SONARJAVA-2511 S4030: Do not raise if there is no usage at all (#5208)

Co-authored-by: Dorian Burihabwa <75226315+dorian-burihabwa-sonarsource@users.noreply.github.com>

Author: tomasz-tylenda-sonarsource

java-checks-test-sources/default/src/main/java/checks/unused/UnusedCollectionCheckSample.java

@@ -17,6 +17,13 @@ int getLengthComp(String a, String b, String c) {
     return a.length() + b.length() + c.length();
   }
 
+  int getLengthNoUsageAtAll(String a, String b, String c) {
+    // Do not report in this case: lack of usage may indicate unreliable data about the symbol
+    // and this case should also be covered by S1481 and S1854.
+    List<String> strings = new ArrayList<>();
+    return a.length() + b.length() + c.length();
+  }
+
   int getLengthCompTested(String a, String b, String c) {
     List<String> strings = new ArrayList<>();
     strings.add(a);

java-checks/src/main/java/org/sonar/java/checks/unused/UnusedCollectionCheck.java

@@ -33,7 +33,8 @@
 import org.sonar.plugins.java.api.tree.VariableTree;
 
 /**
- * Check for collections that are new read, that is the only operations are adding and removing elements.
+ * Check for collections whose content is not used,
+ * that is, the only operations on them are adding and removing elements.
  */
 @Rule(key = "S4030")
 public class UnusedCollectionCheck extends IssuableSubscriptionVisitor {
@@ -52,6 +53,9 @@ public void visitNode(Tree tree) {
       if (symbol.type().isSubtypeOf("java.util.Collection") &&
         symbol.isLocalVariable() &&
         isInitializedByConstructor(variableTree.initializer()) &&
+        // Do not raise without any usage: usages() may be unreliable,
+        // moreover, this case is in scope of S1481 and S1854 (unused variable and assignment)
+        !symbol.usages().isEmpty() &&
         symbol.usages().stream().allMatch(UnusedCollectionCheck::isWriteOnly)) {
         reportIssue(variableTree.simpleName(), "Consume or remove this unused collection");
       }