The answer from Anton Tykhyy works splendidly; though in the meantime I have found another way to solve this problem, by casting to a c_void type as shown below:

let cid: CLIENT_ID = CLIENT_ID {
    UniqueProcess: HANDLE(pid as *mut core::ffi::c_void),
    UniqueThread: HANDLE(0 as *mut core::ffi::c_void)
};