From 4e494ccd688cd44f92d010cdd6fa46412339f69e Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <m@mitchellh.com>
Date: Sat, 14 Mar 2026 14:20:23 -0700
Subject: [PATCH] lib: lib.Struct can convert packed structs to extern structs

---
 src/lib/struct.zig | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/lib/struct.zig b/src/lib/struct.zig
index d494da2e6..134f6bebc 100644
--- a/src/lib/struct.zig
+++ b/src/lib/struct.zig
@@ -1,6 +1,17 @@
 const std = @import("std");
+const testing = std.testing;
 const Target = @import("target.zig").Target;
 
+/// Create a struct type that is C ABI compatible from a Zig struct type.
+///
+/// When the target is `.zig`, the original struct type is returned as-is.
+/// When the target is `.c`, the struct is recreated with an `extern` layout,
+/// ensuring a stable, C-compatible memory layout.
+///
+/// This handles packed structs by resolving zero alignments to the natural
+/// alignment of each field's type, since extern structs require explicit
+/// alignment. This means packed struct fields like `bool` will take up
+/// their full size (1 byte) rather than being bit-packed.
 pub fn Struct(
     comptime target: Target,
     comptime Zig: type,
@@ -16,7 +27,7 @@ pub fn Struct(
                     .type = field.type,
                     .default_value_ptr = field.default_value_ptr,
                     .is_comptime = field.is_comptime,
-                    .alignment = field.alignment,
+                    .alignment = if (field.alignment > 0) field.alignment else @alignOf(field.type),
                 };
             }
 
@@ -29,3 +40,20 @@ pub fn Struct(
         },
     };
 }
+
+test "packed struct converts to extern with full-size bools" {
+    const Packed = packed struct {
+        flag1: bool,
+        flag2: bool,
+        value: u8,
+    };
+
+    const C = Struct(.c, Packed);
+    const info = @typeInfo(C).@"struct";
+
+    try testing.expectEqual(.@"extern", info.layout);
+    try testing.expectEqual(@as(usize, 1), @sizeOf(@FieldType(C, "flag1")));
+    try testing.expectEqual(@as(usize, 1), @sizeOf(@FieldType(C, "flag2")));
+    try testing.expectEqual(@as(usize, 1), @sizeOf(@FieldType(C, "value")));
+    try testing.expectEqual(@as(usize, 3), @sizeOf(C));
+}