One extreme case of this would be rustls-openssl-compat which is basically what if you take a Rust TLS implementation but you give it the same ABI as the openssl C library so (where this works, which is far from everywhere) it's a drop-in replacement.

e.g. you can run curl, and go fetch https://example.com/ but using this ersatz openssl rather than your C openssl implementation. That's a thing which works today, albeit not a supported configuration from the point of view of Curl's author.