Synthesize default action of () if type is ()

lalrpop/src/grammar/repr.rs

@@ -164,6 +164,13 @@ pub enum TypeRepr {
 }
 
 impl TypeRepr {
+    pub fn is_unit(&self) -> bool {
+        match *self {
+            TypeRepr::Tuple(ref v) => v.is_empty(),
+            _ => false,
+        }
+    }
+
     pub fn usize() -> TypeRepr {
         TypeRepr::Nominal(NominalTypeRepr {
             path: Path::usize(),

lalrpop/src/normalize/lower/mod.rs

@@ -257,7 +257,17 @@ impl LowerState {
     {
         let action = match action {
             Some(s) => s,
-            None => format!("(<>)"),
+            None => {
+                // If the user declared a type `()`, or we inferred
+                // it, then there is only one possible action that
+                // will type-check (`()`), so supply that. Otherwise,
+                // default is to include all selected items.
+                if nt_type.is_unit() {
+                    format!("()")
+                } else {
+                    format!("(<>)")
+                }
+            }
         };
 
         // Note that the action fn takes ALL of the symbols in `expr`