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Ownership II: Borrowing and the Borrow Checker

Rust

`&`/`&mut` and aliasing XOR mutability. `.clone()` is withdrawn here — the training wheels come off. The borrow checker as a proof system, not a linter.

This track is written for Rust, which isn't the mode you're browsing in.

0 / 21 solved · 3 articles
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  1. 1. Not solved yet. Move or borrow: the two ways to pass a value
  2. 2. Not solved yet. &mut: exclusive access, and why `mut` appears twice
  3. 3. Aliasing XOR mutability: the one rule Read
  4. 4. Not solved yet. How to read a borrow-check diagnostic
  5. 5. Not solved yet. Non-lexical lifetimes: a borrow ends at its last use
  6. 6. Not solved yet. E0502: cannot borrow as mutable because also borrowed as immutable
  7. 7. Not solved yet. E0499: cannot borrow as mutable more than once
  8. 8. Not solved yet. The owner is frozen too: E0503, E0505, E0506
  9. 9. Not solved yet. Slices are borrows
  10. 10. Not solved yet. &str not &String, &[T] not &Vec<T> — deref coercion and ptr_arg
  11. 11. Not solved yet. ref, ref mut, and default binding modes in edition 2024
  12. 12. Not solved yet. Iterator invalidation, and the bug class that stopped existing
  13. 13. Not solved yet. Indexing in a loop is usually a borrow-checker workaround — and clippy knows
  14. 14. Not solved yet. iter_mut(): mutating every element without ever holding two &mut
  15. 15. Not solved yet. Split borrows: the checker reasons about places, not variables
  16. 16. Not solved yet. split_at_mut and the disjointness-proof APIs
  17. 17. Not solved yet. Reborrowing: why &mut sometimes moves and sometimes doesn't
  18. 18. Not solved yet. Two-phase borrows: why v.push(v.len()) compiles
  19. 19. Not solved yet. What a &mut parameter actually promises
  20. 20. Not solved yet. NLL Problem Case #3, the entry API, and the limits of today's checker
  21. 21. Not solved yet. Closures hold their borrows
  22. 22. Not solved yet. Interior mutability: moving the check from compile time to run time
  23. 23. Why the rule exists: aliasing, noalias, and the optimizer Read
  24. 24. The borrow checker is a proof system, not a linter Read

Check yourself

4 questions · one attempt each

These do not count toward finishing the track. They are here to catch the things that are easy to read past.

0 / 4

This pattern is famously rejected. Why? 

fn get_or_insert(m: &mut HashMap<K, V>, k: K) -> &V {
    if let Some(v) = m.get(&k) { return v; }
    m.insert(k, V::default());
    m.get(&k).unwrap()
}
Question 1 of 4