Fix Queriable for empty enum

Summary:
Turns out that reference to empty enum is consider inhabited: rust-lang/rust#80651

Fix by adding a unreachable clause.

Reviewed By: brianc118

Differential Revision: D36850027

fbshipit-source-id: 83814b331d51bc843629885ed070d527792e9736

Author: Leon Yang (Containers)

below/below_derive/src/qenum.rs

@@ -77,6 +77,7 @@ pub fn enum_to_string_derive_impl(ast: &DeriveInput) -> syn::Result<TokenStream>
             fn to_string(&self) -> ::std::string::String {
                 match self {
                     #(#variant_to_string_arms)*
+                    _ => unreachable!(),
                 }
             }
         }

below/below_derive/src/queriable.rs

@@ -266,6 +266,7 @@ pub fn queriable_derive_impl(ast: &DeriveInput) -> syn::Result<TokenStream> {
             fn query(&self, field_id: &Self::FieldId) -> ::std::option::Option<Field> {
                 match field_id {
                     #(#queriable_match_arms)*
+                    _ => unreachable!(),
                 }
             }
         }