From 7b4a4a870c4bbeffa4625a54fb82947b303054c7 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Tue, 22 Mar 2016 04:22:42 -0400
Subject: [PATCH] Synthesize default action of `()` if type is `()`

---
 lalrpop/src/grammar/repr.rs        |  7 +++++++
 lalrpop/src/normalize/lower/mod.rs | 12 +++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/lalrpop/src/grammar/repr.rs b/lalrpop/src/grammar/repr.rs
index dbcef9f..de7d01c 100644
--- a/lalrpop/src/grammar/repr.rs
+++ b/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(),
diff --git a/lalrpop/src/normalize/lower/mod.rs b/lalrpop/src/normalize/lower/mod.rs
index d828b1e..8782130 100644
--- a/lalrpop/src/normalize/lower/mod.rs
+++ b/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`