Before Phoenix 1.7 error_tag was a function defined for you in the ErrorHelpers module (source).
Since 1.7 there is a CoreComponents module. If has an <.input /> component that renders an input field with error feedback using <.error />. So you could use that:
<.input field={@form[:some_field]} type="hidden" />
This will generate a hidden input field and the error feedback.
If you’d rather display the error yourself you can just access it directly: @form[:some_field].errors and render it as you like.
The Phoenix Form Bindings guide discusses using phx-feedback attribute for styling and hiding error messages.
Updated: Just noticed LiveView 1.0 will drop support for phx-feedback.
