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|
use quote::{quote, ToTokens};
use syn::{parse_quote, Attribute, Meta, NestedMeta};
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")
}
if !s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
s.add_impl_generic(parse_quote! { 'tcx });
}
s.add_bounds(synstructure::AddBounds::Generics);
s.bind_with(|_| synstructure::BindStyle::Move);
let body_fold = s.each_variant(|vi| {
let bindings = vi.bindings();
vi.construct(|_, index| {
let bind = &bindings[index];
// retain value of fields with #[type_foldable(identity)]
let fixed = bind
.ast()
.attrs
.iter()
.map(Attribute::parse_meta)
.filter_map(Result::ok)
.flat_map(|attr| match attr {
Meta::List(list) if list.path.is_ident("type_foldable") => list.nested,
_ => Default::default(),
})
.any(|nested| match nested {
NestedMeta::Meta(Meta::Path(path)) => path.is_ident("identity"),
_ => false,
});
if fixed {
bind.to_token_stream()
} else {
quote! {
::rustc_middle::ty::fold::TypeFoldable::try_fold_with(#bind, __folder)?
}
}
})
});
s.bound_impl(
quote!(::rustc_middle::ty::fold::TypeFoldable<::rustc_middle::ty::TyCtxt<'tcx>>),
quote! {
fn try_fold_with<__F: ::rustc_middle::ty::fold::FallibleTypeFolder<::rustc_middle::ty::TyCtxt<'tcx>>>(
self,
__folder: &mut __F
) -> Result<Self, __F::Error> {
Ok(match self { #body_fold })
}
},
)
}
|
