Struct passed by-value from C to Rust get's corrupted (using FFI and .dll from Visual Studio)

[Matthias247]

I'm interfacing an external DLL which I'm creating with Visual Studio 2012.

I've tried to pass a struct by-value from C to Rust in a callback.
The struct and the callback are defined as follows:

pub struct Value {
    a: u32,
    b: i32,
    c: i32,
    d: u8,
    e: u8,
    f: u8,
    g: u8
}

#[deriving(Clone,Eq)]
pub struct Notification {
    notification_type: i32,
    value_id: Value,
    data: u8
}

extern "C" fn on_notification(notification_target: *mut LibWrap, notification: Notification) {
  ...
}

However the struct received in on_notification is corrupted and contains not the data which is sent in C++.

I wrote a test program to isolate and reproduce the issue.
The gist therefore is here: https://gist.github.com/Matthias247/8283183
It can be cloned from https://gist.github.com/8283183.git
It includes the source code for the library with a visual studio project to built it and the rust test program.
I also included make.sh to build it on Linux. However in Linux I can't reproduct the issue. The struct get's copied correctly there.

This is the output from my test program:

Starting
rust notification size: 24
rust value size: 16
Initializing library
C++ notification size: 24
C++ value size: 16
 1  0  0  0  2  0  0  0  3  0  0  0  4  0  0  0  5  6  7  8  9 CC CC CC
 1  0  0  0  2  0  0  0  3  0  0  0  4  0  0  0  5  9 48 64 80 CC CC CC
Notification: Notification{notification_type: 1i32, value_id: Value{a: 2u32, b:
3i32, c: 4i32, d: 5u8, e: 9u8, f: 72u8, g: 100u8}, data: 128u8}

The first line is the content of the notification in bytes in C++, the second the content in Rust.
You can see that the last bytes of the notification structure are different in C++ in Rust. Interesting hereby is also that the last field (notification.data == 9) gets copied into notification.value_id.d (verified this with using other values for notification.data) and the remaining notification fields are garbage.

[Matthias247]

@sanxiyn on IRC mentioned that this could be related to #9205
However I'm not sure because that issue is about structs as return values. In my case the struct is a function argument and not the return value.

[Matthias247]

Issue #11198 is also similar.
However I could successfully pass structs from Rust to C before - but mine didn't contain u8 values but only pointers and i32/u32 values.
Maybe the issue is related to u8/i8 types in structs? Because in my example the problem also exists at the first u8 location.

[Matthias247]

Update:

As @eddyb on IRC suggested I have created a more compact example and included LLVM IR: https://gist.github.com/Matthias247/8376787
The error happens on all structs with more than a single u8 or u16 value.
The new test tests are passing a struct with 4 u8 or 4 u16 parameters from Rust to C as well as from C to Rust.

I also switched the C side from a C++ project with extern C declarations to a pure C project, so that this issue can be ruled out.

[ghost]

You can bypass the problem by using external LLVM to compile bitcode. I wrote small utility to do that:
https://gist.github.com/krzat/8374078 , and it worked with rsfml.