Emit proper compile error while trying to #[export] Gd<T> or DynGd<T, D>.

[Yarwin]

Changes compile error from:

error[E0271]: type mismatch resolving `<Node as Bounds>::Memory == MemRefCounted`
  --> src/lib.rs:15:5
   |
15 |     #[export]
   |     ^ expected `MemRefCounted`, found `MemManual`
   |
   = note: required for `godot::prelude::Gd<godot::prelude::Node>` to implement `Default`

error[E0277]: `#[var]` properties require `Var` trait; #[export] ones require `Export` trait
  --> src/lib.rs:16:20
   |
16 |     faulty_export: Gd<Node>,
   |                    ^^^^^^^^ type cannot be used as a property
   |
   = help: the trait `Export` is not implemented for `godot::prelude::Gd<godot::prelude::Node>`
   = note: see also: https://godot-rust.github.io/book/register/properties.html
   = help: the following other types implement trait `Export`:
             Aabb
             Array<DynGd<T, D>>
             Array<T>
             Array<godot::prelude::Gd<T>>
             Basis
             Color
             Dictionary
             GString
           and 42 others

Some errors have detailed explanations: E0271, E0277.
For more information about an error, try `rustc --explain E0271`.
error: could not compile `docstest` (lib) due to 3 previous errors

to:

error: `Gd` and `DynGd` cannot be used directly as properties because they are non-nullable. Use `OnEditor<T>` or `Option<T>` instead.
  --> src/lib.rs:16:20
   |
16 |     faulty_export: Gd<Node>,
   |                    ^^

error: could not compile `docstest` (lib) due to 1 previous error

in case if one tries to export non-nullable property such as:

    #[export]
    faulty_export: Gd<Node>,

I'm not sure if adding extra info/note to diagnostic::on_unimplemented wouldn't be better choice though 🤔. It doesn't seem to break or change anything 🤔 (checked with two medium-sized projects + our tests obviously). Gd<T> couldn't be an #[export] anyway, so 🤔

It provides a bit better IDE experience (notes are omitted by Zed and RustRover):

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1243

[Bromeon]

The result is very nice. But we should generally decide how much we want proc-macros to introspect concrete types, especially in cases where it's "only" informational. For example, initially removing #[base] for Base<T> fields #577 was a bit controversial (also on Discord) due to the "magic", and I added #[hint] attributes to override macro inference.

Here, there's tiny small chance that the macro rejects valid code because other types happen to be named Gd/DynGd as well. This is very unlikely though, so I'm not categorically against it, if the user experience is overall positive.

However, I also want to be careful about setting a precedent to do all sorts of validations on the proc-macro level.

---

I'm not sure if adding extra info/note to diagnostic::on_unimplemented wouldn't be better choice though 🤔

That lint doesn't work for specific attempts (impls that are tried but are absent), does it? So on the type-system level there isn't a better message than the generic one 🤔

[Bromeon]

Also clarified semantics in #1244.

[Yarwin]

Here, there's tiny small chance that the macro rejects valid code because other types happen to be named Gd/DynGd as well. This is very unlikely though, so I'm not categorically against it, if the user experience is overall positive.

Yeah, it irks me as well – I can't really say what (if anything) can go wrong and I would rather avoid something which might have weird side effects 🤔. It is also yet another thing which can go wrong or stop working during refactors or after heavier changes.

I decided to add an extra note to Export diagnostic instead (shame we can't control types shown in second help section):

error[E0277]: `#[var]` properties require `Var` trait; #[export] ones require `Export` trait
  --> src/lib.rs:16:20
   |
16 |     faulty_export: Gd<Node>,
   |                    ^^^^^^^^ type cannot be used as a property
   |
   = help: the trait `Export` is not implemented for `godot::prelude::Gd<godot::prelude::Node>`
   = note: see also: https://godot-rust.github.io/book/register/properties.html
   = note: `Gd` and `DynGd` cannot be used directly as properties because they are non-nullable. In such a case use `OnEditor<T>` or `Option<T>` instead.
   = help: the following other types implement trait `Export`:
             Aabb
             Array<DynGd<T, D>>
             Array<T>
             Array<godot::prelude::Gd<T>>
             Basis
             Color
             Dictionary
             GString
           and 42 others

[Bromeon]

The note is too long, it should ideally not need horizontal scrollbar here.

Would it be possible to have

   = note: `Gd` and `DynGd` cannot be exported directly; wrap them in `Option<...>` or `OnEditor<...>`.

[Yarwin]

The note is too long, it should ideally not need horizontal scrollbar here.

Changed; Aye, we don't have to be so mouthful after docs has been updated.