Add a warning when a copy value is used in a higher scope than it was created in (#2771)

packages/signals/docs/hoist/error.rs

@@ -0,0 +1,31 @@
+#[component]
+fn Counters() -> Element {
+    let counts = use_signal(Vec::new);
+    let mut children = use_signal(|| 0);
+
+    rsx! {
+        button { onclick: move |_| children += 1, "Add child" }
+        button { onclick: move |_| children -= 1, "Remove child" }
+        // A signal from a child is read or written to in a parent scope
+        "{counts:?}"
+        for _ in 0..children() {
+            Counter {
+                counts
+            }
+        }
+    }
+}
+
+#[component]
+fn Counter(mut counts: Signal<Vec<Signal<i32>>>) -> Element {
+    let mut signal_owned_by_child = use_signal(|| 0);
+    // Moving the signal up to the parent may cause issues if you read the signal after the child scope is dropped
+    use_hook(|| counts.push(signal_owned_by_child));
+
+    rsx! {
+        button {
+            onclick: move |_| signal_owned_by_child += 1,
+            "{signal_owned_by_child}"
+        }
+    }
+}