Ownership II: Borrowing and the Borrow Checker

Rust

Use shared and mutable references to work with data without taking ownership. Learn to resolve conflicting borrows and choose appropriate function signatures.

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With rustc 1.95.0, why does insert trigger a borrowing error in this function?

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use std::collections::HashMap;

fn get_or_insert(map: &mut HashMap<u32, u32>, key: u32) -> &u32 {
    if let Some(value) = map.get(&key) {
        return value;
    }
    map.insert(key, 0);
    map.get(&key).unwrap()
}

fn main() {
    let mut map = HashMap::new();
    println!("{}", get_or_insert(&mut map, 7));
}

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