Borrow checker
- Rust course
- Expert
- Lesson 4/46
- 4 min
Learn how the borrow checker runs on MIR.
The borrow checker
The borrow checker runs on MIR and checks three things: that no reference outlives the value it points to (lifetimes), that there's no overlapping &mut access at the same time, and that no moved or uninitialized value is used. In some cases there are refinements that fit the rules to how code is written; for example two-phase borrows:
let mut v = vec![1, 2];
v.push(v.len()); // &mut v is taken, but it "activates" after the argument is computedWithout this, the &v needed for v.len() would clash with push's &mut v.
Polonius
There are some programs the current checker rejects that are actually safe. The best known is the pattern that returns what it found in a map and inserts when it doesn't find it. The next-generation checker Polonius is designed to accept these cases; until it arrives, the entry API or a two-step lookup is used:
// today's solution: a single lookup with entry
let deger = harita.entry(anahtar).or_insert_with(|| hesapla());Practice Two-phase borrow
For let mut v = vec![1, 2];, run v.push(v.len()); and print the vector with {:?}.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let mut v = vec![1, 2];
v.push(v.len());
println!("{:?}", v);
}Expected output
[1, 2, 2]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When the borrow checker finds an error, what's the effect on the program's runtime?
- Memory grows
- None; all checking is at compile time, with no runtime cost Correct answer
- The program slows down
- Runtime checks are added
Explanation: Borrow checking is a fully static analysis; runtime checking like RefCell exists only when explicitly chosen.
2 Two-phase borrow
let mut v = vec![1, 2];
v.push(v.____()); // [1, 2, 2]let mut v = vec![1, 2];
v.push(v.len()); // [1, 2, 2]Explanation: The argument is computed first; the &mut borrow activates afterwards.
3 Check and error code
| Term | Meaning |
|---|---|
Use of a moved value |
E0382 |
Overlapping &mut |
E0499 |
Okurken yazma |
E0502 |
Short-lived value |
E0597 |