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;
+use std::{cell::Cell, collections::BTreeSet};
 use sycamore::prelude::*;
 use web_sys::{console, wasm_bindgen::JsValue}; /* DEBUG */
 
@@ -13,6 +13,10 @@ pub type ConstraintKey = usize;
 
 pub type ElementColor = [f32; 3];
 
+thread_local! {
+    static NEXT_ELEMENT_SERIAL: Cell<u64> = Cell::new(0);
+}
+
 #[derive(Clone, PartialEq)]
 pub struct Element {
     pub id: String,
@@ -20,6 +24,10 @@ pub struct Element {
     pub color: ElementColor,
     pub representation: Signal<DVector<f64>>,
     pub constraints: Signal<BTreeSet<ConstraintKey>>,
+
+    // a serial number, assigned by `Element::new`, that uniquely identifies
+    // each element (until `NEXT_ELEMENT_SERIAL` wraps around)
+    pub serial: u64,
     
     // the configuration matrix column index that was assigned to this element
     // last time the assembly was realized
@@ -33,12 +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: serial,
             column_index: 0
         }
     }

app-proto/src/outline.rs

@@ -200,7 +200,7 @@ pub fn Outline() -> View {
                 view=|(key, elt)| view! {
                     ElementOutlineItem(key=key, element=elt)
                 },
-                key=|(key, _)| key.clone()
+                key=|(_, elt)| elt.serial
             )
         }
     }