state-shift 2.0.2

Macros for implementing Type-State-Pattern on your structs and methods
Documentation 
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use syn::{punctuated::Punctuated, Ident, PathArguments, ReturnType, Token, Type, TypePath};

pub fn switch_to_inner(
    fn_output: &ReturnType,
    parsed_args: &Punctuated<Ident, Token![,]>,
    struct_name: &Ident,
    fn_name: &Ident,
) -> ReturnType {
    let generic_idents: Vec<syn::GenericArgument> = parsed_args
        .iter()
        .map(|i| {
            syn::GenericArgument::Type(Type::Path(TypePath {
                qself: None,
                path: i.clone().into(),
            }))
        })
        .collect();

    let original_return_type = match &fn_output {
        ReturnType::Type(_, ty) => &**ty,
        _ => panic!(
            "Function `{}`: Expected a return type with explicit type annotation (e.g., '-> Type'), but found none.",
            fn_name
        ),
    };

    let mut modified_return_type = original_return_type.clone();

    // Recursively modify the return type, using the struct_name to match
    recursively_modify_return_type(
        &mut modified_return_type,
        generic_idents,
        struct_name,
        fn_name,
    );

    ReturnType::Type(Default::default(), Box::new(modified_return_type))
}

fn recursively_modify_return_type(
    ty: &mut Type,
    generic_idents: Vec<syn::GenericArgument>,
    struct_name: &Ident,
    fn_name: &Ident,
) {
    match ty {
        Type::Path(type_path) => {
            let last_segment = type_path.path.segments.last_mut().unwrap();

            if last_segment.ident == *struct_name {
                // Match the provided struct name (e.g., Player)
                modify_type_path(type_path, generic_idents, fn_name);
            } else {
                // Handle cases where it's a wrapper type like Result<Player> or Option<Player>
                if let PathArguments::AngleBracketed(arguments) = &mut last_segment.arguments {
                    for arg in &mut arguments.args {
                        if let syn::GenericArgument::Type(inner_type) = arg {
                            // Recurse into the inner type to check for Player (or any target struct)
                            recursively_modify_return_type(
                                inner_type,
                                generic_idents.clone(),
                                struct_name,
                                fn_name,
                            );
                        }
                    }
                }
            }
        }
        _ => panic!(
            "Function `{}`: Expected a type path (like MyStruct or std::option::Option) in the return type, but found a different type variant (such as array, reference, or tuple).",
            fn_name
        ),
    }
}

fn modify_type_path(
    type_path: &mut TypePath,
    generic_idents: Vec<syn::GenericArgument>,
    fn_name: &Ident,
) {
    let last_segment = type_path.path.segments.last_mut().unwrap();

    match &mut last_segment.arguments {
        PathArguments::AngleBracketed(arguments) => {
            arguments.args.extend(generic_idents);
        }
        PathArguments::None => {
            last_segment.arguments =
                PathArguments::AngleBracketed(syn::AngleBracketedGenericArguments {
                    args: generic_idents.into_iter().collect(),
                    colon2_token: None,
                    lt_token: Default::default(),
                    gt_token: Default::default(),
                });
        }
        _ => panic!(
            "Function `{}`: Unsupported path arguments in return type of the function.",
            fn_name
        ),
    }
}