From 238e9ac3a74031551d2398fe32d0ec30d30ef14f Mon Sep 17 00:00:00 2001
From: Thomas Hebb <tommyhebb@gmail.com>
Date: Mon, 17 Mar 2025 18:38:03 -0400
Subject: [PATCH] Don't wrap struct padding in __bindgen_marker_Opaque

Just as with zero-sized structs, this usage of blob() doesn't mean the
type is opaque: padding is perfectly safe to allocate, modify, and pass
around. By marking it with __bindgen_marker_Opaque, we prevent autocxx
from making padded structs POD.

This hasn't cropped up so far because explicit padding fields are quite
rare in bindgen, but they do show up when a struct ends with a bitfield.
I'm about to contribute POD bitfield support to autocxx, which this fix
helps enable.
---
 bindgen/codegen/mod.rs           | 9 +++------
 bindgen/codegen/struct_layout.rs | 7 ++++++-
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/bindgen/codegen/mod.rs b/bindgen/codegen/mod.rs
index 998c4ca2da..d2e95ca373 100644
--- a/bindgen/codegen/mod.rs
+++ b/bindgen/codegen/mod.rs
@@ -2362,12 +2362,9 @@ impl CodeGenerator for CompInfo {
 
             if has_address {
                 let layout = Layout::new(1, 1);
-                // Normally for opaque data we use helpers::blob,
-                // which may wrap it in __bindgen_marker_Opaque<T>.
-                // Downstream tools may then use this as a sign that
-                // the type is complex or can't be stack-allocated,
-                // etc. But in this case this one-byte field is harmless
-                // so we'll just represent it without that extra marker.
+                // Don't use __bindgen_marker_Opaque for this harmless one-byte
+                // field. See StructLayoutTracker::padding_field() for a longer
+                // explanation of blob_inner() vs blob().
                 let ty = helpers::blob_inner(ctx, Layout::new(1, 1), false);
                 struct_layout.saw_field_with_layout(
                     "_address",
diff --git a/bindgen/codegen/struct_layout.rs b/bindgen/codegen/struct_layout.rs
index f7f66373fb..94f164fbd2 100644
--- a/bindgen/codegen/struct_layout.rs
+++ b/bindgen/codegen/struct_layout.rs
@@ -393,7 +393,12 @@ impl<'a> StructLayoutTracker<'a> {
     }
 
     fn padding_field(&mut self, layout: Layout) -> proc_macro2::TokenStream {
-        let ty = helpers::blob(self.ctx, layout, false);
+        // Normally for opaque data we use helpers::blob(), which may wrap it
+        // __bindgen_marker_Opaque<T>. Downstream tools may then use this as a
+        // sign that the type is complex or can't be stack-allocated, etc. But
+        // in this case the padding is just plain data, so we want to represent
+        // it without the extra marker.
+        let ty = helpers::blob_inner(self.ctx, layout, false);
         let padding_count = self.padding_count;
 
         self.padding_count += 1;