Create rule S7473: Unused variables in pattern matching should be replaced with unnamed variables

Author: romainbrenguier

rules/S7473/java/metadata.json

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+{
+  "title": "Unused variables in pattern matching should be replaced with unnamed variables",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "1min"
+  },
+  "tags": [
+    "unused",
+    "java22"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-7473",
+  "sqKey": "S7473",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "targeted",
+  "code": {
+    "impacts": {
+      "MAINTAINABILITY": "MEDIUM"
+    },
+    "attribute": "CONVENTIONAL"
+  }
+}

rules/S7473/java/rule.adoc

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+== Why is this an issue?
+
+Unused variables in pattern matching can make the code harder to understand and cause confusion.
+This may lead to potential errors.
+
+For example, the variable that is declared but not used, may be mistakenly assumed to have significance in the code.
+This can lead to misunderstandings and make the code less readable.
+
+== How to fix it
+
+Replace unused variables with the unnamed variable `_`.
+This will not change the behavior of the program, but it will reduce clutter and make the code clearer.
+
+=== Exceptions
+
+Unnamed variables are only available since Java 22, so the rule ignores projects configured with earlier
+Java versions.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,java,diff-id=1,diff-type=noncompliant]
+----
+public void foo(Ball ball){
+ switch (ball) {
+   case RedBall   red   -> process(ball);    // Noncompliant, Use _ instead
+   case BlueBall  blue  -> process(ball);    // Noncompliant, Use _ instead
+   case GreenBall green -> stopProcessing(); // Noncompliant, Use _ instead
+ }
+ if (r instanceof ColoredPoint(Point(int x, int y), Color c)) { // Noncompliant, Use _ instead
+   System.out.println("x: " + x);
+  }
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=1,diff-type=compliant]
+----
+public void foo(Ball ball){
+ switch (ball) {
+   case RedBall   _    -> process(ball);      // Compliant, unused variable has been replaced with _
+   case BlueBall  blue -> processColor(blue); // Compliant, variable blue is used
+   case GreenBall _    -> stopProcessing();   // Compliant
+ }
+ if (r instanceof ColoredPoint(Point(int x, _), _)) {  // Compliant, no unused named variables
+   System.out.println("x: " + x);
+  }
+}
+----
+
+== Resources
+
+* https://openjdk.org/jeps/456[JEP 456: Unnamed Variables & Patterns]
+

rules/S7473/metadata.json

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+{
+}