Why is one mutable borrow allowed while this could also cause a race condition?

1

Hello,

I know that the below code does as described in the comments. But why can the Rust compiler
"catch" a potential race condition between 2 variables but not between 3 or more? Thanks!

fn main() {
    let mut x = 2;
    let y = &mut x; // 1st borrow works
    let z = &mut x; // 2nd borrow throws an error
    *y += 1;
    println!("{}",x);
}
2

You mean that usage of y, being a mutable borrow of x, could race with usage of x itself? It can't, since we can't use x while y is still active:

fn main() {
    let mut x = 2;
    let y = &mut x;
    x = 1; // error: cannot assign to `x` because it is borrowed
    *y += 1;
    println!("{}", x);
}
2 Likes
3

oh youre right I forgot about that, thanks!

4

FWIW I'm being pedantic but this is not a race condition -- you would need threads and as soon as you attempt to send the variable across a thread boundary you'd get an error. This is just a generic violation of Rust's "one mutable reference"/noalias rule.

5

thanks :slight_smile:

6

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