Dataframe feature for plugins (#3424)

crates/nu_plugin_post/Cargo.toml

@@ -21,4 +21,8 @@ serde_json = "1.0.57"
 surf = "1.0.3"
 url = "2.1.1"
 
+[features]
+dataframe = ["nu-protocol/dataframe"]
+
+
 [build-dependencies]

crates/nu_plugin_post/src/post.rs

@@ -400,6 +400,14 @@ pub fn value_to_json_value(v: &Value) -> Result<serde_json::Value, ShellError> {
         }
 
         UntaggedValue::Table(l) => serde_json::Value::Array(json_list(l)?),
+        #[cfg(feature = "dataframe")]
+        UntaggedValue::Dataframe(_) => {
+            return Err(ShellError::labeled_error(
+                "Cannot convert dataframe",
+                "Cannot convert dataframe",
+                &v.tag,
+            ))
+        }
         UntaggedValue::Error(e) => return Err(e.clone()),
         UntaggedValue::Block(_) | UntaggedValue::Primitive(Primitive::Range(_)) => {
             serde_json::Value::Null