Symfony Forms: HTML5 datalist

First, add your new FormType for the field:.

<?php
// src/Acme/Form/Type/DatalistType
namespace Acme\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\OptionsResolver\OptionsResolver;

class DatalistType extends AbstractType
{
    public function getParent()
    {
        return TextType::class;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setRequired(['choices']);
    }

    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        $view->vars['choices'] = $options['choices'];
    }

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

In services.yml:

form.type.datalist_type:
    class: Acme\Form\Type\DatalistType
    tags:
        -  { name: form.type, alias: datalist }

Do you have a form theme? If yes, skip to the next step, if no, create a new one in app/Resources/views/Form/fields.html.twig and change your default Twig theme to it:

# app/config/config.yml
twig:
    form_themes:
        - ':Form:fields.html.twig'

Now define a template for your new field in the form theme:

{% block datalist_widget %}
    <input list="{{ id }}_list" {{ block('widget_attributes') }}{% if value is not empty %}value="{{ value }}"{% endif %}>
    <datalist id="{{ id }}_list">
        {% for choice in choices %}
            <option value="{{ choice }}"></option>
        {% endfor %}
    </datalist>
{% endblock %}

Use your field in FormType:

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder->add('country', DatalistType::class, ['choices' => ['a', 'b']]); 
}

Instead of ['a', 'b'] You need to load your choices from DB somehow, I’d suggest passing them in form options as the easiest solution.