Why borrowing is required in Rust - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to borrow a reference to the string without taking ownership.

Rust

fn main() {
    let s = String::from("hello");
    let len = calculate_length([1]);
    println!("Length: {}", len);
}

fn calculate_length(s: &String) -> usize {
    s.len()
}

Drag options to blanks, or click blank then click option'

A&s

Bs

C*s

Dref s

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable without & causes ownership to move.

Using *s tries to dereference but s is not a reference here.

Using &s borrows a reference to s without taking ownership, allowing calculate_length to read the string.

2fill in blank

medium

Complete the code to borrow a mutable reference and modify the string.

Rust

fn main() {
    let mut s = String::from("hello");
    change([1]);
    println!("{}", s);
}

fn change(s: &mut String) {
    s.push_str(", world");
}

Drag options to blanks, or click blank then click option'

A&s

B*s

Cs

D&mut s

Attempts:

3 left

💡 Hint

Common Mistakes

Using &s instead of &mut s causes a compile error when trying to modify.

Passing s moves ownership, which is not intended.

Using &mut s borrows a mutable reference, allowing the function to modify the string.

3fill in blank

hard

Fix the error by borrowing the string slice correctly.

Rust

fn main() {
    let s = String::from("hello");
    let first_word = first_word([1]);
    println!("First word: {}", first_word);
}

fn first_word(s: &str) -> &str {
    let bytes = s.as_bytes();
    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }
    &s[..]
}

Drag options to blanks, or click blank then click option'

Aref s

Bs

C&s

D*s

Attempts:

3 left

💡 Hint

Common Mistakes

Passing s instead of &s causes a type mismatch error.

Using *s or ref s are invalid in this context.

Borrowing &s passes a string slice reference, matching the function parameter type &str.

4fill in blank

hard

Fill in the blank to create a HashMap that maps words to their lengths only if length is greater than 3.

Rust

fn main() {
    let words = vec!["apple", "cat", "banana", "dog"];
    let lengths: std::collections::HashMap<_, _> = words.iter()
        .filter(|&&word| word.len() [1] 3)
        .map(|&word| (word, word.len()))
        .collect();
    println!("{:?}", lengths);
}

Drag options to blanks, or click blank then click option'

A<

B>

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < or == selects wrong words.

Using != does not filter correctly.

The filter condition word.len() > 3 selects words longer than 3 characters.

5fill in blank

hard

Fill all three blanks to create a mutable reference, modify the string, and print the updated value.

Rust

fn main() {
    let mut s = String::from("hello");
    let r = [1] s;
    r.push_str([2]);
    println!("{}", [3]);
}

Drag options to blanks, or click blank then click option'

A&mut

B", world"

Cs

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of &mut causes a compile error when modifying.

Printing the reference instead of the original variable may confuse output.

Use &mut to borrow a mutable reference, append the string ", world", and print the original string s.

Practice

1. Why does Rust require borrowing when accessing data instead of always transferring ownership?

easy

A. To allow multiple parts of code to read data without copying it

B. To make the code run slower for safety

C. To force programmers to write more code

D. To prevent any data from being changed

Solution

1. Step 1: Understand ownership and borrowing

   Ownership means one variable controls the data. Borrowing lets others use data without owning it.

2. Step 2: Why borrowing is useful

   Borrowing allows multiple parts of code to read data safely without making copies or moving ownership.

3. Final Answer:

   To allow multiple parts of code to read data without copying it -> Option A

4. Quick Check:

   Borrowing = safe shared access [OK]

Hint: Borrowing means sharing data without moving ownership [OK]

Common Mistakes:

• Thinking borrowing slows down code
• Believing borrowing prevents all data changes
• Confusing borrowing with copying data

2. Which of the following is the correct syntax to borrow a variable immutably in Rust?

easy

A. let x = ref value;

B. let x = &value;

C. let &x = value;

D. let mut x = &mut value;

Solution

1. Step 1: Identify borrowing syntax

   In Rust, borrowing immutably uses the ampersand before the variable: &value.

2. Step 2: Match correct syntax

   The correct way to bind a reference is let x = &value;. Other options are invalid or mean something else.

3. Final Answer:

   let x = &value; -> Option B

4. Quick Check:

   Immutable borrow = &value [OK]

Hint: Use & before variable to borrow immutably [OK]

Common Mistakes:

• Using let x = ref value; invalid syntax
• Confusing &x with &value
• Using mutable borrow (&mut) for immutable access

3. What will be the output of this Rust code?

fn main() {
    let s = String::from("hello");
    let r1 = &s;
    let r2 = &s;
    println!("{} and {}", r1, r2);
}

medium

A. hello and hello

B. Compilation error due to multiple mutable borrows

C. hello and error

D. Runtime error

Solution

1. Step 1: Analyze borrowing in code

   Two immutable references r1 and r2 borrow s simultaneously, which is allowed.

2. Step 2: Check output of println!

   Both references print the string "hello", so output is "hello and hello".

3. Final Answer:

   hello and hello -> Option A

4. Quick Check:

   Multiple immutable borrows print values [OK]

Hint: Multiple immutable borrows allowed, prints values [OK]

Common Mistakes:

• Thinking multiple immutable borrows cause errors
• Confusing mutable and immutable borrow rules
• Expecting runtime errors for borrowing

4. Identify the error in this Rust code and why borrowing is required:

fn main() {
    let mut s = String::from("hello");
    let r1 = &s;
    let r2 = &mut s;
    println!("{} and {}", r1, r2);
}

medium

A. No error, code runs fine

B. Cannot borrow `s` as immutable because it is mutable

C. Cannot borrow `s` as mutable because it is already borrowed as immutable

D. Cannot print two variables in println!

Solution

1. Step 1: Check borrowing rules

   Rust forbids mutable borrow while immutable borrow exists to prevent data races.

2. Step 2: Identify error cause

   Here, r1 immutably borrows s, then r2 tries mutable borrow, causing compile error.

3. Final Answer:

   Cannot borrow `s` as mutable because it is already borrowed as immutable -> Option C

4. Quick Check:

   Mutable borrow blocked by existing immutable borrow [OK]

Hint: Mutable borrow blocked if immutable borrow exists [OK]

Common Mistakes:

• Thinking mutable borrow is allowed with immutable borrow
• Ignoring borrow checker errors
• Assuming println! causes error

5. You want to write a function that changes a string passed to it without taking ownership. Which borrowing type should you use and why?

hard

A. Immutable borrow (&) because it allows reading the string safely

B. Copy the string inside the function to modify it

C. Ownership transfer because borrowing is not needed for modification

D. Mutable borrow (&mut) because it allows modifying the string without ownership

Solution

1. Step 1: Understand function parameter needs

   To modify data inside a function without taking ownership, you must borrow it mutably.

2. Step 2: Choose correct borrow type

   Mutable borrow &mut allows changing the original data safely without moving ownership.

3. Final Answer:

   Mutable borrow (&mut) because it allows modifying the string without ownership -> Option D

4. Quick Check:

   Modify data = use &mut borrow [OK]

Hint: Use &mut to change data without ownership transfer [OK]

Common Mistakes:

• Using immutable borrow when modification is needed
• Transferring ownership unnecessarily
• Copying data instead of borrowing