A PHP Class for inferring variables from a Twig template

Twig2Schema.php

<?php

class Twig2Schema
{
    /**
     * @param \Twig\Environment $twig - A twig environment containing loaded templates
     * @param $twigTemplateName - The name of the template to infer variables from
     * @param $config - A configuration object for this function
     * @return array
     */
    public function infer(\Twig\Environment $twig, $twigTemplateName)
    {
        $source = $twig->getLoader()->getSourceContext($twigTemplateName);
        $tokens = $twig->tokenize($source);
        $ast = $twig->parse($tokens);
        return $this->inferFromAst($ast);
    }

    /**
     * @param \Twig\Node\ModuleNode $ast - An abstract syntax tree parsed from Twig
     * @return array - The variables used in the Twig template
     */
    public function inferFromAst(\Twig\Node\ModuleNode $ast)
    {
        $keys = $this->visit($ast);

        foreach ($keys as $key => $value) {
            if ($value['always_defined'] || $key === '_self') {
                unset($keys[$key]);
            }
        }

        return $keys;
    }

    /**
     * @param \Twig\Node\Node $ast - The tree to traverse and extract variables
     * @return array - The variables found in this tree
     */
    private function visit(\Twig\Node\Node $ast)
    {
        $vars = [];
        switch (get_class($ast)) {
            case \Twig\Node\Expression\AssignNameExpression::class:
            case \Twig\Node\Expression\NameExpression::class:
                $vars[$ast->getAttribute('name')] = [
                    'type' => get_class($ast),
                    'always_defined' => $ast->getAttribute('always_defined'),
                    'is_defined_test' => $ast->getAttribute('is_defined_test'),
                    'ignore_strict_check' => $ast->getAttribute('ignore_strict_check')
                ];
                break;
            case \Twig\Node\ForNode::class:
                foreach ($ast as $key => $node) {
                    switch ($key) {
                        case 'value_target':
                            $vars[$node->getAttribute('name')] = [
                                'for_loop_target' => true,
                                'always_defined' => $node->getAttribute('always_defined')
                            ];
                            break;
                        case 'body':
                            $vars = array_merge($vars, $this->visit($node));
                            break;
                        default:
                            break;
                    }
                }
                break;
            case \Twig\Node\IfNode::class:
                foreach ($ast->getNode('tests') as $key => $test) {
                    $vars = array_merge($vars, $this->visit($test));
                }
                foreach ($ast->getNode('else') as $key => $else) {
                    $vars = array_merge($vars, $this->visit($else));
                }
                break;
            default:
                if ($ast->count()) {
                    foreach ($ast as $key => $node) {
                        $vars = array_merge($vars, $this->visit($node));
                    }
                }
                break;
        }
        return $vars;
    }
}

Thanks a lot for posting this!

It solves a problem I have perfectly, I just needed to add:

if ($ast->hasNode('else') {
  (...)
}

in the IfNode::class case, as it would throw a fatal error if there was no else node in the template.