Assign each element a serial number

For future thread-safety, keep the next serial number in a static atomic
variable.

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::{collections::BTreeSet, sync::atomic::{AtomicU64, Ordering}};
 use sycamore::prelude::*;
 use web_sys::{console, wasm_bindgen::JsValue}; /* DEBUG */
 
@@ -13,6 +13,8 @@ pub type ConstraintKey = usize;
 
 pub type ElementColor = [f32; 3];
 
+static NEXT_ELEMENT_SERIAL: AtomicU64 = AtomicU64::new(0);
+
 #[derive(Clone, PartialEq)]
 pub struct Element {
     pub id: String,
@@ -21,6 +23,10 @@ pub struct Element {
     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
     column_index: usize
@@ -39,6 +45,7 @@ impl Element {
             color: color,
             representation: create_signal(representation),
             constraints: create_signal(BTreeSet::default()),
+            serial: NEXT_ELEMENT_SERIAL.fetch_add(1, Ordering::SeqCst),
             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
             )
         }
     }