Rust

Write idiomatic Rust avoiding ownership pitfalls, lifetime confusion, and common borrow checker battles.

Quick Reference

Topic	File	Key Trap
Ownership & Borrowing	ownership-borrowing.md	Move semantics catch everyone
Strings & Types	types-strings.md	String vs &str, UTF-8 indexing
Errors & Iteration	errors-iteration.md	unwrap() in production, lazy iterators
Concurrency & Memory	concurrency-memory.md	Rc not Send, RefCell panics
Advanced Traps	advanced-traps.md	unsafe, macros, FFI, performance

---

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)

Error	Cause	Fix
value moved here	Used after move	Clone or borrow
cannot borrow as mutable	Already borrowed	Restructure or RefCell
missing lifetime specifier	Ambiguous reference	Add <'a>
the trait bound X is not satisfied	Missing impl	Check trait bounds
type annotations needed	Can't infer	Turbofish or explicit type
cannot move out of borrowed content	Deref moves	Clone or pattern match

---

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