1 files changed, 36 insertions, 0 deletions
diff --git a/compiler/rustc_macros/src/type_foldable.rs b/compiler/rustc_macros/src/type_foldable.rs
new file mode 100644
index 000000000..23e619221
--- /dev/null
+++ b/compiler/rustc_macros/src/type_foldable.rs
@@ -0,0 +1,36 @@
+use quote::quote;
+use syn::parse_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")
+    }
+
+    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];
+            quote! {
+                ::rustc_middle::ty::fold::TypeFoldable::try_fold_with(#bind, __folder)?
+            }
+        })
+    });
+
+    s.bound_impl(
+        quote!(::rustc_middle::ty::fold::TypeFoldable<'tcx>),
+        quote! {
+            fn try_fold_with<__F: ::rustc_middle::ty::fold::FallibleTypeFolder<'tcx>>(
+                self,
+                __folder: &mut __F
+            ) -> Result<Self, __F::Error> {
+                Ok(match self { #body_fold })
+            }
+        },
+    )
+}