Add fragment debug id

components/layout_2020/layout_debug.rs

@@ -10,10 +10,15 @@ use serde_json::{to_string, to_value, Value};
 use std::cell::RefCell;
 use std::fs::File;
 use std::io::Write;
+#[cfg(debug_assertions)]
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
 
 thread_local!(static STATE_KEY: RefCell<Option<State>> = RefCell::new(None));
 
+#[cfg(debug_assertions)]
+static DEBUG_ID_COUNTER: AtomicUsize = AtomicUsize::new(0);
+
 pub struct Scope;
 
 #[macro_export]
@@ -80,6 +85,12 @@ impl Drop for Scope {
     }
 }
 
+/// Generate a unique ID for Fragments.
+#[cfg(debug_assertions)]
+pub fn generate_unique_debug_id() -> u16 {
+    DEBUG_ID_COUNTER.fetch_add(1, Ordering::SeqCst) as u16
+}
+
 /// Begin a layout debug trace. If this has not been called,
 /// creating debug scopes has no effect.
 pub fn begin_trace(root: Arc<FragmentTreeRoot>) {