Hi, the reason for using C99 is that it is fully compatible with other languages. If you write a library in Rust you might be tempted to use Rust only feature which will then make it hard to wrap the library for other languages. And if you don't use those features from Rust in your library people will comment that your library isn't Rust enough. Same thing applies to C++. To that extent my advice is to write C in C rather than C in Rust or C++.

Also if people don't have a Rust toolchain setup, compiling the library and using it from source would be hard. In some cases integrating Rust in their toolchain could be hard.

Regarding metal and vulkan I will edit the article to mention Vulkan there too alongside metal. Thanks for your feedback.