Report error about wrong C-style type definition for all fields

[ankddev]

Problem

Currently with following code:

pub type User {
  name: String,
  surname: String
}

Gleam gives this error:

error: Syntax error
  ┌─ C:\Users\user\projects\deps_test\src\deps_test.gleam:2:3
  │
2 │   name: String
  │   ^^^^ I was not expecting this

Each custom type variant must have a constructor:

pub type User {
  User(
    name: String,
  )
}

As you see, it says only about one field, which is wrong, I think.

Desired behaviour

It should say about all fields in the error.

Motivation

Firstly, this will improve DX, since user will see full error.
Secondly, this will allow implementation of #3479, which is blocked by this.

Possible solution

I don't know, if this is possible inside parser, since it's fault tolerance skips the entire expression, I think. In case it's possible, we need to say about each field in error.

Things to consider

0. Is it possible in parser?
1. How to display errors about each field (just one I was not expecting this or for each one?)?

[giacomocavalieri]

Whether the solution to this might look like I think we should show a better error rather than repeating the "unexpected field" error for all of them. A single error that highlights the whole structure pointing out that's not how it's written would be better!

[lpil]

It's not wrong, more it's an example of what the syntax looks like. It could be upgraded to parse this non-Gleam syntax in order to show what it would look like in Gleam syntax.

We could also change the error like so:

pub type User {
  User(
    name: String,
    // Other fields here...
  )
}

I don't think highlighting the whole definition is overly clearer, as generally I prefer to point to precisely where the issue is rather than to the definition that contains it. If we are to highlight the whole definition we'll need to decide how the compiler should identify the end of the definition, which would require us to parse some non-Gleam syntaxes and then convert them back into Gleam.

[ankddev]

The main reason to highlight the whole definition is #3479, which needs information about all of fields. Currently it's not possible, or at least I've no idea how to implement it other way.