\xXX is very misleading in Rust since it actually works exactly like \u00XX instead of the way it works in C, C++ and other languages. Example:

// this FAILS because left is [195, 191]
assert_eq!( bytes!( "\xFF"), bytes!( 255 ) ); 
// this SUCCEEDS
assert_eq!( bytes!( "\xFF"), bytes!( "\u00FF" ) );  

I understand the reasoning behind this (Rust strings are always UTF-8), but then \xXX shouldn't exist in the language. It brings nothing but confusion and it's functionality as implemented is the same as \u00XX.