Function include_paths 

pub fn include_paths(
    target: TargetTuple,
    host: Option<TargetTuple>,
    options: &Options<'_>,
) -> Vec<Entry>

The directories to search for an included file, in order.

host is what the compiler is running on, and it is an argument rather than something read from the environment so that the rule can be tested for a host it is not running on. Passing None says the host is unknown, which is treated as not being the target: an unknown host cannot be proved to be the target, and guessing yes is the contamination this function is written against.