quest – Rust: Double borrows

Borrow checker Advanced

27 Double borrows

Two classics that look fine but borrow the same collection twice. The fixes are idioms you'll reuse constantly: the HashMap entry API, and collect-then-extend instead of mutating while iterating.

Step 2 / 2

Fix echo_evens: collect the doubled evens first, then extend after the loop.

Hints

Build the additions in a separate Vec first -- then nothing borrows v when you push.
let doubled: Vec<i32> = v.iter().filter(..).map(..).collect(); then v.extend(doubled);.

27_double_borrow.rs

// BROKEN: the for loop borrows v to iterate while push needs a
// mutable borrow. Run it to see the error, then collect first.
fn echo_evens(v: &mut Vec<i32>) {
    for x in v.iter() {
        if x % 2 == 0 {
            v.push(x * 2);
        }
    }
}

Reset to starterCompiled in a sandbox; hidden checks must pass.

Reveal solution

fn echo_evens(v: &mut Vec<i32>) {
    let doubled: Vec<i32> = v.iter().filter(|&&x| x % 2 == 0).map(|&x| x * 2).collect();
    v.extend(doubled);
}