Add test for span

ast/src/span.rs

@@ -31,3 +31,23 @@ impl<'i> From<SpanDef<'i>> for Span<'i> {
         Span::new(def.input, def.start, def.end).unwrap()
     }
 }
+
+#[test]
+fn test_span_def() {
+    // Wrapper serializable JSON struct
+    #[derive(Serialize)]
+    struct Element<'ast> {
+        #[serde(with = "SpanDef")]
+        span: Span<'ast>,
+    }
+
+    // Starting value
+    let span = Span::new("hello world", 1, 5).unwrap();
+    let element = Element { span };
+
+    // Attempt to serialize span to string.
+    let output = serde_json::to_string(&element).unwrap();
+
+    let expected_output = "{\"span\":{\"input\":\"ello\",\"start\":1,\"end\":5}}";
+    assert_eq!(expected_output, output);
+}