[derive] Fix IntoBytes on repr(C) DST (#2836)

Closes #2835

Author: joshlf

src/util/macro_util.rs

@@ -393,6 +393,12 @@ macro_rules! repr_c_struct_has_padding {
         );
         layout.requires_static_padding() || layout.requires_dynamic_padding()
     }};
+    (@field ([$t:ty])) => {
+        <[$t] as $crate::KnownLayout>::LAYOUT
+    };
+    (@field ($t:ty)) => {
+        $crate::DstLayout::for_unpadded_type::<$t>()
+    };
     (@field [$t:ty]) => {
         <[$t] as $crate::KnownLayout>::LAYOUT
     };

zerocopy-derive/src/lib.rs

@@ -1777,7 +1777,7 @@ impl<'a, D: DataExt> ImplBlockBuilder<'a, D> {
             .map(|check| {
                 let variant_types = variants.iter().map(|var| {
                     let types = var.iter().map(|(_vis, _name, ty)| ty);
-                    quote!([#(#types),*])
+                    quote!([#((#types)),*])
                 });
                 let validator_context = check.validator_macro_context();
                 let (trt, validator_macro) = check.validator_trait_and_macro_idents();

zerocopy-derive/src/output_tests.rs

@@ -480,7 +480,7 @@ fn test_into_bytes_struct() {
                 u8: ::zerocopy::IntoBytes,
                 (): ::zerocopy::util::macro_util::PaddingFree<
                     Self,
-                    { ::zerocopy::struct_padding!(Self, [u8, u8]) },
+                    { ::zerocopy::struct_padding!(Self, [(u8), (u8)]) },
                 >,
             {
                 fn only_derive_is_allowed_to_implement_this_trait() {}
@@ -504,7 +504,7 @@ fn test_into_bytes_struct() {
                 [Trailing]: ::zerocopy::IntoBytes,
                 (): ::zerocopy::util::macro_util::DynamicPaddingFree<
                     Self,
-                    { ::zerocopy::repr_c_struct_has_padding!(Self, [u8, [Trailing]]) },
+                    { ::zerocopy::repr_c_struct_has_padding!(Self, [(u8), ([Trailing])]) },
                 >,
             {
                 fn only_derive_is_allowed_to_implement_this_trait() {}

zerocopy-derive/tests/issue_2835.rs

@@ -0,0 +1,22 @@
+// Copyright 2025 The Fuchsia Authors
+//
+// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+
+use zerocopy::{IntoBytes, Unalign};
+
+#[allow(unused)]
+#[derive(IntoBytes)]
+#[repr(C)]
+struct Struct {
+    leading: Unalign<u32>,
+    trailing: [u8],
+}
+
+#[test]
+fn test_issue_2835() {
+    // Compilation is enough to verify the fix
+}