Compiling Rust with GCC: an update

This overstates the situation. There are a bunch of promises Rust makes which, if it turns out rustc doesn't keep that promise in some corner case, they'll fix rustc. This is especially the case in safe Rust.

There are though - as you imply - a lot of crucial things which are not specified, particularly in unsafe Rust. For example the atomics model in Rust is basically "You see what your C++ compiler actually does? That". But then the reality in C++ is similar, your C++ compiler's behaviour, even in C++ 11 "mode" is a lot more like the documented C++ 20 atomics than the C++ 11 atomics. That's not because the compiler is doing it wrong in C++ 11 mode, it's just because the C++ 11 documentation was (more) wrong.

We're at the edge of our understanding is the problem. I'm comfortable with this remaining true in unsafe Rust, because the compiler internals and ultimately even CPU internals we're dealing with actually are this murky - but yes this needs to be resolved for safe Rust.