Rust features feature

[blackliner]

I wanted to check the pulse on usage of the rust features feature in safety critical code. For C/C++, preprocessor macros and defines are to be avoided, for multiple reasons. I think the same reasons apply for rust, and usage of cfg should be reduced to a minimum. Thoughts?

    #[cfg(feature = "feature_a")]
    foo
    #[cfg(not(feature = "feature_a"))]
    bar