My understanding is that LTO (and thus any cross-language inlining) takes place using a low level IR where language barriers aren't relevant. The GCC and LLVM backends both have full support as far as I know. Hypothetically it's simple to implement support in a given frontend, but apparently it proved to be a bit tricky in practice for Rust (http://blog.llvm.org/2019/09/closing-gap-cross-language-lto-...).