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.

Oh, that’s awesome. That actually works great, idk why I didn’t think of that
Thanks!

After implementing this, it does work but I’m also getting warnings in the console about it. Do you just ignore them? Lol

     warning: attribute "type" in component NuzzleWeb.CoreComponents.input/1 must be one of ["checkbox", "color", "date", "datetime-local", "email", "file", "month", "number", "password", "range", "search", "select", "tel", "text", "textarea", "time", "url", "week"], got: "hidden"
     │
 142 │             <.input type="hidden" field={@form[:gender]} />
     │             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     │
     └─ lib/nuzzle_web/live/quote.ex:142: (file)

<.input> bundles a bunch of markup around labels and errors together, which doesn’t really make sense for hidden inputs. You can just use a normal input and the <.error> component separately.

Are you aware of an example somewhere that shows what this would look like?
The only example I have come across with the <.error> component is this, which doesn’t seem relevant for showing errors for a single input:
<.error :for={msg <- @errors}><%= msg %></.error>

Not sure how this wouldn’t seem relevant, but if you follow what happens as part of <.input> it would show that it does the following eventually:

<.error :for={msg <- Enum.map(@form[:gender].errors, &translate_error(&1)}><%= msg %></.error>