Syn data structures do not implement std::fmt::debug

[dutchmartin]

While trying to build a new procedural macro, I wanted to see what data I was working with, so I tried to use the debug macro and the print line macro, but this does not compile (rust 1.32) since the Debug trait is not implemented.
For reference, see the code below:

extern crate proc_macro;

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput};


#[proc_macro_derive(do_stuff)]
pub fn do_stuff(input: TokenStream) -> TokenStream {
// Parse the input tokens into a syntax tree
    let input = parse_macro_input!(input as DeriveInput);

    let name = &input.ident;
    // this line below does not compile.
    dbg!(input);

    let expanded = quote! {
        impl #name {
            pub fn do_some_stuff()  {
                println!("macro fn executed");
            }
        }
        };

    TokenStream::from(expanded)
}

Looking at the declaration of the struct, it should have been added since the ast_struct macro Adds the debug derive. So I am clueless what causes this.

TLDR: syn::derive::DeriveInput cannot be formatted using {:?} because it doesn't implement `std::fmt::Debug``

[Dushistov]

Look at README, optional features sections, you should enable "extra-traits" feature.

[dtolnay]

The "extra-traits" cfg exposes the Debug impl. The feature is off by default because having all those impls increases compile time quite substantially, so you would normally only want it during debugging.

From your second link:

syn/src/macros.rs

Line 37 in 6f1b7f2

#[cfg_attr(feature = "extra-traits", derive(Debug, Eq, PartialEq, Hash))]

the derive(Debug) only applies if "extra-traits" is enabled.