Rust

Quick Reference

---

Critical Traps (High-Frequency Failures)

Ownership — #1 Source of Compiler Errors

• Variable moved after use — clone explicitly or borrow with &
• for item in vec moves vec — use &vec or .iter() to borrow
• String moved into function — pass &str for read-only access

Borrowing — The Borrow Checker Always Wins

• Can't have &mut and & simultaneously — restructure or interior mutability
• Returning reference to local fails — return owned value instead
• Mutable borrow through &mut self blocks all access — split struct or RefCell

Lifetimes — When Compiler Can't Infer

• 'static means CAN live forever, not DOES — String is 'static capable
• Struct with reference needs <'a> — struct Foo<'a> { bar: &'a str }
• Function returning ref must tie to input — fn get<'a>(s: &'a str) -> &'a str

Strings — UTF-8 Surprises

• s[0] doesn't compile — use .chars().nth(0) or .bytes()
• .len() returns bytes, not chars — use .chars().count()
• s1 + &s2 moves s1 — use format!("{}{}", s1, s2) to keep both

Error Handling — Production Code

• unwrap() panics — use ? or match in production
• ? needs Result/Option return type — main needs -> Result<()>
• expect("context") > unwrap() — shows why it panicked

Iterators — Lazy Evaluation

• .iter() borrows, .into_iter() moves — choose carefully
• .collect() needs type — collect::<Vec<_>>() or typed binding
• Iterators are lazy — nothing runs until consumed

Concurrency — Thread Safety

• Rc is NOT Send — use Arc for threads
• Mutex lock returns guard — auto-unlocks on drop, don't hold across await
• RwLock deadlock — reader upgrading to writer blocks forever

Memory — Smart Pointers

• RefCell panics at runtime — if borrow rules violated
• Box for recursive types — compiler needs known size
• Avoid Rc<RefCell<T>> spaghetti — rethink ownership

---

Common Compiler Errors (NEW)

---

Cargo Traps (NEW)

• cargo update updates Cargo.lock, not Cargo.toml — manual version bump needed
• Features are additive — can't disable a feature a dependency enables
• [dev-dependencies] not in release binary — but in tests/examples
• cargo build --release much faster — debug builds are slow intentionally