Layers: one next_layer_name, in the document (review of #341)

The engine kept its own copy of Document::next_layer_name after #353; the
document's now counts sublayers too, so the engine uses it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/doc/src/lib.rs

@@ -813,20 +813,14 @@ impl Document {
     /// The default name of a new layer or sublayer, "Layer N". Layers and sublayers share the
     /// numbering: it starts after the number of layers there are and skips any layer's name.
     pub fn next_layer_name(&self) -> String {
-        let mut names = Vec::new();
+        let (mut count, mut taken) = (0, std::collections::HashSet::new());
         self.walk(|n| {
             if n.is_layer() {
-                names.push(n.name.as_deref().unwrap_or_default());
+                count += 1;
+                taken.extend(n.name.as_deref());
             }
         });
-        let mut i = names.len() + 1;
-        loop {
-            let name = format!("Layer {i}");
-            if !names.contains(&name.as_str()) {
-                return name;
-            }
-            i += 1;
-        }
+        (count + 1..).map(|i| format!("Layer {i}")).find(|name| !taken.contains(name.as_str())).unwrap_or_default()
     }
 
     /// Visit every node depth first in paint order (bottom to top).

crates/engine/src/cmd/layer.rs

@@ -228,24 +228,10 @@ fn all_layers(d: &Document) -> Vec<&Node> {
     out
 }
 
-/// The next unused `Layer N` name, counting sublayers too.
-pub(crate) fn next_layer_name(d: &Document) -> String {
-    let layers = all_layers(d);
-    let taken: HashSet<&str> = layers.iter().filter_map(|l| l.name.as_deref()).collect();
-    let mut i = layers.len() + 1;
-    loop {
-        let name = format!("Layer {i}");
-        if !taken.contains(name.as_str()) {
-            return name;
-        }
-        i += 1;
-    }
-}
-
 /// A new layer named `name` (default: the next `Layer N`) in the next layer colour.
 pub(crate) fn make_layer(d: &mut Document, name: Option<&str>) -> Node {
     let n = all_layers(d).len();
-    let name = name.map(str::to_string).unwrap_or_else(|| next_layer_name(d));
+    let name = name.map(str::to_string).unwrap_or_else(|| d.next_layer_name());
     let id = d.alloc_id();
     Node::layer(id, &name, LayerColor::Preset((n % LAYER_COLORS.len()) as u8))
 }