Prevent conversion of empty tuples to unit structs

Fixes #1408

src/patterns.rs

@@ -260,8 +260,8 @@ fn rewrite_tuple_pat(pats: &[ptr::P<ast::Pat>],
     }
 
     if pat_vec.is_empty() {
-        path_str
-    } else {
+        return Some(format!("{}()", try_opt!(path_str)));
+    }
     // add comma if `(x,)`
     let add_comma = path_str.is_none() && pat_vec.len() == 1 && dotdot_pos.is_none();
 
@@ -310,7 +310,6 @@ fn rewrite_tuple_pat(pats: &[ptr::P<ast::Pat>],
                  })
         }
     }
-    }
 }
 
 fn count_wildcard_suffix_len(items: &[ListItem]) -> usize {

tests/target/empty-tuple-no-conversion-to-unit-struct.rs

@@ -0,0 +1,13 @@
+enum TestEnum {
+    Arm1(),
+    Arm2,
+}
+
+
+fn foo() {
+    let test = TestEnum::Arm1;
+    match test {
+        TestEnum::Arm1() => {}
+        TestEnum::Arm2 => {}
+    }
+}