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Rust II: data, choices and confidence · Lesson 02 / 12
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Borrow to read and change

A function can inspect or update a label without taking ownership. Its signature shows which permission it requests.

Learning paths12 min

Let's discover together

  1. 1

    A reference & accesses a value without taking ownership. A function that only reads text can accept &str; &name converts the String borrow when needed.

  2. 2

    &mut String permits changing that String. Both the original variable and the borrow must permit mutation. text.push_str needs no explicit * because method calls resolve the reference.

  3. 3

    Shared references to the same value can coexist; a mutable borrow needs exclusive access while in use. Scenario 2 keeps a read reference in use until after the change.

  4. 4

    In many cases the compiler ends a borrow after its last use, before the closing brace. Separating reading and writing into stages helps more than adding clone without understanding the conflict.

FROM LESSON TO PRACTICE

A guide to follow at your pace

Read the steps before using the reference below. Commands run in the indicated system’s terminal; signup happens on the official site. Rullst Academy exercises are practice and do not verify your installation or external account.

fn length(text: &str) -> usize { text.len() }
fn mark(text: &mut String) { text.push_str("!"); }
fn main() {
    let mut name = String::from("Tico");
    println!("{}", length(&name));
    mark(&mut name);
    println!("{}", length(&name));
}

Official links and next step

✦
YOUR MISSION

Use &str and &mut String and recognise conflicting borrows.

Now it's your turn

Discovery workshop

Read, predict, investigate

Predict before revealing. This panel shows prepared results; it does not compile your code. Compiler errors are summarised; exact wording can vary with the compiler version.
Mini experiment

Learning simulation: compare cards and check your predictions. The browser is not compiling Rust or starting a server.

fn length(text: &str) -> usize { text.len() }
fn mark(text: &mut String) { text.push_str("!"); }
fn main() {
    let mut name = String::from("Tico");
    println!("{}", length(&name));
    mark(&mut name);
    println!("{}", length(&name));
}
Build your answers and check when you are ready.
A moment to understand

Which contract permits changing the borrowed String?

I need a hint

&mut communicates exclusive mutable access while the borrow is in use.

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Take it with you

Move println!("{view}") before name.push_str in scenario 2. Compile and explain why use order matters. Write a function that appends two signs and check the length.

RUST II · .rs

Your Rust II workbench

Read, predict and answer here. To actually compile, extract the activity ZIP into a new folder, open a terminal beside Cargo.toml and use the commands below. Rust and your operating system tools must be installed.

cargo check --offline
cargo test --offline
cargo run --offline
My files and GitHub

The editor below saves a draft in this browser and lets you download or publish your .rs. To run it in the project, save the edited file as src/main.rs. The ZIP contains the starter version; prepared results do not change with your edits. Online grading does not verify local execution.

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At your own pace

My files and GitHub

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