use std::convert::TryInto;
// *** before: ***
// a narrowing as cast silently cut the number down
let big: i64 = 300;
let small = big as u8; // 44, with nothing to say it went wrong
// *** in version 1.34: ***
// a conversion that may fail returns a Result
let big: i64 = 300;
match big.try_into() {
Ok(small) => println!("it fits: {}", small as u8),
Err(_) => println!("does not fit into u8"),
}