downcast-arc

Author: nikomatsakis

crates/formality-macros/src/cast.rs

@@ -13,7 +13,7 @@ pub(crate) fn upcast_impls(s: synstructure::Structure) -> Vec<TokenStream> {
         .filter(|v| num_variants == 1 || has_isa_attr(v.ast().attrs))
         .map(|v: &VariantInfo<'_>| upcast_to_variant(&s, v))
         .chain(Some(self_upcast(&s)))
-        .chain(Some(arc_upcast(&s)))
+        .chain(Some(arc_cast(&s)))
         .collect()
 }
 
@@ -29,13 +29,14 @@ fn self_upcast(s: &synstructure::Structure) -> TokenStream {
     })
 }
 
-fn arc_upcast(s: &synstructure::Structure) -> TokenStream {
+fn arc_cast(s: &synstructure::Structure) -> TokenStream {
     let type_name = &s.ast().ident;
     let (impl_generics, type_generics, where_clauses) = s.ast().generics.split_for_impl();
 
     quote_spanned! { type_name.span() =>
         const _: () = {
             use formality_core::Upcast;
+            use formality_core::DowncastFrom;
             use std::sync::Arc;
 
             impl #impl_generics Upcast<Arc<Self>> for #type_name #type_generics
@@ -44,6 +45,14 @@ fn arc_upcast(s: &synstructure::Structure) -> TokenStream {
                     Arc::new(self)
                 }
             }
+
+
+            impl #impl_generics DowncastFrom<Arc<Self>> for #type_name #type_generics
+            #where_clauses {
+                fn downcast_from(t: &Arc<Self>) -> Option<Self> {
+                    Some(Self::clone(t))
+                }
+            }
         };
     }
 }