Figured out why dropLast call was producing an argument mismatch:

- Although list_drop_last's LowLevel operation modeling (body) was correct, the defn() CALLED that body with an extra argument for index (a copy-paste error from dropAt).
- List.dropAt works now :)

compiler/can/src/builtins.rs

@@ -2043,7 +2043,7 @@ fn list_drop_last(symbol: Symbol, var_store: &mut VarStore) -> Def {
 
     defn(
         symbol,
-        vec![(list_var, Symbol::ARG_1), (index_var, Symbol::ARG_2)],
+        vec![(list_var, Symbol::ARG_1)],
         var_store,
         body,
         list_var,

compiler/gen_llvm/src/llvm/build_list.rs

@@ -344,20 +344,28 @@ pub fn list_drop_at<'a, 'ctx, 'env>(
 /// List.dropLast : List elem -> List elem
 pub fn list_drop_last<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
-    original_wrapper: StructValue<'ctx>,
-    count: IntValue<'ctx>,
-    element_layout: &Layout<'a>,
+    list: BasicValueEnum<'ctx>,
+    list_layout: &Layout<'a>,
+    update_mode: UpdateMode,
 ) -> BasicValueEnum<'ctx> {
-    let dec_element_fn = build_dec_wrapper(env, layout_ids, element_layout);
+    let element_layout = match *list_layout {
+        Layout::Builtin(Builtin::EmptyList) => {
+            // this pointer will never actually be dereferenced
+            Layout::Builtin(Builtin::Int64)
+        }
+
+        Layout::Builtin(Builtin::List(elem_layout)) => *elem_layout,
+
+        _ => unreachable!("Invalid layout {:?} in List.dropLast", list_layout),
+    };
+
     call_bitcode_fn_returns_list(
         env,
         &[
-            pass_list_cc(env, original_wrapper.into()),
-            env.alignment_intvalue(element_layout),
-            layout_width(env, element_layout),
-            count.into(),
-            dec_element_fn.as_global_value().as_pointer_value().into(),
+            pass_list_cc(env, list),
+            env.alignment_intvalue(&element_layout),
+            layout_width(env, &element_layout),
+            pass_update_mode(env, update_mode),
         ],
         bitcode::LIST_DROP_LAST,
     )