Function flag_to_backend_features 

pub fn flag_to_backend_features<'a>(
    sess: &'a Session,
    extend_backend_features: impl FnMut(&'a str, bool),
)

Translates the -Ctarget-feature flag into a backend target feature list.

extend_backend_features extends the set of backend features (assumed to be in mutable state accessible by that closure) to enable/disable the given Rust feature name.