Warn that a custom type variant is never constructed

[ollie-dot-earth]

It'd be nice to have a warning if a custom type variant is only ever used in say, an update function, and never actually Constructed anywhere.
Ideally this would work for both private and pub opaque types.

My specific usecase would be Message variants, commonly in lustre applications, where the functions emitting them were removed in a refactor at some point.
But since the update function still handles it there is no warning telling me that the variant is never used.
This makes it pretty annoying to find out if a message is still relevant. Especially in larger applications.

Real world example

https://tangled.org/ollie.earth/inventory/blob/f362445debcefa5f7d2a4d2c8c43f32feae9756c/client/src/inventory/page/project/list.gleam#L308

UserSelectedProject is one of the ones that got deleted in more recent commits.

see: product.gleam for a module with more messages

Constructed example

pub fn main() {
  let assert Ok(_) = lustre.start(lustre.simple(init, update, view), Nil)
}

type Model = ...

fn init(_flags) -> Model {
  todo
}

type Message {
  OldButtonPressed // <- never constructed
  NewButtonPressed
  InputChanged(String)
}

fn update(message: Message, model: Model) -> Model {
  case message {
    OldButtonPressed -> todo
    NewButtonPressed -> todo
    InputChanged(_) -> todo
}

fn view(_model) {
  html.div([], [
    html.button([event.on_click(NewButtonPressed)], [...]),
    html.input([event.on_change(InputChanged)])
  ])
}