Handle generics in NativeScript derive and #[methods]

[chitoyuu]

The derive macros are now aware of both lifetime and type parameters. 'static bounds are added to both kinds, which improves the error message shown when a user tries to put a lifetime into a NativeClass.

This behavior is chosen because unlike type parameters, lifetime parameters aren't very useful in this context: only 'static would be supported anyway, and any attempted usage is hightly likely to have originated from user error.

Also added the #[skip] attribute for skipping fields in FromVarargs. This is useful mainly for skipping marker fields in generic code.

Close #601

Error message

The error message that appears when lifetime parameters are declared for NativeClass types now clearly indicate where the problem lies and what the expectations are:

error[E0478]: lifetime bound not satisfied
 --> tests/ui/derive_fail_lifetime.rs:3:10
  |
3 | #[derive(NativeClass)]
  |          ^^^^^^^^^^^
  |
note: lifetime parameter instantiated with the lifetime `'a` as defined here
 --> tests/ui/derive_fail_lifetime.rs:4:12
  |
4 | struct Foo<'a> {
  |            ^^
  = note: but lifetime parameter must outlive the static lifetime
  = note: this error originates in the derive macro `NativeClass` (in Nightly builds, run with -Z macro-backtrace for more info)

Compares this with the old error message:

error[E0726]: implicit elided lifetime not allowed here
 --> tests/ui/derive_fail_lifetime.rs:4:8
  |
4 | struct Foo<'a> {
  |        ^^^ expected lifetime parameter
  |
  = note: assuming a `'static` lifetime...
help: indicate the anonymous lifetime
  |
4 | struct Foo<'_><'a> {
  |           ++++

error[E0106]: missing lifetime specifier
 --> tests/ui/derive_fail_lifetime.rs:4:8
  |
4 | struct Foo<'a> {
  |        ^^^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
3 ~ #[derive(NativeClass<'a>)]
4 ~ struct Foo<'a><'a> {
  |

error[E0277]: the trait bound `&gdnative::prelude::Node: OwnerArg<'_, Reference, Shared
>` is not satisfied
 --> tests/ui/derive_fail_lifetime.rs:3:10
  |
3 | #[derive(NativeClass)]
  |          ^^^^^^^^^^^ the trait `OwnerArg<'_, Reference, Shared>` is not implemented
 for `&gdnative::prelude::Node`
  |
  = help: the following other types implement trait `OwnerArg<'a, T, Own>`:
            &'a T
            TRef<'a, T, Own>
  = note: this error originates in the derive macro `NativeClass` (in Nightly builds, r
un with -Z macro-backtrace for more info)

Close #174

[Bromeon]

Very nice addition! I especially like how you found a way to use the natural type-aliases to register monomorphizations:

#[gdnative::derive::monomorphize]
type IntPair = Pair<i32>;

I was first under the impression this would need to happen in the scope of the NativeClass macro itself.

Proc-macro code for To/FromVariant has reached quite a complexity already, but I think that's in the nature of proc-macros. Those two traits have become very powerful in the meantime, with lots of serde-like features! 💪

Would you add monomorphize to the prelude, or do you think it's rare enough to deserve a qualified import/path?

[Bromeon]

Maybe add newline at end of file?

[chitoyuu]

Proc-macro code for To/FromVariant has reached quite a complexity already, but I think that's in the nature of proc-macros. Those two traits have become very powerful in the meantime, with lots of serde-like features! muscle

Admittedly, there is indeed a lot of complexity in those two macros. It's more of an unfortunate consequence of Godot having a richer data model than what serde allows (Rids, pointers, etc.). The changes this PR made are mainly to the NativeScript and #[methods] macros though.

Would you add monomorphize to the prelude, or do you think it's rare enough to deserve a qualified import/path?

I don't think it's prelude-worthy. It isn't something you'd expect to use in every other file in the average project.

[chitoyuu]

I'll be merging this in a day, in case there are no further comments.

[chitoyuu]

bors try

[bors]

try

Build succeeded:

• full-ci

[chitoyuu]

bors r+

[bors]

Build succeeded:

• full-ci