derive_display 0.0.2

A convenient attribute to derive `Display` implementation from another implementation. Currently supports `ToTokens`.
Documentation 
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use proc_macro::TokenStream;

use quote::{quote, ToTokens};
use syn::{parse_str, Generics};
// TODO: use the one in proc_macro2 crate
use syn::ItemImpl;
use syn::__private::TokenStream2;

#[proc_macro_attribute]
pub fn derive_display(_attr: TokenStream, item: TokenStream) -> TokenStream {
    // panic!("item: \n {} \n", item);
    let mut result = TokenStream2::new();
    let orig_item = item.clone();
    TokenStream2::from(item).to_tokens(&mut result);

    let parsed: ItemImpl = syn::parse(orig_item).expect("Expected a valid ItemImpl block");
    // let (_, tr, _) = parsed.trait_.expect("Expected there to be a trait in an ItemImpl");
    let impl_generics: Generics = parsed.generics;
    let ty = *parsed.self_ty;
    let inner_s: TokenStream2 = parse_str("#s").expect("Expected a valid TokenStream");
    let q = quote! {
        impl #impl_generics Display for #ty {
            fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
                let s = &self;
                let q = quote!(#inner_s);
                f.write_fmt(format_args!("{}", q))
            }
        }
    };
    q.to_tokens(&mut result);
    result.into()
}