feat: Concat does not generate mutant when both operands are the same (…

…#1602)

* feat: Concat does not generate mutant when both operands are the same

Closes #1601

* Update ConcatTest.php

* use prettyPrint instead of prettyPrintExpr

* add clone per review suggestion

* suppress psalm issue - ImpureMethodCall

- per @maks-rafalko

* Update Concat.php

* suppress issue in psalm.xml

psalm.xml

@@ -14,4 +14,12 @@
            <directory name="src/PhpParser" />
         </ignoreFiles>
     </projectFiles>
+
+    <issueHandlers>
+        <ImpureMethodCall>
+            <errorLevel type="suppress">
+                <file name="src/Mutator/Operator/Concat.php"/>
+            </errorLevel>
+        </ImpureMethodCall>
+    </issueHandlers>
 </psalm>

src/Mutator/Operator/Concat.php

@@ -40,6 +40,7 @@
 use Infection\Mutator\Mutator;
 use Infection\Mutator\MutatorCategory;
 use PhpParser\Node;
+use PhpParser\PrettyPrinter\Standard;
 
 /**
  * @internal
@@ -81,13 +82,19 @@ public static function getDefinition(): ?Definition
      */
     public function mutate(Node $node): iterable
     {
+        $printer = new Standard();
+
         if ($node->left instanceof Node\Expr\BinaryOp\Concat) {
             $left = new Node\Expr\BinaryOp\Concat($node->left->left, $node->right);
             $right = $node->left->right;
-
-            yield new Node\Expr\BinaryOp\Concat($left, $right);
         } else {
-            yield new Node\Expr\BinaryOp\Concat($node->right, $node->left);
+            [$left, $right] = [$node->right, $node->left];
+        }
+
+        $newNode = new Node\Expr\BinaryOp\Concat($left, $right);
+
+        if ($printer->prettyPrint([clone $node]) !== $printer->prettyPrint([$newNode])) {
+            yield $newNode;
         }
     }
 

tests/phpunit/Mutator/Operator/ConcatTest.php

@@ -154,5 +154,37 @@ public function mutationsProvider(): iterable
                 ,
             ],
         ];
+
+        yield 'Does not flip the same variable' => [
+            <<<'PHP'
+<?php
+
+$a = 'foo';
+$a . $a;
+PHP
+            ,
+            [],
+        ];
+
+        yield 'Does not flip the same variable - multiple concatenation' => [
+            <<<'PHP'
+<?php
+
+$a = 'foo';
+$a . $a . $a;
+PHP
+            ,
+            [],
+        ];
+
+        yield 'Does not flip the same value' => [
+            <<<'PHP'
+<?php
+
+'foo' . 'foo';
+PHP
+            ,
+            [],
+        ];
     }
 }