There's some subtleties there, on both the C++ and Rust side. I won't speak to the C++ stuff, but on the Rust side, it's more "not yet, and we don't know when, and maybe never, we'll see" than it is "no."

It would be accurate to say that the only stable ABI Rust currently supports is the C FFI ABI, right?

I would say that "the Rust ABI is not stable, but Rust also supports other ABIs." https://doc.rust-lang.org/stable/reference/items/external-bl...

Do you think that there will ever be reified generics on the ABI boundary?

No idea, ABIs aren't my area of expertise.

A large amount of cruft and performance issues in the C++ standard library remain because the committee is too afraid of breaking ABI compatibility, so it's not so much as a "no" as "oh no." Even MSVC hasn't broken ABI in a bit.