ import net.sourceforge.pmd.lang.java.ast.ASTLocalVariableDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTLoopStatement;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodCall;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodDeclaration;
+import net.sourceforge.pmd.lang.java.ast.ASTRecordComponent;
 import net.sourceforge.pmd.lang.java.ast.ASTResourceList;
 import net.sourceforge.pmd.lang.java.ast.ASTReturnStatement;
 import net.sourceforge.pmd.lang.java.ast.ASTStatement;
@@ -818,6 +819,23 @@ public final class DataflowPass {
             return data;
         }
 
+        @Override
+        public SpanInfo visit(ASTCompactConstructorDeclaration node, SpanInfo data) {
+            super.visit(node, data);
+
+            // mark any write to a variable that is named like a record component as usage
+            // record compact constructors do an implicit assignment at the end.
+            for (ASTRecordComponent component : node.getEnclosingType().getRecordComponents()) {
+                node.descendants(ASTAssignmentExpression.class)
+                        .descendants(ASTVariableAccess.class)
+                        .filter(v -> v.getAccessType() == AccessType.WRITE)
+                        .filter(v -> v.getName().equals(component.getVarId().getName()))
+                        .forEach(varAccess -> data.use(varAccess.getReferencedSym(), null));
+            }
+
+            return data;
+        }
+
         /**
          * Whether the variable has an implicit initializer, that is not
          * an expression. For instance, formal parameters have a value

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/bestpractices/xml/UnusedAssignment.xml

@@ -3543,4 +3543,43 @@ public class UnusedAssignmentUnusedVariableFP {
 }
         ]]></code>
     </test-code>
+
+    <test-code>
+        <description>[java] UnusedAssignment false positive in record compact constructor #4603</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public class Main {
+    public static void main(String[] args) {
+        System.out.println(new TestRecord(2).foo());
+    }
+}
+
+record TestRecord(int foo) {
+    TestRecord {
+        // src/Main.java:10:        UnusedAssignment:       The value assigned to variable 'foo' is never used
+        foo = Math.min(foo, 1); // <!--- violation here
+        // implicit this.foo = foo;
+    }
+}
+]]></code>
+    </test-code>
+
+    <test-code>
+        <description>[java] Verify explicit canonical record constructor #4603</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public class Main {
+    public static void main(String[] args) {
+        System.out.println(new TestRecord(2).foo());
+    }
+}
+
+record TestRecord(int foo) {
+    TestRecord(int foo) {
+        foo = Math.min(foo, 1);
+        this.foo = foo; // explicit
+    }
+}
+]]></code>
+    </test-code>
 </test-data>