Create a generic HashStable derive.

Author: cjgillot

src/librustc_macros/src/hash_stable.rs

@@ -47,6 +47,44 @@ fn parse_attributes(field: &syn::Field) -> Attributes {
     attrs
 }
 
+pub fn hash_stable_generic_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
+    let generic: syn::GenericParam = parse_quote!(__CTX);
+    s.add_bounds(synstructure::AddBounds::Generics);
+    s.add_impl_generic(generic);
+    let body = s.each(|bi| {
+        let attrs = parse_attributes(bi.ast());
+        if attrs.ignore {
+             quote!{}
+        } else if let Some(project) = attrs.project {
+            quote!{
+                &#bi.#project.hash_stable(__hcx, __hasher);
+            }
+        } else {
+            quote!{
+                #bi.hash_stable(__hcx, __hasher);
+            }
+        }
+    });
+
+    let discriminant = match s.ast().data {
+        syn::Data::Enum(_) => quote! {
+            ::std::mem::discriminant(self).hash_stable(__hcx, __hasher);
+        },
+        syn::Data::Struct(_) => quote! {},
+        syn::Data::Union(_) => panic!("cannot derive on union"),
+    };
+
+    s.bound_impl(quote!(::rustc_data_structures::stable_hasher::HashStable<__CTX>), quote!{
+        fn hash_stable(
+            &self,
+            __hcx: &mut __CTX,
+            __hasher: &mut ::rustc_data_structures::stable_hasher::StableHasher) {
+            #discriminant
+            match *self { #body }
+        }
+    })
+}
+
 pub fn hash_stable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
     let generic: syn::GenericParam = parse_quote!('__ctx);
     s.add_bounds(synstructure::AddBounds::Generics);

src/librustc_macros/src/lib.rs

@@ -24,4 +24,9 @@ pub fn symbols(input: TokenStream) -> TokenStream {
 }
 
 decl_derive!([HashStable, attributes(stable_hasher)] => hash_stable::hash_stable_derive);
+decl_derive!(
+    [HashStable_Generic, attributes(stable_hasher)] =>
+    hash_stable::hash_stable_generic_derive
+);
+
 decl_derive!([TypeFoldable, attributes(type_foldable)] => type_foldable::type_foldable_derive);