What you just described has already been there for months, before the C++ port happened, however, we still needed a C++ port so that we can draw in some experienced devs who were mainly at home in C/C++ land to do the Vulkan part, and also because it is easier to compile for both our buildbot and other users.

Anyway, you can use the external Rust renderer through this C-based API, or the C++ integrated renderer. The main point is to keep both codebases more or less in sync with each other.

Libretro as a project generally favors C/C++ codebases. There is one or two Rust-based cores, though, which happen to be done by the same author, simias (same guy the renderer here comes from).

Rust <-> C++ via a C interface is a lot of work and not a lot of fun.