Make get_or_create_layer more robust

At the cost of some efficiency (now iterates through layers twice)

src/layer.rs

@@ -136,14 +136,6 @@ impl LayerSet {
         self.layers.iter().map(|l| &l.name)
     }
 
-    fn create_layer(&mut self, name: Name) -> &mut Layer {
-        let path = util::default_file_name_for_layer_name(&name, &self.path_set);
-        let layer = Layer::new(name, path);
-        self.path_set.insert(layer.path.to_string_lossy().to_lowercase());
-        self.layers.push(layer);
-        self.layers.last_mut().unwrap()
-    }
-
     /// Returns a new layer with the given name.
     pub fn new_layer(&mut self, name: &str) -> Result<&mut Layer, NamingError> {
         if name == DEFAULT_LAYER_NAME {
@@ -152,7 +144,11 @@ impl LayerSet {
             Err(NamingError::Duplicate(name.to_string()))
         } else {
             let name = Name::new(name).map_err(|_| NamingError::Invalid(name.into()))?;
-            Ok(self.create_layer(name))
+            let path = util::default_file_name_for_layer_name(&name, &self.path_set);
+            let layer = Layer::new(name, path);
+            self.path_set.insert(layer.path.to_string_lossy().to_lowercase());
+            self.layers.push(layer);
+            Ok(self.layers.last_mut().unwrap())
         }
     }
 
@@ -161,13 +157,7 @@ impl LayerSet {
         let index = self.layers.iter().position(|l| l.name.as_str() == name);
         match index {
             Some(its_here) => Ok(&mut self.layers[its_here]),
-            None => {
-                // We can avoid the formal checks that new_layer does because by checking if it's already in the list,
-                // we know that name is not the default name or a duplicate
-                // Skipping it also avoids iterating through self.layers a second time
-                let name = Name::new(name)?;
-                Ok(self.create_layer(name))
-            }
+            None => self.new_layer(name),
         }
     }
 
@@ -772,6 +762,10 @@ mod tests {
         let result = ufo.layers.get_or_create_layer("\t");
         assert!(matches!(result, Err(NamingError::Invalid(_))));
         assert_eq!(ufo.layers.len(), 3);
+
+        let result = ufo.layers.get_or_create_layer(DEFAULT_LAYER_NAME);
+        assert!(matches!(result, Err(NamingError::ReservedName)));
+        assert_eq!(ufo.layers.len(), 3);
     }
 
     #[test]