extern crate proc_macro;

#[proc_macro]
pub fn unelide_lifetimes(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    use quote::ToTokens;
    use syn::parse::{Parse, ParseStream};
    use syn::visit_mut::VisitMut;

    struct Input {
        lt: syn::Lifetime,
        ty: syn::Type,
    }

    impl Parse for Input {
        fn parse(input: ParseStream) -> syn::Result<Self> {
            let lt: syn::Lifetime = input.parse()?;
            let _: syn::Token!(;) = input.parse()?;
            let ty: syn::Type = input.parse()?;
            Ok(Self { lt, ty })
        }
    }

    struct UnelideLifetimes(syn::Lifetime);

    impl VisitMut for UnelideLifetimes {
        fn visit_lifetime_mut(&mut self, i: &mut syn::Lifetime) {
            if i.ident == "_" {
                *i = self.0.clone();
            }
        }
    }

    let mut input = syn::parse_macro_input!(input as Input);
    UnelideLifetimes(input.lt).visit_type_mut(&mut input.ty);
    input.ty.to_token_stream().into()
}