read from file only when it's not provided from CLI

Signed-off-by: onur-ozkan <work@onurozkan.dev>

mm2src/mm2_main/src/mm2.rs

@@ -303,22 +303,23 @@ pub fn mm2_main(version: String, datetime: String) {
 /// Parses and returns the `first_arg` as JSON.
 /// Attempts to load the config from `MM2.json` file if `first_arg` is None
 pub fn get_mm2config(first_arg: Option<&str>) -> Result<Json, String> {
-    let conf_path = common::kdf_config_file().map_err(|e| e.to_string())?;
-    let conf_from_file = slurp(&conf_path);
     let conf = match first_arg {
-        Some(s) => s,
+        Some(s) => s.to_owned(),
         None => {
+            let conf_path = common::kdf_config_file().map_err(|e| e.to_string())?;
+            let conf_from_file = slurp(&conf_path);
+
             if conf_from_file.is_empty() {
                 return ERR!(
                     "Config is not set from command line arg and {} file doesn't exist.",
                     conf_path.display()
                 );
             }
-            try_s!(std::str::from_utf8(&conf_from_file))
+            try_s!(String::from_utf8(conf_from_file))
         },
     };
 
-    let mut conf: Json = match json::from_str(conf) {
+    let mut conf: Json = match json::from_str(&conf) {
         Ok(json) => json,
         // Syntax or io errors may include the conf string in the error message so we don't want to take risks and show these errors internals in the log.
         // If new variants are added to the Error enum, there can be a risk of exposing the conf string in the error message when updating serde_json so