Get all variable names defined in a Twig template

twig-getTemplateVars.php

<?php
tokens = $twig->tokenize($loader->getSourceContext('index.html'));
$templateVars = [];
while (!$tokens->isEOF()) {
    $token = $tokens->next();
    if ($token->getType() === \Twig_Token::VAR_START_TYPE) {
        $templateVars[] = $tokens->next()->getValue();
        $tokens->next();
    }
}

It is better to handle missing values INSIDE the template (but I didn't want to waste this piece of code)

It depends. If you have to check a template for missing vars, you have to check it outside the template.
Moreover, you can even report if there are unhandled vars in a template to make a sort of check before uploading like "unused declared var" in common programming languages.