C++: Non-canonical lists lowering reuses the C++ result vector

[cpetig]

This can be seen in the tests/runtime/lists example, in function wasi_http_headers_roundtrip.

Here the C++ result vector storage is also used to lower the elements. These types may or may not have the same size and might disagree on layout. For sure in-depth copying and overwriting the elements in place is not a good idea. The lowered result should be allocated before the copy and the lifted result array freed after the copy. I only found this because my compiler had a different idea of the C++ type layout.

This error is subtle and doesn't show up in the test.

[cpetig]

It looks like the only noticeable difference is due to g++ storing tuples' elements in reverse order, while clang++ uses normal order.

[cpetig]

Update: https://stackoverflow.com/questions/9641699/why-is-it-not-good-to-use-recursive-inheritance-for-stdtuple-implementations/9643480#9643480 is probably the best explanation of the situation.

<rant> Another C++ type which gives zero guarantees about the binary representation and thus is incompatible between compiler variants and thus unsuitable for FFI.