Even if it were possible to avoid interop via C, there are still some areas of the languages that impact FFI:

Trivial relocatability

Cannot safely move C++ objects on the Rust side; must pin or keep in C++ heap.

In Rust, object movement, which occurs during assignment or by being passed by value, always copies values bit by bit.

C++ allows users to define their own semantics by allowing them to overload the assignment operator and create move and copy constructors.

This impacts interop because self-referential types become natural in high-performance C++. Custom constructors can uphold safety invariants even when the object moves its position in memory.

Objects with the same semantics are impossible to define in Rust.

Destruction safety

Moved-from C++ object semantics don’t map; must prevent Rust from “moving” C++ types.

Exception safety

Neither can cross into the other safely; both must catch at the boundary.