No warning when mutating a reference to temporary

help
1

Hi,

I believe the compiler should warn me that I am trying to modify a reference to a temporary clone

2

It’s temporary lifetime extension.

struct A {
    i: i32,
}

impl A {
    fn get_i(&self) -> i32 {
        self.i
    }
}


fn main() {
    let a = A{i:1};
    let i = &mut a.get_i();  // Takes mutable reference to copy of a.i, extends lifetime to lifetime of i
    *i = 2; // Modifies the copy, not a.i
 
    println!("i = {}", a.i);
}
3

OK, I feel like mutating such reference should generate a warning. It could prevent bugs, as I don't think mutating a temporary copy is usually the intention.

4

Then this could be something for clippy?

1 Like
5

This probably should generate an "unused value" warning, since you mutate the value and don't use the new value. However, it's very hard to detect.

6

This actually does what you seem to want to do. Notice that if you remove the mut declaration before a the compiler will warn you. It doesn't warn you in your case because there's nothing to warn about (strictly, since no unwanted mutation is going on) .

7

Wow, I think you may have just shown me why I was scratching my head over a weird bug for a week.

I had a function that added items to a vector and then returned the vector. Immediately prior to the return a println! showed the vector was changed. Immediately back at the call site a `println!' shows the vector was empty.

I can't show my code that demonstrated that but it was a symptom that sounds like what is described here. A temporary being updated but not the real thing.

Not knowing were the problem was I rearranged a lot of code till it went away?

Perhaps some warning may have helped.

8

I've made that mistake myself. Fortunately, it was only hard to find the first 6 times. +1 for a warning.

10

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