Fix Issue fizyk20#30 (Empty arr![] compiler error)

Added example to arr![] doc
Return a helpful error when empty macro is invoked. Syntax error hack.
Added some test cases to make sure actual zero and unit length arrays weren't broken.

src/arr.rs

@@ -42,10 +42,12 @@ macro_rules! arr_impl {
 }
 
 /// Macro allowing for easy generation of Generic Arrays.
+/// Example: `let test = arr![u32; 1, 2, 3];`
 #[macro_export]
 macro_rules! arr {
     ($T:ty; $($x:expr),*) => (
         arr_impl!($T; U0, [], [$($x),*])
     );
-    ($($x:expr,)*) => (arr![$($x),*])
+    ($($x:expr,)+) => (arr![$($x),*]);
+    () => ("""Macro requires a type, e.g. `let array = arr![u32; 1, 2, 3];`")
 }

tests/mod.rs

@@ -64,6 +64,23 @@ fn test_iter_flat_map() {
     assert!((0..5).flat_map(|i| arr![i32; 2 * i, 2 * i + 1]).eq(0..10));
 }
 
+#[test]
+fn test_unit_macro(){
+    let arr = arr![f32; 3.14];
+    assert_eq!(arr[0], 3.14);
+}
+
+#[test]
+fn test_empty_macro(){
+    let arr = arr![f32;];
+}
+
+/// This test should cause a helpful compile error if uncommented.
+// #[test]
+// fn test_empty_macro2(){
+//     let arr = arr![];
+// }
+
 #[cfg(feature="serde")]
 mod impl_serde {
     extern crate serde_json;