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Add Check Support for Java 21 Unnamed Variables & Patterns Syntax: UnusedLocalVariable #14950

@mahfouz72

Description

@mahfouz72

child of #14942 , #14960

we need to update UnusedLocalVariableCheck to support unnamed variables in local variables declaration and in enhanced/basic for.

I have read check documentation: https://checkstyle.org/checks/coding/unusedlocalvariable.html
I have downloaded the latest checkstyle from: https://checkstyle.org/cmdline.html#Download_and_Run
I have executed the cli and showed it below, as cli describes the problem better than 1,000 words

PS D:\CS\test> javac src/Test.java
PS D:\CS\test> cat src/Test.java
import java.util.PriorityQueue;
import java.util.Queue;

class Test {
    void test() {
        Queue<Integer> q = new PriorityQueue<>();
        q.add(1);
        while (!q.isEmpty()) {
            var _ = q.poll();   // violation
        }
    }
}
PS D:\CS\test> cat config.xml
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
          "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
          "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">

<module name="Checker">
    <property name="charset" value="UTF-8"/>
 <module name="TreeWalker">
    <module name="UnusedLocalVariable">

    </module>
 </module>
</module>
PS D:\CS\test> java  -jar checkstyle-10.14.2-all.jar -c config.xml  src/Test.java
Starting audit...
[ERROR] D:\CS\test\src\Test.java:9:13: Unused local variable '_'. [UnusedLocalVariable]
Audit done.
Checkstyle ends with 1 errors.

Describe what you expect in detail.

underscore is used to declare variables that are not intended to be used. So, I consider this example as a false positive. the check should not violate unnamed variables


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