Add HashStable_NoContext to simplify HashStable implementations in rustc_type_ir

compiler/rustc_macros/src/hash_stable.rs

@@ -64,6 +64,50 @@ pub(crate) fn hash_stable_generic_derive(
     )
 }
 
+pub(crate) fn hash_stable_no_context_derive(
+    mut s: synstructure::Structure<'_>,
+) -> proc_macro2::TokenStream {
+    let generic: syn::GenericParam = parse_quote!(__CTX);
+    s.add_bounds(synstructure::AddBounds::Fields);
+    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! {
+            #[inline]
+            fn hash_stable(
+                &self,
+                __hcx: &mut __CTX,
+                __hasher: &mut ::rustc_data_structures::stable_hasher::StableHasher) {
+                #discriminant
+                match *self { #body }
+            }
+        },
+    )
+}
+
 pub(crate) fn hash_stable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
     let generic: syn::GenericParam = parse_quote!('__ctx);
     s.add_bounds(synstructure::AddBounds::Generics);

compiler/rustc_macros/src/lib.rs

@@ -48,6 +48,13 @@ decl_derive!(
     [HashStable_Generic, attributes(stable_hasher)] =>
     hash_stable::hash_stable_generic_derive
 );
+decl_derive!(
+    [HashStable_NoContext] =>
+    /// `HashStable` implementation that has no `HashStableContext` bound and
+    /// which adds `where` bounds for `HashStable` based off of fields and not
+    /// generics. This is suitable for use in crates like `rustc_type_ir`.
+    hash_stable::hash_stable_no_context_derive
+);
 
 decl_derive!([Decodable] => serialize::decodable_derive);
 decl_derive!([Encodable] => serialize::encodable_derive);