Add support for `#[repr(align(x))]` on bridge structs #902

mxxo · 2021-07-21T17:00:58Z

Add support for #[repr(align(x))] on bridge structs, closing #835. For the example code:

#[cxx::bridge]
mod ffi {
    #[repr(align(4))]
    struct S {
        b: [u8; 4],
    }
}

the following output was obtained using the cxx code generators:

// -- snip 
#[repr(C)]
#[repr(align(4))]
pub struct S {
    pub b: [u8; 4],
}
// -- snip

#include <array>
#include <cstdint>
#include <type_traits>

struct S;

#ifndef CXXBRIDGE1_STRUCT_S
#define CXXBRIDGE1_STRUCT_S
struct alignas(4) S final {
  ::std::array<::std::uint8_t, 4> b;

  using IsRelocatable = ::std::true_type;
};
#endif // CXXBRIDGE1_STRUCT_S

I believe it would be straightforward to extend this implementation for #[repr(packed)] as well.

I was wondering whether to add support for enums but decided against it for now and made it an "unsupported" error for the time being. enum alignas(X) is accepted by all the C++ compilers I tried, but I also found a defect report from 2017 (2534) that removes the ability to apply alignas to enums: https://wg21.cmeerw.net/cwg/issue2354.

dyangelo-grullon · 2023-09-20T12:31:48Z

@dtolnay is there any feedback on this change? What is the path to getting this merged? I’m contemplating closing the last mile or putting up a bounty for this specific feature.

dtolnay

Thanks!

dtolnay · 2025-08-09T18:26:23Z

gen/src/write.rs

@@ -238,7 +238,12 @@ fn write_struct<'a>(out: &mut OutFile<'a>, strct: &'a Struct, methods: &[&Extern
    for line in strct.doc.to_string().lines() {
        writeln!(out, "//{}", line);
    }
-    writeln!(out, "struct {} final {{", strct.name.cxx);
+    let alignment = if let Some(Alignment::Align(x)) = strct.alignment {
+        format!("alignas({}) ", x)


This is not sufficient for alignment values that are smaller than the alignment of one of the fields, which Rust allows and C++ does not: "If the strictest (largest) alignas on a declaration is weaker than the alignment it would have without any alignas specifiers (that is, weaker than its natural alignment or weaker than alignas on another declaration of the same object or type), the program is ill-formed."

It needs to expand to something like:

struct alignas(max( N, alignof(Field1), alignof(Field2), ... )) Struct final {

dtolnay · 2025-08-09T18:28:18Z

gen/src/write.rs

+    let alignment = if let Some(Alignment::Align(x)) = strct.alignment {
+        format!("alignas({}) ", x)
+    } else {
+        String::from("")
+    };
+    writeln!(out, "struct {}{} final {{", alignment, strct.name.cxx);


Suggested change

let alignment = if let Some(Alignment::Align(x)) = strct.alignment {

format!("alignas({}) ", x)

} else {

String::from("")

};

writeln!(out, "struct {}{} final {{", alignment, strct.name.cxx);

write!(out, "struct");

if let Some(Alignment::Align(align)) = strct.alignment {

write!(out, " alignas({})", align);

}

writeln!(out, " {} final {{", strct.name.cxx);

dtolnay · 2025-08-09T18:30:11Z

syntax/attrs.rs

+fn parse_repr_attribute(input: ParseStream) -> Result<Repr> {
+    input.parse::<Repr>()
 }


Unnecessary function, it can use attr.parse_args::<Repr>() above instead of attr.parse_args_with(parse_repr_attribute).

dtolnay · 2025-08-09T18:30:40Z

syntax/mod.rs

+pub enum Alignment {
+    Align(u32),
+}


This enum is not serving any purpose. An alignment can be stored directly as u32 or LitInt below.

dtolnay · 2025-08-09T18:31:07Z

syntax/mod.rs

@@ -92,6 +97,7 @@ pub struct ExternType {
 pub struct Struct {
    pub doc: Doc,
    pub derives: Vec<Derive>,
+    pub alignment: Option<Alignment>,


Suggested change

pub alignment: Option<Alignment>,

pub alignment: Option<LitInt>,

dtolnay · 2025-08-09T18:34:36Z

syntax/parse.rs

+    let repr = match repr {
+        Some(Repr::Atom(atom)) => Some(atom),
+        Some(Repr::Align(_)) => {
+            cx.error(&item, "repr(align) on enums is not supported");


It should be clarified that this is a limitation of C++ enums and not cxx.

Suggested change

cx.error(&item, "repr(align) on enums is not supported");

cx.error(&item, "C++ does not support custom alignment on an enum");

Also the error should point to just the align attribute, not the entire item.

dtolnay · 2025-08-09T18:35:21Z

syntax/repr.rs

+use syn::parse::{Error, Parse, ParseStream, Result};
+use syn::{Ident, LitInt};
+
+#[derive(Copy, Clone, PartialEq)]


None of these trait impls are used anywhere.

Suggested change

#[derive(Copy, Clone, PartialEq)]

dtolnay · 2025-08-09T18:35:54Z

syntax/repr.rs

+        } else if ident == "align" {
+            let content;
+            syn::parenthesized!(content in input);
+            let alignment: u32 = content.parse::<LitInt>()?.base10_parse()?;


This would accept a suffixed integer literal such as #[repr(align(2int))]. That should not be allowed.

dtolnay · 2025-08-09T18:39:09Z

syntax/repr.rs

+                    "invalid `repr(align)` attribute: not a power of two",
+                ));
+            }
+            if alignment > 2u32.pow(29) {


I don't think there is a guarantee that C++ compilers support alignments in this whole range. For example Clang:

struct alignas(1<<19) A {}; struct alignas(1<<29) B {}; auto a() { return alignof(A); } // 524288 auto b() { return alignof(B); } // 1

This needs to use a smaller limit until somebody needs otherwise. As is, this could lead to C++ passing an insufficiently aligned reference to Rust which would be UB.

dtolnay · 2025-08-09T18:40:13Z

syntax/repr.rs

+                }
+                _ => {}
+            }
+        } else if ident == "align" {


It would be helpful to give a better error message if a repr attribute is written with a non-literal expression, since C++ allows this but Rust does not. #[repr(align(1 << 6))]

mxxo added 2 commits July 21, 2021 12:26

Add repr(align) support for bridge structs

91a28fe

Add error for repr(align) on enums for now

9c23707

dtolnay requested changes Aug 9, 2025

View reviewed changes

dtolnay merged commit 1b4f7e2 into dtolnay:master Aug 9, 2025

dtolnay mentioned this pull request Aug 9, 2025

Improve repr(align) implementation #1567

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Add support for `#[repr(align(x))]` on bridge structs #902

Add support for `#[repr(align(x))]` on bridge structs #902

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mxxo commented Jul 21, 2021 •

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	pub alignment: Option<Alignment>,
	pub alignment: Option<LitInt>,

	cx.error(&item, "repr(align) on enums is not supported");
	cx.error(&item, "C++ does not support custom alignment on an enum");

Add support for #[repr(align(x))] on bridge structs #902

Add support for #[repr(align(x))] on bridge structs #902

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Add support for `#[repr(align(x))]` on bridge structs #902

Add support for `#[repr(align(x))]` on bridge structs #902

mxxo commented Jul 21, 2021 •

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