Implement support for flexible self types

[chitoyuu]

Instance and TInstance can now be used as self types where applicable. Additionally, Arc is supported when ArcData is used as the user-data wrapper.

The self type can be provided through the standard arbitrary_self_types syntax, or a #[self] attribute on the argument when unavailable.

This is technically a breaking change because all UserData errors are now required to implement std::error::Error, although the possibility of actual breakage should be pretty low -- the wrapper types we ship should be powerful enough to cover most if not all valid use cases.

Close #974

[chitoyuu]

bors try

This PR would allow users to use the Instance and TInstance types as receivers for their exported methods. Additionally, Arc<Self> can also be used for types using the ArcData wrapper, since it's already exposed via ArcData::into_inner. All of the following declarations are now valid:

#[methods]
impl Foo {
    #[method]
    fn none() {}

    #[method]
    fn arc(self: Arc<Self>) {}

    #[method]
    fn instance(#[self] this: Instance<Self>) {}

    #[method]
    fn t_instance(#[self] this: TInstance<Self>) {}
}

@B-head Please see if this satisfies your main needs as mentioned here: https://github.com/godot-rust/gdnative/pull/1033#issuecomment-1430654640

[bors[bot]]

try

Build failed:

• full-ci

[chitoyuu]

CI failed because GATs were stablized in Rust 1.65, while our current MSRV is 1.63. We can probably bump it directly to the latest 1.67 unless there's a significant reason not to.

[B-head]

About 80% looks good. (My code uses Rc<T>, so I have not tested it yat)

Can you support the emplace construct feature? It is useful to remove Option<T> in the structure.

[chitoyuu]

Can you support the emplace construct feature? It is useful to remove Option<T> in the structure.

This works with the existing system. Everything that works still do.

You can also still use Rc<T> in your types. There's nothing restricting that.

[chitoyuu]

bors try

[bors[bot]]

try

Build succeeded:

• full-ci

[chitoyuu]

Since we have bumped the MSRV and there's a general tendency for nightly clippy lints to be left unfixed until they become breaking, I've taken the liberty to change the Default impl to a derived one for a type in gdnative-derive.

Merging since this appears to solve the the original requirements that motivated the other PR.

bors r+

[bors[bot]]

Build succeeded:

• full-ci