Use features for minimal NIF version

[filmor]

• Make rustler_sys forward compatible
• Use Cargo features to define the minimal required NIF version
• Stop using RUSTLER_NIF_VERSION in rustler_mix

Another side-effect of this change is that I have completely removed the Erlang call from the build process now as we do not attempt to auto-discover the highest available NIF version anymore. Instead, we compile by default for NIF 2.14, if anything higher is required, it has to be given explicitly as either RUSTLER_NIF_VERSION or by defining the respective feature.

I would suggest that we eventually (with rustler-0.29, rustler-sys-2.3.0) retire RUSTLER_NIF_VERSION in favour of users defining their rustler (or rustler-sys) dependency with the minimal required NIF version. This has also the huge advantage of being able to handle situations where rustler is used in a dependency gracefully.

[philss]

It makes totally sense to me! 👍

I think this way is much easier to control features based on the NIF version. My question is if we really need to deprecate the env var support.

My concern is that it's much easier today to set the NIF version by using the env var in the context of a precompilation workflow (we do it here).

The way using only features may lead to the need of defining custom features - in the user projects - to activate the correct nif_version_*_* on rustler, if the user is planning to support multiple NIF versions, right?

[filmor]

I'm not sure I get what you are concerned about. If a library itself supports multiple NIF versions (making use of newer functions if available), it should also put this support behind feature flags. The environment variable makes this handling much more error prone and leads to binaries that are needlessly xompiled to higher NIF version levels.

It's actually a simplification of the precompilation as in almost all cases it will suffice to provide a single binary to support all relevant BEAM versions.

[philss]

It's actually a simplification of the precompilation as in almost all cases it will suffice to provide a single binary to support all relevant BEAM versions.

Got it. Makes sense!