introduce Upcast impls for Arc

Author: nikomatsakis

book/src/SUMMARY.md

@@ -2,11 +2,12 @@
 
 - [Intro](./intro.md)
 - [`formality_core`: the Formality system](./formality_core.md)
-    - [Defining your lang](./formality_core/lang.md)
-    - [Defining terms with the `term` macro](./formality_core/terms.md)
-        - [Parsing](./formality_core/parse.md)
-        - [Customizing debug](./formality_core/debug.md)
-    - [Variables](./formality_core/variables.md)
-    - [Collections](./formality_core/collections.md)
-    - [Judgment functions and inference rules](./formality_core/judgment_fn.md)
-    - [FAQ and troubleshooting](./formality_core/faq.md)
+  - [Defining your lang](./formality_core/lang.md)
+  - [Defining terms with the `term` macro](./formality_core/terms.md)
+    - [Parsing](./formality_core/parse.md)
+    - [Customizing debug](./formality_core/debug.md)
+  - [Constructors](./formality_core/constructors.md)
+  - [Variables](./formality_core/variables.md)
+  - [Collections](./formality_core/collections.md)
+  - [Judgment functions and inference rules](./formality_core/judgment_fn.md)
+  - [FAQ and troubleshooting](./formality_core/faq.md)

crates/formality-macros/src/cast.rs

@@ -11,8 +11,9 @@ pub(crate) fn upcast_impls(s: synstructure::Structure) -> Vec<TokenStream> {
     s.variants()
         .iter()
         .filter(|v| num_variants == 1 || has_isa_attr(v.ast().attrs))
-        .map(|v| upcast_to_variant(&s, v))
+        .map(|v: &VariantInfo<'_>| upcast_to_variant(&s, v))
         .chain(Some(self_upcast(&s)))
+        .chain(Some(arc_upcast(&s)))
         .collect()
 }
 
@@ -28,6 +29,25 @@ fn self_upcast(s: &synstructure::Structure) -> TokenStream {
     })
 }
 
+fn arc_upcast(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 std::sync::Arc;
+
+            impl #impl_generics Upcast<Arc<Self>> for #type_name #type_generics
+            #where_clauses {
+                fn upcast(self) -> Arc<Self> {
+                    Arc::new(self)
+                }
+            }
+        };
+    }
+}
+
 fn upcast_to_variant(s: &synstructure::Structure, v: &VariantInfo) -> TokenStream {
     let binding_tys: Vec<&Type> = v.bindings().iter().map(|b| &b.ast().ty).collect();
     let binding_names: Vec<&syn::Ident> = v.bindings().iter().map(|b| &b.binding).collect();