ret_ty_attr 0.1.2

Specify the return type depending on configuration conditional checks.
Documentation 
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use proc_macro as pm;
use proc_macro2 as pm2;
use quote::ToTokens;

#[proc_macro_attribute]
pub fn ret_ty(meta: pm::TokenStream, input: pm::TokenStream) -> pm::TokenStream {
    fn inner(meta: pm2::TokenStream, input: pm2::TokenStream) -> syn::Result<pm2::TokenStream> {
        let mut input = syn::parse2::<syn::ItemFn>(input)?;
        let ret_ty: syn::ReturnType = if meta.is_empty() {
            syn::ReturnType::Default
        } else {
            let ret_ty = syn::parse2::<syn::Type>(meta)?;
            syn::ReturnType::Type(
                syn::token::RArrow {
                    spans: [pm2::Span::call_site(), pm2::Span::call_site()],
                },
                Box::new(ret_ty),
            )
        };
        let syn::ItemFn {
            sig: syn::Signature { output, .. },
            ..
        } = &mut input;
        *output = ret_ty;
        Ok(input.into_token_stream())
    }
    match inner(meta.into(), input.into()) {
        Ok(output) => output.into(),
        Err(error) => error.into_compile_error().into(),
    }
}