Integer operations

Integer overflows

Although some verification is performed by Rust regarding potential integer overflows, precautions should be taken when executing arithmetic operations on integers.

In particular, it should be noted that using debug or release compilation profile changes integer overflow behavior. In debug configuration, overflow cause the termination of the program (panic), whereas in the release configuration the computed value silently wraps around the maximum value that can be stored.

This last behavior can be made explicit by using the Wrapping generic type, or the overflowing_<op> and wrapping_<op> operations on integers (the <op> part being add, mul, sub, shr, etc.).

use std::num::Wrapping;
use std::panic;

fn main() {
let x: u8 = 242;

let result = panic::catch_unwind(|| {
println!("{}", x + 50);                      // panics in debug, prints 36 in release.
});
if result.is_err() { println!("panic"); }
println!("{}", x.overflowing_add(50).0);     // always prints 36.
println!("{}", x.wrapping_add(50));          // always prints 36.
println!("{}", Wrapping(x) + Wrapping(50));  // always prints 36.

// always panics:
let (res, c) = x.overflowing_add(50);
let result = panic::catch_unwind(|| {
if c { panic!("custom error"); }
else { println!("{}", res); }
});
if result.is_err() { println!("panic"); }
}

Rule LANG-ARITH

When assuming that an arithmetic operation can produce an overflow, the specialized functions overflowing_<op>, wrapping_<op>, or the Wrapping type must be used.