Debug FPS display with rendering backend info

src/settings.rs

@@ -150,6 +150,7 @@ pub struct Settings {
     pub ui_factor: u8,   // interface size: 0 = automatic, 1..4 = fixed integer factor
     pub render_scale: u8, // 3D world resolution divisor: 1 = full, 2..16 = pixel look
     pub touch_ui: u8,     // touch interface: 0 = automatic (after the first touch), 1 = on, 2 = off
+    pub debug_fps: bool,  // show FPS and rendering backend
     // game
     pub game_speed: f32, // 1 or 2
     pub hints: bool,
@@ -179,6 +180,7 @@ impl Default for Settings {
             ui_factor: 0,
             render_scale: 1,
             touch_ui: 0,
+            debug_fps: false,
             game_speed: 1.0,
             hints: true,
             difficulty: 1,
@@ -225,3 +227,19 @@ impl Settings {
         serde_json::from_str(s).ok()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn debug_fps_round_trips() {
+        let mut s = Settings::default();
+        assert!(!s.debug_fps);
+        s.debug_fps = true;
+        let l = Settings::from_json(&s.to_json()).expect("settings serialize back");
+        assert!(l.debug_fps);
+        let old = Settings::from_json(r#"{"language":"en","map_w":64,"map_h":64}"#).expect("old settings remain readable");
+        assert!(!old.debug_fps);
+    }
+}