skip generating constructors if there are no args

Author: nikomatsakis

book/src/formality_core/constructors.md

@@ -3,5 +3,6 @@
 Unless you include `#[customize(constructors)]`, the `#[term]` macro automatically creates constructors as follows:
 
 - For a `struct`, defines a `new` method that takes an `impl Upcast<T>` for each of your fields.
-- For an `enum`, defines a method per variant (converted to snake-case).
+- For an `enum`, defines a method per variant that has fields (converted to snake-case).
   - If the name of the variant is a Rust keyword like `Struct`, the method will be called `struct_`.
+  - We do not generate constructors for variants with no arguments.

crates/formality-macros/src/constructors.rs

@@ -51,6 +51,11 @@ fn derive_new_for_variant(
     let type_name = &s.ast().ident;
     let (impl_generics, type_generics, where_clauses) = s.ast().generics.split_for_impl();
 
+    // If there are no bindings, not worth it.
+    if v.bindings().is_empty() {
+        return TokenStream::default();
+    }
+
     let binding_names = v.bindings().iter().map(|b| &b.binding).collect::<Vec<_>>();
     let binding_types = v.bindings().iter().map(|b| &b.ast().ty).collect::<Vec<_>>();
     let construct = v.construct(|_b, i| {