From 484cc582fc5a11defcb6de772db5c0f48f28bfda Mon Sep 17 00:00:00 2001
From: Richard Feldman <oss@rtfeldman.com>
Date: Sun, 7 Aug 2022 00:32:52 -0400
Subject: [PATCH] Add some comments

---
 crates/glue/src/rust_glue.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/crates/glue/src/rust_glue.rs b/crates/glue/src/rust_glue.rs
index 513a9c2660..492d993a19 100644
--- a/crates/glue/src/rust_glue.rs
+++ b/crates/glue/src/rust_glue.rs
@@ -388,7 +388,7 @@ fn add_tag_union(
     target_info: TargetInfo,
     type_id: TypeId,
     tags: &[(String, Option<TypeId>)],
-    null_tag_index: Option<usize>,
+    null_tag_index: Option<usize>, // used only in the nullable-wrapped case
     discriminant_size: u32,
     discriminant_offset: u32,
     types: &Types,
@@ -891,6 +891,10 @@ pub struct {name} {{
             &discriminant_name,
             &mut drop_payload,
             |_index, tag_name, opt_payload_id| {
+                // Note: we don't need to check Some(tag_index) != null_tag_index
+                // because the null tag is guaranteed to have no payload,
+                // and so it will already be handled correctly by the logic of
+                // "don't drop it if it has no payload."
                 match opt_payload_id {
                     Some(payload_id) if cannot_derive_copy(types.get_type(payload_id), types) => {
                         format!("unsafe {{ core::mem::ManuallyDrop::drop(&mut {actual_self_mut}.{tag_name}) }},",)