Make the next element serial number thread-local

In the last revision, the next element serial number was thread-safe,
but that might be overdoing it. I suspect that elements will only ever
be created from the main thread.

app-proto/src/assembly.rs

@@ -1,7 +1,7 @@
 use nalgebra::{DMatrix, DVector};
 use rustc_hash::FxHashMap;
 use slab::Slab;
-use std::{collections::BTreeSet, sync::atomic::{AtomicU64, Ordering}};
+use std::{cell::Cell, collections::BTreeSet};
 use sycamore::prelude::*;
 use web_sys::{console, wasm_bindgen::JsValue}; /* DEBUG */
 
@@ -13,7 +13,9 @@ pub type ConstraintKey = usize;
 
 pub type ElementColor = [f32; 3];
 
-static NEXT_ELEMENT_SERIAL: AtomicU64 = AtomicU64::new(0);
+thread_local! {
+    static NEXT_ELEMENT_SERIAL: Cell<u64> = Cell::new(0);
+}
 
 #[derive(Clone, PartialEq)]
 pub struct Element {
@@ -39,13 +41,17 @@ impl Element {
         color: ElementColor,
         representation: DVector<f64>
     ) -> Element {
+        // take the next serial number
+        let serial = NEXT_ELEMENT_SERIAL.get();
+        NEXT_ELEMENT_SERIAL.set(serial.wrapping_add(1));
+        
         Element {
             id: id,
             label: label,
             color: color,
             representation: create_signal(representation),
             constraints: create_signal(BTreeSet::default()),
-            serial: NEXT_ELEMENT_SERIAL.fetch_add(1, Ordering::SeqCst),
+            serial: serial,
             column_index: 0
         }
     }