Copy function body to new file

[chx]

I am trying to write a rule which copies the body in a function into a method. I know this is not the most usual use case here but I am trying because Drupal uses Drupal Rector and I'd rather integrate with that than ship an entirely separate utility.

So I have:

    public function getNodeTypes(): array
    {
        return [\PhpParser\Node\Stmt\Function_::class];
    }

    public function refactor(Node $node): ?Node
    {
      $prettyPrinter = new BetterStandardPrinter();
      echo $prettyPrinter->printFormatPreserving($node->getStmts(), $node->getStmts(), $this->file->getOldTokens());
        return $node;
    }

While echo $prettyPrinter->print($node) prints the function body, printFormatPreserving called like this prints the entire file instead of just the function body. Please advise.