hex parser in rust

hex.rs

macro_rules! hex {
    (0) => {48};
    (9) => {57};
    (A) => {65};
    (F) => {70};
    (a) => {97};
    (z) => {102};
    (0..9) => {48...57};
    (A..F) => {65...70};
    (a..f) => {97...102};
}

pub fn parse_hex_address(string: &str) -> Option<usize> {
  debug_assert!(!string.starts_with("0x"));
  let mut value: usize = 0;
  let mut base: usize = 1;
  for byte in string.bytes().rev() {
    let digit = match byte {
      hex!(0..9) => byte - hex!(0),
      hex!(A..F) => 10 + (byte - hex!(A)),
      hex!(a..f) => 10 + (byte - hex!(a)),
      _ => return None,
    };
    value += base * (digit as usize);
    base *= 16;
  }
  Some(value)
}