Feature request: "Missing initial state" error when using `Self`

[mx00s]

When I had this trait for transitions relating to my Opened state everything worked as expected with the new function making Opened the start state.

pub trait Opened {
    fn new() -> Opened;
    // ...
}

However, changing the return value for new from Opened to Self causes the macro to abort with several errors, beginning with:

error: Missing initial state. To declare an initial state you can use a function with signature like fn f() -> T where T is a declared state.

It would be nicer, I think, if I could use -> Self because it helps make the initial state stand out.

[jmg-duarte]

Hello! Thank you for submitting this issue!

The fix should be simple enough. However, the part of the code that handles that needs a bit of refactoring.

The following function and friends should have more context on the current state:

typestate-rs/typestate-proc-macro/src/visitors/transition.rs

Lines 295 to 312 in ad6c902

	fn extract_output_kind(&self, states: &HashSet<Ident>) -> OutputKind {
	let fn_out = &self.output;
	match fn_out {
	ReturnType::Default => OutputKind::Unit,
	ReturnType::Type(_, ty) => match **ty {
	Type::Path(ref path) => {
	if let Some(ident) = path.path.segments.first() {
	let ident = &ident.ident;
	if states.contains(ident) {
	return OutputKind::State(ident.clone());
	}
	}
	OutputKind::Other
	}
	_ => OutputKind::Other,
	},
	}
	}

If the function knows which state is it part of, it can return the correct ident