> even staying within the bounds of the standard might result in different outcomes in the code

If that's important, why not pretend that your project is in "Clang C++" rather than C++? Clang compiles to at least all the platforms that Rust does.

> not to mention the whole mess around dependencies in C++ code

Agreed. I feel like the proliferation of header-only libraries is a sign that something's seriously wrong here.