'temporary value dropped while borrowed' when creating Iterator based in condition

Question

I am trying to build an Iterator in Rust that returns the values and indexes of non-null elements in an array. This Iterator should be reversible based on a bool parameter.

I managed to build the Iterator without the reverse condition :

fn main() {
    let arr: [u8; 8] = [0, 2, 0, 0, 0, 1, 0, 5];

    for (i, x) in arr.iter().enumerate().filter(|(_, &x)| x!=0) {
        println!("index: {}, value: {}", i, x);
    }
}

>>> index: 1, value: 2
>>> index: 5, value: 1
>>> index: 7, value: 5

However, when trying to build an Iterator based on a boolean condition (based on this post), I get an error when compiling : temporary value dropped while borrowed consider using a let binding to create a longer lived value rustc(E0716).

fn main() {
    // Array to observe
    let arr: [u8; 8] = [0, 2, 0, 0, 0, 1, 0, 5];
    // Reverse parameter
    let reverse: bool = true;

    // Building Iterator based on `reverse` parameter
    let iter: &mut dyn Iterator<Item = (usize, &u8)> = match reverse {
        // forward Iterator
        false => {
            &mut arr.iter().enumerate().filter(|(_, &x)| x!=0)
        }
        // Reversed iterator
        true => {
            &mut arr.iter().enumerate().filter(|(_, &x)| x!=0).rev()
        }
    };

    // Print indices and values of non 0 elements in the array
    for (i, x) in iter {
        println!("index: {}, value: {}", i, x);
    }
}

I have tried cloning the array, or declaring the Iterator with the let keyword as suggested by the compiler, but none of it seemed to work. Any idea?

You can solve this by declaring variables to hold the temporaries outside of the `match` statement, same as [here](https://stackoverflow.com/a/70847700/401059): [playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=2968c413ca46b43e7453802d6ee5e71d). — Caesar, May 13 '22 at 01:52

score 0 · Answer 1 · answered May 10 '22 at 18:07

The problem in your code is that you take a reference to a temporary variable.

While trying to fix it, you will have to work around several other issues to do with iterators, such as that using rev() on one changes its type.

I think your code is best written as something like:

use std::fmt::Display;

fn main() {
    // Array to observe
    let arr: [u8; 8] = [0, 2, 0, 0, 0, 1, 0, 5];
    // Reverse parameter
    let reverse: bool = true;

    // Building Iterator
    let iter = arr.iter().enumerate().filter(|(_, &x)| x != 0);

    // Print indices and values of non 0 elements in the array
    if reverse {
        print_iter(iter.rev());
    } else {
        print_iter(iter);
    }
}

fn print_iter<I: Display, T: Display>(iter: impl Iterator<Item = (I, T)>) {
    for (i, x) in iter {
        println!("index: {}, value: {}", i, x);
    }
}

score 0 · Accepted Answer · answered May 11 '22 at 05:34

I found a pretty compact solution using the Either crate, inspired by this answer.

extern crate either;
use either::Either;

fn main() {
    // Array to observe
    let arr: [u8; 8] = [0, 2, 0, 0, 0, 1, 0, 5];
    // Reverse parameter
    let reverse: bool = true;

    // Pick the right iterator based on reverse
    let iter = match reverse {
        true => Either::Left(arr.iter().enumerate().filter(|(_, &x)| x!=0).rev()),
        false => Either::Right(arr.iter().enumerate().filter(|(_, &x)| x!=0))
    };

    // Print indices and values of non 0 elements in the array
    for (i, x) in iter {
        println!("index: {}, value: {}", i, x);
    }
}

>>> index: 7, value: 5
>>> index: 5, value: 1
>>> index: 1, value: 2

This solution solves the problem mentioned by hkBst (types of an Iterator and its reverse being different), as well as the original 'temporary value dropped while borrowed' compile error.

'temporary value dropped while borrowed' when creating Iterator based in condition

2 Answers2