error[E0588]: packed type cannot transitively contain a #[repr(align)] type

[olivier-fs]

Hi!

I'm trying to build the bindgen tutorial example from :
https://rust-lang.github.io/rust-bindgen/tutorial-0.html
=> https://github.com/fitzgen/bindgen-tutorial-bzip2-sys
=> https://fitzgeraldnick.com/2016/12/14/using-libbindgen-in-build-rs.html

It's an exemple binding to bzip2 (just as an exemple ; the author knows about bzip2 and bzip2-sys crates).

I'm using :

• rustc 1.64.0 (a55dd71 2022-09-19)
• bindgen-0.60.1
• LLVM-15.0.1 (clang version 15.0.1, Target: x86_64-pc-windows-msvc, InstalledDir: C:\dev\llvm-15.0.1\bin)
• Windows 10
• MSYS2/Mingw64 : clang64/mingw-w64-clang-x86_64-bzip2 1.0.8-2 as the provider of C package bzip2

After adding some bindgen::Builder config in build.rs to set path to included <bzlib.h> :

.clang_arg("-IC:/dev/msys64/clang64/include")

I'm getting this error :

$ cargo build
   Compiling bindgen-tutorial-bzip2-sys v0.1.0 (C:\Users\olivier\dev\rust\www-sample\interop\c\bindgen-tutorial-bzip2-sys)
error[E0588]: packed type cannot transitively contain a `#[repr(align)]` type
     --> C:\Users\olivier\dev\rust\www-sample\interop\c\bindgen-tutorial-bzip2-sys\target\debug\build\bindgen-tutorial-bzip2-sys-03a76bb986e88418\out/bind

ings.rs:64916:1
      |
64916 | pub struct _IMAGE_TLS_DIRECTORY64 {
      | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
note: `_IMAGE_TLS_DIRECTORY64__bindgen_ty_1__bindgen_ty_1` has a `#[repr(align)]` attribute
     --> C:\Users\olivier\dev\rust\www-sample\interop\c\bindgen-tutorial-bzip2-sys\target\debug\build\bindgen-tutorial-bzip2-sys-03a76bb986e88418\out/bind

ings.rs:64933:1
      |
64933 | pub struct _IMAGE_TLS_DIRECTORY64__bindgen_ty_1__bindgen_ty_1 {
      | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: `_IMAGE_TLS_DIRECTORY64` contains a field of type `_IMAGE_TLS_DIRECTORY64__bindgen_ty_1`
     --> C:\Users\olivier\dev\rust\www-sample\interop\c\bindgen-tutorial-bzip2-sys\target\debug\build\bindgen-tutorial-bzip2-sys-03a76bb986e88418\out/bind

ings.rs:64922:9
      |
64922 |     pub __bindgen_anon_1: _IMAGE_TLS_DIRECTORY64__bindgen_ty_1,
      |         ^^^^^^^^^^^^^^^^
note: ...which contains a field of type `_IMAGE_TLS_DIRECTORY64__bindgen_ty_1__bindgen_ty_1`
     --> C:\Users\olivier\dev\rust\www-sample\interop\c\bindgen-tutorial-bzip2-sys\target\debug\build\bindgen-tutorial-bzip2-sys-03a76bb986e88418\out/bind

ings.rs:64928:9
      |
64928 |     pub __bindgen_anon_1: _IMAGE_TLS_DIRECTORY64__bindgen_ty_1__bindgen_ty_1,
      |         ^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0588`.
error: could not compile `bindgen-tutorial-bzip2-sys` due to previous error

I tried with nightly toolchain (currently 1.66.0)

• with cargo +nightly build
• by adding some bindgen::Builder config in build.rs : .rust_target(bindgen::RustTarget::Nightly)
  but I get the same error.

I see that issue #67383 with the same problem has been closed.

• Is rustc complain valid (i.e. bindgen has generated some "invalid" code), or is rustc hitting an error despite of valid bingen output ?
• Is this error anyway related to rust/rustc, or should I forward the issue to bindgen ?
• Otherwise, what to do to solve this, as I tried with latest stable rust and bindgen versions ?

[olivier-fs]

Excerpt from bindgen generated bindings.rs :

#[repr(C, packed(4))]
#[derive(Copy, Clone)]
pub struct _IMAGE_TLS_DIRECTORY64 {
    pub StartAddressOfRawData: ULONGLONG,
    pub EndAddressOfRawData: ULONGLONG,
    pub AddressOfIndex: ULONGLONG,
    pub AddressOfCallBacks: ULONGLONG,
    pub SizeOfZeroFill: DWORD,
    pub __bindgen_anon_1: _IMAGE_TLS_DIRECTORY64__bindgen_ty_1,
}
#[repr(C)]
#[derive(Copy, Clone)]
pub union _IMAGE_TLS_DIRECTORY64__bindgen_ty_1 {
    pub Characteristics: DWORD,
    pub __bindgen_anon_1: _IMAGE_TLS_DIRECTORY64__bindgen_ty_1__bindgen_ty_1,
}
#[repr(C)]
#[repr(align(4))]   /* <=== THIS LINE */
#[derive(Debug, Copy, Clone)]
pub struct _IMAGE_TLS_DIRECTORY64__bindgen_ty_1__bindgen_ty_1 {
    pub _bitfield_align_1: [u32; 0],
    pub _bitfield_1: __BindgenBitfieldUnit<[u8; 4usize]>,
}

So these are nested struct, and rustc complains about the repr(align(4)) of the innermost struct, when the outer struct has repr(pack(4)).

Can someone with better understanding than I have of this packed vs align conflict confirm that rustc is rightfully raising an error ? I would then forward the issue to the bindgen crate.

Or is that a rustc error ?

[asquared31415]

This is intended behavior as described by The Reference

a packed type cannot transitively contain another aligned type.

bindgen's code is very strange here, is it emitting both a repr(align) attribute and a dummy align field, at most one of those should be necessary.

[olivier-fs]

Very clear now, this is an issue with the bindgen generated code ; and it's even not a matter of relative values of the packed/align parameters.
As always, machine-generated code looks strange/non-optimal for humans ; but bindgen spares a lot of hassle of doing manual bindings.
I guess this section of code is the result of a different bindgen code path than the one fixed in the other issue.
I'll forward the issue to rust-lang/rust-bindgen.
I close this issue.
Thank you !