src/librustc_macros/src/type_foldable.rs

use synstructure;
use syn;
use quote::quote;

pub fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
    if let syn::Data::Union(_) = s.ast().data {
        panic!("cannot derive on union")
    }

    s.add_bounds(synstructure::AddBounds::Generics);
    let body_fold = s.each_variant(|vi| {
        let bindings = vi.bindings();
        vi.construct(|_, index| {
            let bind = &bindings[index];
            quote!{
                ::rustc::ty::fold::TypeFoldable::fold_with(#bind, __folder)
            }
        })
    });
    let body_visit = s.fold(false, |acc, bind| {
        quote!{ #acc || ::rustc::ty::fold::TypeFoldable::visit_with(#bind, __folder) }
    });

    s.bound_impl(quote!(::rustc::ty::fold::TypeFoldable<'tcx>), quote!{
        fn super_fold_with<__F: ::rustc::ty::fold::TypeFolder<'tcx>>(
            &self,
            __folder: &mut __F
        ) -> Self {
            match *self { #body_fold }
        }

        fn super_visit_with<__F: ::rustc::ty::fold::TypeVisitor<'tcx>>(
            &self,
            __folder: &mut __F
        ) -> bool {
            match *self { #body_visit }
        }
    })
}