_form.html.slim - Final

= f.label :email, required: true, class: "block text-xxs uppercase text-gray-500" do |label_builder|
  = label_builder.translation
  span.ml-2.normal-case.text-orange-400.text-xxs.font-semibold= t("required")
  = f.email_field :email, autocomplete: false, required: true, class: "form-input mt-1 block w-full text-field focus:shadow-outline focus:border-green-300", placeholder: "richard@piedpiper.com"
  
  # Or with the form builder
  = f.label :email, required: true, class: "block text-xxs uppercase text-gray-500", required_class: "ml-2 normal-case text-orange-400 text-xxs font-semibold"
  = f.email_field :email, autocomplete: false, required: true, class: "form-input mt-1 block w-full text-field focus:shadow-outline focus:border-green-300", placeholder: "richard@piedpiper.com"

_form.html.slim - Hackish

= f.label :email, required: true, class: "block text-xxs uppercase text-gray-500" do
  = f.object.class.human_attribute_name(:email)
  span.ml-2.normal-case.text-orange-400.text-xxs.font-semibold= t("required")
  = f.email_field :email, autocomplete: false, required: true, class: "form-input mt-1 block w-full text-field focus:shadow-outline focus:border-green-300", placeholder: "richard@piedpiper.com"

_form.html.slim - Starting point

= f.label :email, required: true, class: "block text-xxs uppercase text-gray-500"
= f.email_field :email, autocomplete: false, required: true, class: "form-input mt-1 block w-full text-field focus:shadow-outline focus:border-green-300", placeholder: "richard@piedpiper.com"

requiring_form_builder.rb

# In case you want to put that in a form builder
class RequiringFormBuilder < ActionView::Helpers::FormBuilder
  def label(method, text = nil, options = {}, &block)
    text_is_options = text.is_a?(Hash)
    required = text_is_options ? text[:required] : options[:required]
    if required
      required_class = text_is_options ? text.delete(:required_class) : options.delete(:required_class)
      super(method, text, options) do |label_builder|
        @template.concat label_builder.translation
        @template.concat @template.content_tag(:span, I18n.t("required", scope: :helpers), class: required_class)
        @template.concat(@template.capture(label_builder, &block)) if block_given?
      end
    else
      super(method, text, options, &block)
    end
  end
end