Add some comments and TODOs

compiler/builtins/bitcode/src/list.zig

@@ -1301,6 +1301,10 @@ inline fn listReplaceInPlaceHelp(
     element: Opaque,
     element_width: usize,
 ) ?[*]u8 {
+    // TODO: figure out how to return an element and a List.
+    // We only know the elment size at runtime.
+    // This code is currently the same as listSet.
+
     // the element we will replace
     var element_at_index = (bytes orelse undefined) + (index * element_width);
 
@@ -1321,6 +1325,10 @@ inline fn listReplaceImmutable(
     element: Opaque,
     element_width: usize,
 ) ?[*]u8 {
+    // TODO: figure out how to return an element and a List.
+    // We only know the elment size at runtime.
+    // This code is currently the same as listSet.
+
     const data_bytes = length * element_width;
 
     var new_bytes = utils.allocateWithRefcount(data_bytes, alignment);

compiler/gen_llvm/src/llvm/build_list.rs

@@ -294,19 +294,22 @@ pub fn list_drop_at<'a, 'ctx, 'env>(
 /// List.replace : List elem, Nat, elem -> List elem
 pub fn list_replace<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    layout_ids: &mut LayoutIds<'a>,
+    _layout_ids: &mut LayoutIds<'a>,
     list: BasicValueEnum<'ctx>,
     index: IntValue<'ctx>,
     element: BasicValueEnum<'ctx>,
     element_layout: &Layout<'a>,
     update_mode: UpdateMode,
 ) -> BasicValueEnum<'ctx> {
+    // TODO: This or elsewhere needs to deal with building the record that gets returned.
     let (length, bytes) = load_list(
         env.builder,
         list.into_struct_value(),
         env.context.i8_type().ptr_type(AddressSpace::Generic),
     );
 
+    // Assume the bounds have already been checked earlier
+    // (e.g. by List.replace or List.set, which wrap List.#replaceUnsafe)
     let new_bytes = match update_mode {
         UpdateMode::InPlace => call_bitcode_fn(
             env,