Crash while trying to do bit shift

Question

I am trying to run this rust code

use std::net::Ipv4Addr;

fn ip_to_int(addr: Ipv4Addr) -> u32 {
        let ip = addr.octets();
        (ip[0] as u32) << 24 + (ip[1] as u32) << 16 + (ip[2] as u32) << 8 + (ip[3] as u32)
    }

fn main() {
    let ip = Ipv4Addr::new(74, 125, 227, 0);
    println!("{}", ip_to_int(ip));
}

This crashes with

thread '<main>' panicked at 'shift operation overflowed', test.rs:5

I did typecast everything to 32 bit ints and no shift is larger than 32 bits. Why does it crash? Also, isn't the compiler supposed to catch this and prevent compilation?

Abhishek@Abhisheks-MacBook-Pro-2:~$ rustc --version
rustc 1.1.0-nightly (21f278a68 2015-04-23) (built 2015-04-24)

Answer 1

According to the the Rust reference, operator + as a stronger precedence than operator <<, meaning that your expression is actually parsed like this:

(ip[0] as u32) << (24 + (ip[1] as u32)) << (16 + (ip[2] as u32)) << (8 + (ip[3] as u32))

Which can pretty easily overflow.

You need to add the appropriates brackets:

((ip[0] as u32) << 24) + ((ip[1] as u32) << 16) + ((ip[2] as u32) << 8) + (ip[3] as u32)