Function rank 

pub fn rank(
    adapter: &GpuAdapterInfo,
    preference: &GpuPreference,
    compositor: Option<&PermanentGpuId>,
) -> GpuRank

Rank one adapter under preference.

LowPower, HighPerformance, Device and Custom are portable; Auto leaves the ordering to the backend, so callers establish their own order first and a stable sort under this key preserves it. compositor is the Linux compositor hint, expressed in the same identity type; a backend with no equivalent passes None.