subtract with overflow in release mode

[ZhangHanDong]

The following code attempt to subtract with overflow in release mode.

fn main() {
    let data  = [0, 1, 2, 3];
    for i in 0..data.len() {
        if data[i] == 0 {
             println!("{}", i - 1); // 18446744073709551615
        }
    }
}

playground link

[sfackler]

What is the problem here? Overflow checks are only present in debug mode.

[ZhangHanDong]

@sfackler I see, thanks.

[ZhangHanDong]

@sfackler The problem is that the following code didn't overflow checks in debug mode.

// data.rs
fn main() {
    let data  = [0, 1, 2, 3];
    for i in 0..data.len() {
        if data[i] == 0 {
             println!("{}", i - 1); 
        }
    }
}

it should be emit warning info when compiled, but it didn't:

$  rustc  data.rs

But it will be panic when run.
playgroud

[sfackler]

The panic is the overflow check.

[ZhangHanDong]

@sfackler overflow check in runtime?

[sfackler]

Yes.

[ZhangHanDong]

the following code will be emit warning info in compile time.

fn main(){
    let m : usize= 0;
    m - 1;
}

like this:

warning: this expression will panic at run-time
  |
3 |     m - 1;
  |     ^^^^^ attempt to subtract with overflow

[ZhangHanDong]

@sfackler I got it. thanks.