Rust does not have offsetof (the real deal, not some pointer-based hack in a third-party crate) and using FFI to call C or C++ practically requires bindgen which is not stable, not part of the standard library, and tricky to configure in a portable way. Rust also doesn't have a stable ABI yet though slow progress is being made.

If you can write pure Rust in a single library/binary these aren't major issues but as a drop-in replacement for C/C++ in many of the areas where those languages are heavily used today, the edges can be surprisingly sharp.