Hello, compiler — From Panic to Production

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Worked examples

Read the claim, then run it and check the machine agrees. One at a time — nothing here is taken on trust.

Example 1 of 3 Hello, and every kind of slot

example.py

fn main() {
    println!("Hello, compiler!");

    let name = "Ferris";
    let year = 2015;
    println!("{name} shipped in {year}");     // inline capture

    println!("{} + {} = {}", 2, 40, 2 + 40);  // positional slots
    println!("pi is roughly {:.2}", 3.14159); // spec: 2 decimals
}

Four println! calls, three slot styles. The macro validates every one at compile time: delete the year argument's binding or add a fourth {} and this stops being a program. {:.2} rounds for display only — the f64 underneath keeps its full value.

Example 2 of 3 Expressions all the way down

example.py

fn main() {
    let cold = -3;
    let label = if cold < 0 { "freezing" } else { "fine" };

    let y = {
        let a = 2;
        a * 21
    };

    println!("{label}, y = {y}");
}

No ternary operator, no temporary mutable variable waiting to be assigned in two branches: if hands its branch's value straight to let. The block computing y shows the same rule one level down — its last expression, a * 21 with no semicolon, is the block's value. Prints freezing, y = 42.

Example 3 of 3 The letter, verbatim

example.py

fn main() {
    let x: i32 = 5;            // annotation and value agree
    println!("x = {x}");

    // Uncomment to receive the letter:
    // let y: i32 = "five";
    //
    // error[E0308]: mismatched types
    //   expected `i32`, found `&str`
    //   (and `--> file:line:col` points at the exact span)
}

Put the bad line back and the program never becomes a binary — E0308 names both sides of the disagreement and flags the i32 annotation as the source of the expectation ("expected due to this"). When a challenge in this course won't compile, that letter appears as your feedback, word for word: read it before touching the code.

Check the concept

One question at a time. Unsure? Revisit the lecture, then answer.

Question 1 of 4

In Rust, `if cold < 0 { "freezing" } else { "fine" }` is…

a statement — you need a ternary operator to get a value out of it an expression: it evaluates to one branch's value and can sit on the right of a let only legal inside println! a compile error — the branches must be statements, not values

The challenge

Pass the quiz to unlock the challenge — your code will still be waiting here.