core: Add GenCall::create_dummy

A simple abstraction for GenCalls that are already handled elsewhere.

nac3core/src/toplevel/builtins.rs

@@ -514,11 +514,9 @@ pub fn get_builtins(unifier: &mut Unifier, primitives: &PrimitiveStore) -> Built
             instance_to_symbol: HashMap::default(),
             instance_to_stmt: HashMap::default(),
             resolver: None,
-            codegen_callback: Some(Arc::new(GenCall::new(Box::new(
-                |_, _, _, _, _| {
-                    unreachable!("handled in gen_expr")
-                },
-            )))),
+            codegen_callback: Some(Arc::new(GenCall::create_dummy(
+                String::from("handled in gen_expr"),
+            ))),
             loc: None,
         })),
         Arc::new(RwLock::new(TopLevelDef::Class {

nac3core/src/toplevel/mod.rs

@@ -53,6 +53,13 @@ impl GenCall {
         GenCall { fp }
     }
 
+    /// Creates a dummy instance of [`GenCall`], which invokes [`unreachable!()`] with the given 
+    /// `reason`.
+    #[must_use]
+    pub fn create_dummy(reason: String) -> GenCall {
+        Self::new(Box::new(move |_, _, _, _, _| unreachable!("{reason}")))
+    }
+
     pub fn run<'ctx>(
         &self,
         ctx: &mut CodeGenContext<'ctx, '_>,