Module 2Borrowing & ReferencesLesson 6 of 918 min

Reading Borrow Checker Errors

Hand-holding through the most common compile errors and how to fix them.

Error patterns you’ll see

1) Use after move

Fix: borrow, clone, or restructure.

2) Cannot borrow as mutable because also borrowed as immutable

Fix: finish read borrows before mut borrow; shrink scopes.

rust

fn main() {
    let mut v = vec![1, 2, 3];
    let first = v[0]; // copy out i32
    v.push(4); // ok — no live ref
    println!("first={first} v={v:?}");
}

Output

first=1 v=[1, 2, 3, 4]

3) Returns reference to local

Fix: return owned data (String, Vec) instead of &str to locals.

Practical strategy

Prefer owned fields in structs for long-lived data
Prefer short borrows in function signatures
Don’t fight the checker with unsafe while learning

Exercise

Fix the broken function in the starter (commented mistakes).

Challenge

Write fn longest<'a>(a: &'a str, b: &'a str) -> &'a str that returns the longer borrowed str.

Tips

If data is created inside the function, return String not &str
Split code into smaller scopes when mut/immut conflict
The checker is teaching aliasing rules used by CPUs and compilers

Try it live

Reading Borrow Checker ErrorsRust 2021

Loading editor…

Output

Press Run to compile and execute.