Function parse_rust_feature_list 

fn parse_rust_feature_list<'a>(
    sess: &'a Session,
    features: &'a str,
    err_callback: impl Fn(&'a str),
    callback: impl FnMut(&'a str, FxHashSet<&'a str>, bool),
)

Parse the value of the target spec features field or -Ctarget-feature, also expanding implied features, and call the closure for each (expanded) Rust feature. If the list contains a syntactically invalid item (not starting with +/-), the error callback is invoked.