[printer] Keep blank lines between statements when a node list is emptied

src/PhpParser/Printer/BetterStandardPrinter.php

@@ -29,8 +29,11 @@
 use PhpParser\Node\InterpolatedStringPart;
 use PhpParser\Node\Scalar\InterpolatedString;
 use PhpParser\Node\Scalar\String_;
+use PhpParser\Node\Stmt\Class_;
 use PhpParser\Node\Stmt\Declare_;
+use PhpParser\Node\Stmt\Enum_;
 use PhpParser\Node\Stmt\InlineHTML;
+use PhpParser\Node\Stmt\Interface_;
 use PhpParser\Node\Stmt\Nop;
 use PhpParser\PrettyPrinter\Standard;
 use PhpParser\Token;
@@ -55,6 +58,19 @@ final class BetterStandardPrinter extends Standard
      */
     private const string EXTRA_SPACE_BEFORE_NOP_REGEX = '#^[ \t]+$#m';
 
+    /**
+     * php-parser cannot emptying a node list, it falls back to a full reprint of the parent node
+     * and drops blank lines between its statements; these keyword tokens let us cut just the
+     * emptied clause instead, e.g. the "implements" of a class that loses its last interface
+     *
+     * @var array<string, int>
+     */
+    private const array EMPTIED_LIST_KEYWORD_TOKENS = [
+        Class_::class . '->implements' => \T_IMPLEMENTS,
+        Enum_::class . '->implements' => \T_IMPLEMENTS,
+        Interface_::class . '->extends' => \T_EXTENDS,
+    ];
+
     public function __construct(
         private readonly ExprAnalyzer $exprAnalyzer,
         private readonly PrivatesAccessor $privatesAccessor,
@@ -230,9 +246,20 @@ protected function pArray(
         string $subNodeName,
         ?int $fixup
     ): ?string {
+        $startPos = $pos;
+
         $content = parent::pArray($nodes, $origNodes, $pos, $indentAdjustment, $parentNodeClass, $subNodeName, $fixup);
         if ($content === null) {
-            return $content;
+            $cutToPos = $this->resolveEmptiedNodeListEndPos($nodes, $origNodes, $startPos, $parentNodeClass, $subNodeName);
+            if ($cutToPos === null) {
+                // restore the position possibly advanced by the parent before falling back
+                $pos = $startPos;
+                return null;
+            }
+
+            // drop the whole clause: whitespace + keyword + type list
+            $pos = $cutToPos;
+            return '';
         }
 
         if (! $this->containsNop($nodes)) {
@@ -242,6 +269,62 @@ protected function pArray(
         return Strings::replace($content, self::EXTRA_SPACE_BEFORE_NOP_REGEX);
     }
 
+    /**
+     * php-parser bails on emptying a node list, which forces the parent node to be fully reprinted
+     * and loses blank lines between statements; when the whole list is removed we can instead cut
+     * just the emptied clause (keyword + types) and keep the rest format-preserved
+     *
+     * Returns the token position right after the removed clause, or null when not handled.
+     *
+     * @param mixed[] $nodes
+     * @param array<int, mixed> $origNodes
+     */
+    private function resolveEmptiedNodeListEndPos(
+        array $nodes,
+        array $origNodes,
+        int $startPos,
+        string $parentNodeClass,
+        string $subNodeName
+    ): ?int {
+        // only the "whole list removed" case is handled here
+        if ($nodes !== [] || $origNodes === []) {
+            return null;
+        }
+
+        if (! $this->origTokens instanceof TokenStream) {
+            return null;
+        }
+
+        $keywordToken = self::EMPTIED_LIST_KEYWORD_TOKENS[$parentNodeClass . '->' . $subNodeName] ?? null;
+        if ($keywordToken === null) {
+            return null;
+        }
+
+        $firstOrigNode = $origNodes[0];
+        $lastOrigNode = $origNodes[count($origNodes) - 1];
+        if (! $firstOrigNode instanceof Node || ! $lastOrigNode instanceof Node) {
+            return null;
+        }
+
+        $firstStartPos = $firstOrigNode->getStartTokenPos();
+        $lastEndPos = $lastOrigNode->getEndTokenPos();
+        if ($firstStartPos < 0 || $lastEndPos < 0) {
+            return null;
+        }
+
+        // the keyword must sit between the previous sub-node and the removed list
+        if (! $this->origTokens->haveTokenInRange($startPos, $firstStartPos, $keywordToken)) {
+            return null;
+        }
+
+        // bail on php tags in the removed range to avoid dropping them
+        if ($this->origTokens->haveTagInRange($startPos, $lastEndPos + 1)) {
+            return null;
+        }
+
+        return $lastEndPos + 1;
+    }
+
     /**
      * Do not add "()" on Expressions
      * @see https://github.com/rectorphp/rector/pull/401#discussion_r181487199

tests/PhpParser/Printer/BetterStandardPrinterTest.php

@@ -11,12 +11,16 @@
 use PhpParser\Node\Expr\Variable;
 use PhpParser\Node\Expr\Yield_;
 use PhpParser\Node\Scalar\String_;
+use PhpParser\Node\Stmt\Class_;
 use PhpParser\Node\Stmt\ClassMethod;
 use PhpParser\Node\Stmt\Expression;
+use PhpParser\NodeTraverser;
+use PhpParser\NodeVisitor\CloningVisitor;
 use PHPUnit\Framework\Attributes\DataProvider;
 use Rector\Configuration\Option;
 use Rector\Configuration\Parameter\SimpleParameterProvider;
 use Rector\NodeTypeResolver\Node\AttributeKey;
+use Rector\PhpParser\Parser\RectorParser;
 use Rector\PhpParser\Printer\BetterStandardPrinter;
 use Rector\Testing\PHPUnit\AbstractLazyTestCase;
 
@@ -49,6 +53,32 @@ public function testAddingCommentOnSomeNodesFail(): void
         );
     }
 
+    public function testKeepBlankLinesBetweenMethodsWhenImplementsRemoved(): void
+    {
+        $fileContent = <<<'PHP'
+<?php
+
+class SomeClass implements \SomeInterface
+{
+    public function first()
+    {
+    }
+
+    public function second()
+    {
+    }
+}
+PHP;
+
+        $printed = $this->printWithRemovedImplements($fileContent);
+
+        // the interface is gone...
+        $this->assertStringNotContainsString('implements', $printed);
+
+        // ...but the blank line between methods is kept
+        $this->assertStringContainsString("    }\n\n    public function second()", $printed);
+    }
+
     public function testStringWithAddedComment(): void
     {
         $string = new String_('hey');
@@ -105,6 +135,26 @@ public function testNoNewlineOnFluentCallWithoutAttribute(): void
         $this->assertSame('$foo->bar()->baz()', $printed);
     }
 
+    private function printWithRemovedImplements(string $fileContent): string
+    {
+        $rectorParser = $this->make(RectorParser::class);
+        $stmtsAndTokens = $rectorParser->parseFileContentToStmtsAndTokens($fileContent);
+
+        $oldStmts = $stmtsAndTokens->getStmts();
+        $tokens = $stmtsAndTokens->getTokens();
+
+        $nodeTraverser = new NodeTraverser(new CloningVisitor());
+        $newStmts = $nodeTraverser->traverse($oldStmts);
+
+        foreach ($newStmts as $newStmt) {
+            if ($newStmt instanceof Class_) {
+                $newStmt->implements = [];
+            }
+        }
+
+        return $this->betterStandardPrinter->printFormatPreserving($newStmts, $oldStmts, $tokens);
+    }
+
     /**
      * @return Iterator<array<int, (Yield_|Expression|string)>>
      */