Create TypeFoldable derive proc-macro.

src/librustc_macros/src/lib.rs

@@ -9,6 +9,7 @@ use synstructure::decl_derive;
 use proc_macro::TokenStream;
 
 mod hash_stable;
+mod type_foldable;
 mod query;
 mod symbols;
 
@@ -23,3 +24,4 @@ pub fn symbols(input: TokenStream) -> TokenStream {
 }
 
 decl_derive!([HashStable, attributes(stable_hasher)] => hash_stable::hash_stable_derive);
+decl_derive!([TypeFoldable, attributes(type_foldable)] => type_foldable::type_foldable_derive);

src/librustc_macros/src/type_foldable.rs

@@ -0,0 +1,39 @@
+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 }
+        }
+    })
+}