> This exact thing is solvable with the new C++ atomic support, but does it guarantee that you'll use it across threads like Rust?

No.

That said, stuff like Clang's thread safety annotations can help: http://clang.llvm.org/docs/ThreadSafetyAnalysis.html

> I left C++ a long time ago

Lucky ;).

C++, and all the supporting tooling around it, is better than it used to be, make no mistake. But C++ is still no Rust - make no mistake on that either.