You can do it via the C ABI, and use opaque pointers to represent higher-level Rust/C++ concepts if you want to.

Firefox is a mixed C++ / Rust codebase with a relatively close coupling between Rust and C++ components in places (layout/dom/script are in C++ while style is in Rust, and a mix of WebRender (Rust) and Skia (C++) are used for rendering with C++ glue code)