C++ interpreter cross-compilation from 64-bit host to 32-bit target broken

[tronical]

Cross-compiling with cmake on 64-bit host to 32-bit target and enabling the interpreter breaks the build complaining that ValueOpaque is missing.

The generated slint_interpreter_internal.h is indeed missing the 32-bit variant:

#if defined(SLINT_TARGET_64)
struct ValueOpaque {
    uintptr_t _0[7];
};
#endif

In ffi.rs this is how it is declared:

#[repr(C)]
#[cfg(target_pointer_width = "64")]
pub struct ValueOpaque([usize; 7]);
#[repr(C)]
#[cfg(target_pointer_width = "32")]
#[repr(align(8))]
pub struct ValueOpaque([usize; 9]);
/// Asserts that ValueOpaque is as large as Value and has the same alignment, to make transmute safe.
const _: [(); std::mem::size_of::<ValueOpaque>()] = [(); std::mem::size_of::<Value>()];
const _: [(); std::mem::align_of::<ValueOpaque>()] = [(); std::mem::align_of::<Value>()];

cbindgen is configured to emit based on the target_pointer_width, which works for many types we emit. But somehow the #[repr(align(8))] breaks that. If I remove it (and comment out the assertion), then the generated header file is correct.

[ogoffart]

I'm thinking we should put C++ Value on the heap.
The current situation is hard to maintain and may even depend on the rust version or something.

[tronical]

Sounds good to me. Thanks for the suggestion .