And yet they claim it's a systems programming language. While I can see the value in doing this, I can see the value in doing it manually. Another criteria can be putting the most frequently used fields together, so accessing one causes the others to load in the same cache line. In many programs the size of the structure is not important, it's the cache behavior that is and that is dependent on a lot more than the size of the elements alone. It's probably important that it can be disabled with rear(C).

C++ also doesn't guarantee a stable ABI, and even the C ABI is only de facto standardized on various platforms, nothing is provided by the language standard itself.

Note that "platform" means e.g. Linux-x86-64 and not Linux. Linux-ppc has different rules, for example.

Rust absolutely could be a systems programming language, but it will need at least repr(C) and such options to interface with: a) hardware, b) code not written in Rust or compiled with a different version of the Rust compiler. Sometimes (b) means "with itself", e.g., via structs saved in files with no additional encoding.