rust (question?): cannot implement static method that returns result<> of resource

[xnuk]

// wit/choco.wit

package xnuk:choco;

interface runner {
	resource state {
		from-string: static func(source: string) -> result<state, string>;
		is-choco: func() -> bool;
	}
}

world choco {
	export runner;
}

// src/lib.rs

use crate::exports::xnuk::choco::runner;

wit_bindgen::generate!("choco");

struct Component;
impl runner::Guest for Component {
	type State = MyState;
}

struct MyState {
	is_choco: bool,
}
impl runner::GuestState for MyState {
	fn from_string(source: String) -> Result<MyState, String> {
	//                                ^^^^^^^^^^^^^^^^^^^^^^^ expected `State`, found `MyState`
		if source.is_empty() {
			return Err("string is empty".to_string());
		}

		Ok(MyState {
			is_choco: source.to_ascii_lowercase() == "choco",
		})
	}
	fn is_choco(&self) -> bool {
		self.is_choco
	}
}

   Compiling mini-wit v0.1.0 (/home/xnuk/mini-wit)
error[E0053]: method `from_string` has an incompatible type for trait
  --> src/lib.rs:14:36
   |
14 |     fn from_string(source: String) -> Result<MyState, String> {
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^ expected `State`, found `MyState`
   |
note: type in trait
  --> src/lib.rs:3:1
   |
 3 | wit_bindgen::generate!("choco");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: expected signature `fn(String) -> Result<State, _>`
              found signature `fn(String) -> Result<MyState, _>`
   = note: this error originates in the macro `wit_bindgen::generate` (in Nightly builds, run with -Z macro-backtrace for more info)
help: change the output type to match the trait
   |
14 -     fn from_string(source: String) -> Result<MyState, String> {
14 +     fn from_string(source: String) -> Result<State, String> {
   |

For more information about this error, try `rustc --explain E0053`.
error: could not compile `mini-wit` (lib) due to 1 previous error

I want to check some logic before creating resources, and reject to create it if logic doesn't match. But wit-bindgen requires Result<runner::State, _> to return, not Result<Self, _>, so it seems impossible to do this. I'm new to the WIT-things, so am I doing wrong? Is there a workaround for this?

Maybe related?: #1114

[pchickey]

#1114 is definitely not related.

Rustc is telling you that you are returning your guest's MyState, but wit-bindgen created a trait that requires you to return the runner::State struct.

On my machine, I invoked wit-bindgen rust choco.wit to generate choco.rs and see exactly what runner::State is. It looks like this, starting at like 17:

        #[derive(Debug)]
        #[repr(transparent)]
        pub struct State{
          handle: _rt::Resource<State>,
        }

        type _StateRep<T> = Option<T>;

        impl State{
          /// Creates a new resource from the specified representation.
          ///
          /// This function will create a new resource handle by moving `val` onto
          /// the heap and then passing that heap pointer to the component model to
          /// create a handle. The owned handle is then returned as `State`.
          pub fn new<T: GuestState>(val: T) -> Self {
            Self::type_guard::<T>();
            let val: _StateRep<T> = Some(val);
            let ptr: *mut _StateRep<T> =
            _rt::Box::into_raw(_rt::Box::new(val));
            unsafe {
              Self::from_handle(T::_resource_new(ptr.cast()))
            }
          }

You have your MyState struct, which impls the GuestState trait. So, if you have one of those and need to return a runner::State, you can do so with runner::State::new(MyState { ... })

[xnuk]

Got it, thanks.