Avoid repeated plan generation and unused diagnostics in layout fuzzers

tests/scenario/mod.rs

@@ -444,12 +444,6 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
     cold.state.root = Some(root);
     cold.frame();
 
-    let places: HashMap<WidgetId, usize> = tree
-        .ids
-        .iter()
-        .enumerate()
-        .map(|(i, &id)| (id, i))
-        .collect();
     let mut drawn = 0;
     for (i, (&w, &c)) in tree.ids.iter().zip(&cold_tree.ids).enumerate() {
         let (got, want) = (warm.region(&w), cold.region(&c));
@@ -457,6 +451,12 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         if got == want {
             continue;
         }
+        let places: HashMap<WidgetId, usize> = tree
+            .ids
+            .iter()
+            .enumerate()
+            .map(|(i, &id)| (id, i))
+            .collect();
         // Where two trees disagree is rarely where the cause is, so the
         // ancestry comes with it, marking the widgets that own a region.
         let mut chain = Vec::new();