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 1 / 2

Fix bump: increment the key's count or insert 1 -- one borrow, via the entry API.

Hints

The entry API does look-up-or-insert in ONE borrow: map.entry(key).
*map.entry(key.to_string()).or_insert(0) += 1; replaces the whole match.

27_double_borrow.rs

use std::collections::HashMap;

// BROKEN: the match holds a mutable borrow of the map through BOTH
// arms, so the insert in the None arm is a second mutable borrow.
// Run it to see the error, then reach for the entry API.
fn bump(map: &mut HashMap<String, i32>, key: &str) {
    match map.get_mut(key) {
        Some(v) => *v += 1,
        None => {
            map.insert(key.to_string(), 1);
        }
    }
}

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

Reveal solution

use std::collections::HashMap;

fn bump(map: &mut HashMap<String, i32>, key: &str) {
    *map.entry(key.to_string()).or_insert(0) += 1;
}