StopwatchExtension.php

<?php

use Symfony\Component\Stopwatch\Stopwatch;

class StopwatchExtension extends Twig_Extension
{
    private $stopwatch;

    public function __construct(Stopwatch $stopwatch = null)
    {
        $this->stopwatch = $stopwatch;
    }

    public function getTokenParsers()
    {
        return array(
            /*
             * {% stopwatch foo %}
             * Some stuff which will be recorded on the timeline
             * {% endstopwatch %}
             */
            new StopwatchTokenParser(),
        );
    }

    public function getName()
    {
        return 'stopwatch';
    }

    public function getStopwatch()
    {
        return $this->stopwatch;
    }

    public function startEvent($name)
    {
        null === $this->stopwatch ?: $this->stopwatch->start($name);
    }

    public function stopEvent($name)
    {
        null === $this->stopwatch ?: $this->stopwatch->stop($name);
    }
}

StopwatchNode.php

<?php

class StopwatchNode extends Twig_Node
{
    public function __construct($name, $body, $lineno = 0, $tag = null)
    {
        parent::__construct(array('body' => $body), array('name' => $name), $lineno, $tag);
    }

    public function compile(Twig_Compiler $compiler)
    {
        $name = $this->getAttribute('name');

        $compiler
            ->write('$this->env->getExtension(\'stopwatch\')->startEvent(\''.$name.'\');')
            ->subcompile($this->getNode('body'))
            ->write('$this->env->getExtension(\'stopwatch\')->stopEvent(\''.$name.'\');')
        ;
    }
}

StopwatchTokenParser.php

<?php

class StopwatchTokenParser extends Twig_TokenParser
{
    public function parse(Twig_Token $token)
    {
        $lineno = $token->getLine();
        $stream = $this->parser->getStream();

        // {% stopwatch bar %}
        $name = $stream->expect(Twig_Token::NAME_TYPE)->getValue();
        $stream->expect(Twig_Token::BLOCK_END_TYPE);

        // {% endstopwatch %}
        $body = $this->parser->subparse(array($this, 'decideStopwatchEnd'), true);
        $stream->expect(Twig_Token::BLOCK_END_TYPE);

        return new StopwatchNode($name, $body, $lineno, $this->getTag());
    }

    public function decideStopwatchEnd(Twig_Token $token)
    {
        return $token->test('endstopwatch');
    }

    public function getTag()
    {
        return 'stopwatch';
    }
}

@matthiasnoback, thank you! I've created a PR ( symfony/symfony#7953 ), I'll see what the stoffs say about the ternary operator (I like it to use it this way)