Prepare for follow-on change

crates/fj-core/src/core.rs

@@ -7,7 +7,6 @@ use crate::{layers::Layers, validation::ValidationConfig};
 /// An instance of the Fornjot core
 ///
 /// This is the main entry point to `fj-core`'s API.
-#[derive(Default)]
 pub struct Core {
     /// The layers of data that make up the state of a core instance
     pub layers: Layers,
@@ -16,7 +15,9 @@ pub struct Core {
 impl Core {
     /// Construct an instance of `Instance`
     pub fn new() -> Self {
-        Self::default()
+        Self {
+            layers: Layers::default(),
+        }
     }
 
     /// Construct an instance of `Instance`, using the provided configuration
@@ -25,3 +26,9 @@ impl Core {
         Self { layers }
     }
 }
+
+impl Default for Core {
+    fn default() -> Self {
+        Self::new()
+    }
+}