attributes.php

<?php

$autoloader = require_once 'autoload.php';

use PhpParser\{Node, ParserFactory};
use Drupal\Core\Extension\Hook;

$parser = (new ParserFactory)->create(ParserFactory::PREFER_PHP7);
$stmts  = $parser->parse(file_get_contents("core/modules/node/src/Hooks/Whatever.php"));
$nameResolver = new PhpParser\NodeVisitor\NameResolver;
$nodeTraverser = new PhpParser\NodeTraverser;
$nodeTraverser->addVisitor($nameResolver);
$stmts = $nodeTraverser->traverse($stmts);

$nodeFinder = new PhpParser\NodeFinder();
$classes = $nodeFinder->findInstanceOf($stmts, Node\Stmt\Class_::class);
foreach ($classes as $class) {
  $class_is_hook = FALSE;
  foreach ($class->attrGroups as $attrGroup) {
    foreach ($attrGroup->attrs as $attribute) {
      if ((string) $attribute->name === Hook::class) {
        $arguments = [];
        foreach ($attribute->args as $arg) {
          $arguments[(string) $arg->name] = $arg->value->value;
        }
        if (isset($arguments['hook'])) {
          if (!isset($arguments['method'])) {
            throw new \LogicException('');
          }
          $implementations[] = new Hook(... $arguments);
        }
      }
    }
  }
  $methods = $nodeFinder->findInstanceOf($class->stmts, Node\Stmt\ClassMethod::class);
  foreach ($methods as $method) {
    foreach ($method->attrGroups as $attrGroup) {
      foreach ($attrGroup->attrs as $attribute) {
        if ((string) $attribute->name === Hook::class) {
          $arguments = [];
          foreach ($attribute->args as $arg) {
            $arguments[(string) $arg->name] = $arg->value->value;
          }
          if (isset($arguments['hook'])) {
            $implementations[] = new Hook(... $arguments)->setMethod((string) $method->name]);
          }
        }
      }
    }
  }
}
var_dump($implementations);