pub type PFN_vkVoidFunction = Option<unsafe extern "system" fn()>;
https://www.khronos.org/registry/vulkan/specs/1.3-extensions/man/html/PFN_vkVoidFunction.html
enum PFN_vkVoidFunction { None, Some(unsafe extern "system" fn()), }
No value.
Some value of type T.
T