## 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 |

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## 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

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## 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