Improve spans; use tuple type for virtual signatures

[Bromeon]

Addresses first part of #641.

Stores the type information as a tuple (type-list) for each virtual function.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1370

[Yarwin]

The outcome adds a new error which still points at #[godot_api]:

after

error[E0050]: method `process` has 1 parameter but the declaration in trait `godot::prelude::INode::process` has 2
  --> src/lib.rs:16:16
   |
16 |     fn process(&mut self) {}
   |                ^^^^^^^^^ expected 2 parameters, found 1
   |
   = note: `process` from trait: `fn(&mut Self, f64)`

error[E0308]: mismatched types
  --> src/lib.rs:14:1
   |
14 | #[godot_api]
   | ^^^^^^^^^^^^
   | |
   | expected `(f64,)`, found `()`
   | expected due to this
   |
   = note:  expected tuple `(f64,)`
           found unit type `()`
   = note: this error originates in the attribute macro `godot_api` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0061]: this function takes 2 arguments but 1 argument was supplied
   --> src/lib.rs:14:1
    |
 14 | #[godot_api]
    | ^^^^^^^^^^^^ argument #2 of type `f64` is missing
    |
note: method defined here
   --> .../target/debug/build/godot-core-b05936b7ac85ab80/out/classes/node.rs:118:12
    |
118 |         fn process(&mut self, delta: f64,) {
    |            ^^^^^^^
    = note: this error originates in the attribute macro `godot_api` (in Nightly builds, run with -Z macro-backtrace for more info)

before

  --> src/lib.rs:16:16
   |
16 |     fn process(&mut self) {}
   |                ^^^^^^^^^ expected 2 parameters, found 1
   |
   = note: `process` from trait: `fn(&mut Self, f64)`

error[E0061]: this function takes 2 arguments but 1 argument was supplied
   --> src/lib.rs:14:1
    |
 14 | #[godot_api]
    | ^^^^^^^^^^^^ argument #2 of type `f64` is missing
    |
note: method defined here
   --> .../target/debug/build/godot-core-b05936b7ac85ab80/out/classes/node.rs:118:12
    |
118 |         fn process(&mut self, delta: f64,) {
    |            ^^^^^^^
    = note: this error originates in the attribute macro `godot_api` (in Nightly builds, run with -Z macro-backtrace for more info)

We can do better though – by adding span to params, like that:

    pub fn arg_decl(&self) -> TokenStream {
        let param_types = &self.param_idents;
        let mut sig = Group::new(Delimiter::Parenthesis, quote! { #(#param_types,)* });
        sig.set_span(Span::call_site().resolved_at(self.span));
        sig.to_token_stream()
    }

    pub fn params_type(&self) -> TokenStream {
        let param_types = &self.param_types;
        let mut sig = Group::new(Delimiter::Parenthesis, quote! { #(#param_types,)* });
        sig.set_span(Span::call_site().resolved_at(self.span));
        sig.to_token_stream()
    }

and using properly spanned Ident for params (instead of dummy tokens):

fn make_forwarding_closure(
...
) -> TokenStream {
...
    let method_name = &signature_info.method_name;
    let params = &signature_info.param_idents;
    let params_decl = &signature_info.arg_decl();
    let mut params_ident = ident("params");
    params_ident.set_span(signature_info.span);
...

...
            quote! {
                |instance_ptr, params| {
                    let #params_decl #sig_tuple_annotation = #params_ident;

                    let storage =
                        unsafe { ::godot::private::as_storage::<#class_name>(instance_ptr) };

                    #instance_decl
                    #before_method_call
                    #method_call
                }
            }

We can point new error to the exact user code:

better

error[E0050]: method `process` has 1 parameter but the declaration in trait `godot::prelude::INode::process` has 2
  --> src/lib.rs:16:16
   |
16 |     fn process(&mut self) {}
   |                ^^^^^^^^^ expected 2 parameters, found 1
   |
   = note: `process` from trait: `fn(&mut Self, f64)`

error[E0308]: mismatched types
  --> src/lib.rs:16:15
   |
14 | #[godot_api]
   | ------------ expected due to this
15 | impl INode for MyClass {
16 |     fn process(&mut self) {}
   |               ^^^^^^^^^^^ expected `(f64,)`, found `()`
   |
   = note:  expected tuple `(f64,)`
           found unit type `()`

(rest errors as they were)

I'll do it in separate PR (it will require some consideration to NOT drown in badly glued spans to everything 😅).

(honestly, we can just store groups directly – it is also a bit pointless in this case, since compiler only cares about #params_ident. I would like to have everything spanned well just-in-case tho, it is required for various rewrites – like Gd<Self> virtual methods)

[Bromeon]

Very nice, thanks! I actually wanted to make it a draft PR, but wasn't sure about the exact missing pieces. Very glad you have an idea of them, a follow-up PR definitely makes sense! 💪

I won't let this close #641 yet, but your PR could maybe do that.