Use derive attributes for uninteresting traversals

compiler/rustc_macros/src/lib.rs

@@ -124,8 +124,27 @@ decl_derive!([TyDecodable] => serialize::type_decodable_derive);
 decl_derive!([TyEncodable] => serialize::type_encodable_derive);
 decl_derive!([MetadataDecodable] => serialize::meta_decodable_derive);
 decl_derive!([MetadataEncodable] => serialize::meta_encodable_derive);
-decl_derive!([TypeFoldable, attributes(type_foldable)] => type_foldable::type_foldable_derive);
-decl_derive!([TypeVisitable, attributes(type_visitable)] => type_visitable::type_visitable_derive);
+decl_derive!(
+    [TypeFoldable, attributes(type_foldable)] =>
+    /// Derives `TypeFoldable` for the annotated `struct` or `enum` (`union` is not supported).
+    ///
+    /// The fold will produce a value of the same struct or enum variant as the input, with
+    /// each field respectively folded using the `TypeFoldable` implementation for its type.
+    /// However, if a field of a struct or an enum variant is annotated with
+    /// `#[type_foldable(identity)]` then that field will retain its incumbent value (and its
+    /// type is not required to implement `TypeFoldable`).
+    type_foldable::type_foldable_derive
+);
+decl_derive!(
+    [TypeVisitable, attributes(type_visitable)] =>
+    /// Derives `TypeVisitable` for the annotated `struct` or `enum` (`union` is not supported).
+    ///
+    /// Each field of the struct or enum variant will be visited in definition order, using the
+    /// `TypeVisitable` implementation for its type. However, if a field of a struct or an enum
+    /// variant is annotated with `#[type_visitable(ignore)]` then that field will not be
+    /// visited (and its type is not required to implement `TypeVisitable`).
+    type_visitable::type_visitable_derive
+);
 decl_derive!([Lift, attributes(lift)] => lift::lift_derive);
 decl_derive!(
     [Diagnostic, attributes(

compiler/rustc_macros/src/type_foldable.rs

@@ -1,5 +1,5 @@
-use quote::quote;
-use syn::parse_quote;
+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 {
@@ -16,8 +16,29 @@ pub fn type_foldable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::
         let bindings = vi.bindings();
         vi.construct(|_, index| {
             let bind = &bindings[index];
-            quote! {
-                ::rustc_middle::ty::fold::ir::TypeFoldable::try_fold_with(#bind, __folder)?
+
+            // 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::ir::TypeFoldable::try_fold_with(#bind, __folder)?
+                }
             }
         })
     });

compiler/rustc_macros/src/type_visitable.rs

@@ -1,11 +1,28 @@
 use quote::quote;
-use syn::parse_quote;
+use syn::{parse_quote, Attribute, Meta, NestedMeta};
 
 pub fn type_visitable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
     if let syn::Data::Union(_) = s.ast().data {
         panic!("cannot derive on union")
     }
 
+    // ignore fields with #[type_visitable(ignore)]
+    s.filter(|bi| {
+        !bi.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_visitable") => list.nested,
+                _ => Default::default(),
+            })
+            .any(|nested| match nested {
+                NestedMeta::Meta(Meta::Path(path)) => path.is_ident("ignore"),
+                _ => false,
+            })
+    });
+
     if !s.ast().generics.lifetimes().any(|lt| lt.lifetime.ident == "tcx") {
         s.add_impl_generic(parse_quote! { 'tcx });
     }