Learn Rust from Ferris himself.

01 · How Ferris helps

Stuck on a compiler error? Ferris walks you through it.

This is level 4.1 on Borrow Bridge. The goal is to make it print ferris has 6 letters, but rustc refuses. Step through what happens next.

fn count_letters(word: String) -> usize {
    word.len()
}

fn main() {
    let word = String::from("ferris");
    let n = count_letters(word);
    println!("{} has {} letters", word, n);
}

error[E0382]: borrow of moved value: `word`
 --> main.rs:8:35
  |
6 |     let word = String::from("ferris");
  |         ---- move occurs because `word` has type `String`,
  |              which does not implement the `Copy` trait
7 |     let n = count_letters(word);
  |                           ---- value moved here
8 |     println!("{} has {} letters", word, n);
  |                                   ^^^^ value borrowed here after move
  |
note: consider changing this parameter type in function
      `count_letters` to borrow instead if owning the value
      isn't necessary
 --> main.rs:1:24
  |
1 | fn count_letters(word: String) -> usize {
  |                        ^^^^^^ this parameter takes ownership

02 · Try a level

🔮 Predict it: does this compile?

Three real levels from Ownership Caves and Borrow Bridge. Pick an answer and watch Ferris react.

fn main() {
    let a = 5;
    let b = a;
    let s1 = String::from("hi");
    let s2 = s1;
    println!("{} {}", a, b);
    println!("{}", s2);
}

fn main() {
    let outer = String::from("outer");
    {
        let inner = String::from("inner");
        println!("{} {}", outer, inner);
    }
    println!("{} {}", outer, inner);
}

fn main() {
    let mut v = vec![1, 2, 3];
    let first = &v[0];
    v.push(4);
    println!("{}", first);
}

03 · A look around

A whole world, drawn in code.

04 · How it works

A real compiler, and a crab who can only talk.

⚙️ Real compiler

Your code runs through your own rustc. The compiler and tests decide whether you pass, not the AI.

🦀 Ferris = Claude

Explanations, hints, reviews and chat go through claude -p, so they use your Claude subscription. No API key.

🧠 Pick a brain

Switch Ferris between Fable, Opus, Sonnet and Haiku, and choose an effort level.

⚡ Energy bar

The “E” bar shows how much of your Claude usage limit is left. Offline mode uses built-in hints and costs nothing.

🔒 Local only

Listens on 127.0.0.1, and its API only answers the game's own page. Each run is limited in memory, CPU time and file size, and stopped after 5 seconds.

✅ Every level verified

cargo run -- verify checks that every starter fails, every solution passes, and every book topic is taught.

05 · The islands

The whole Rust Book, in the order you need it.

The curriculum follows Brown University's interactive edition of The Rust Programming Language.

0. Tutorial BeachWash ashore, meet Ferris, and say your first words in Rust.
1. Variable Villagelet, mut, shadowing, number types, tuples and arrays.
2. Flow Forestif, functions, loop, while and for.
3. Ownership CavesRust's big idea: every value has exactly one owner.
4. Borrow BridgeLend things out without giving them away.
5. Struct SmithyForge your own types.
6. Enum IslesA value that can be one of several things.
7. Collection CitadelLists, text and dictionaries.
8. Error MarshWhen things go wrong, and what to do about it.
9. Guessing Game GrottoBuild the book's guessing game.
10. Pattern PlainsEvery shape a pattern can take.
11. Module MinesOrganize bigger programs into neat tunnels.
12. Testing GroundsProve your code works, automatically.
13. Trait TowerWrite code once, for many types.
14. Lifetime LabyrinthHow long does a reference live?
15. Iterator RapidsLittle functions and conveyor belts.
16. Minigrep HarborBuild a real command-line search tool.
17. Pointer PeaksBox, Rc, RefCell and friends.
18. Object ObservatoryObjects, Rust style.
19. Thread VolcanoDoing many things at once, safely.
20. Async ArchipelagoWaiting for many things without wasting time.
21. Cargo DocksShip your code to the world.
22. Advanced AbyssThe deep end of Rust.
23. Web Server CitadelThe final project: a multithreaded web server.
24. Ferris' LibraryThe book's appendices as quick quizzes.