Mismatched types error in the first example of pointers

[Anderson-Juhasc]

let x: u8 = 1234521; // requires 1 byte(8 bits) to store "x" in memory
let y: &u128 = &x; // In a 64 bit architecture this will require 64 bits to store "y" in memory
let z: &&u128 = &y; // same bits as y

I'm not sure if casting x to u128 is the better way...