Couldn't help but notice,

    F' as u8

is equivalent to

    b'F'

, and

    ['F' as u8, 'R' as u8, 'E' as u8, 'E' as u8]

is equivalent to

    *b"FREE"

which definitely is a lot more concise :). These are called byte string literals.

Edit: The dereference is necessary because b"FREE" on its own has the type &[u8; 4].