Rust interview questions for beginners — Part 4

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Tech Tonic

13 min readMar 20, 2024

In this article series, we’ll go through Rust interview questions for beginners. As a multi-paradigm, general-purpose language, Rust programming language has gained popularity recently by engaging developers with its unique blend of performance, safety, and concurrency. While Rust’s community is growing at an incredible pace, its adoption in the industry is not far behind. Tech giants like Microsoft, Google, and Amazon have recognized Rust’s potential and are actively supporting it. As a result, Rust is increasingly finding its way into real-world applications and products.

This is the fourth article in this series. Without wasting further time, let’s jump into the questions 31 to 40 for beginner interviews.

The other articles are:

Question 31 — Describe the difference between shallow and deep copies in Rust. When would you use each?

While not a built-in concept like in some other languages, copying data in Rust involves understanding ownership and how it’s handled for different data structures. Here’s a breakdown:

Shallow Copy

In Rust, a shallow copy typically refers to copying the structure itself, but not necessarily the data it points to on the heap.
This means the copied data structure holds references (pointers) to the same underlying data as the original.
Modifying the data through one copy can affect the other as they both point to the same memory location.

let data = vec![1, 2, 3];
let shallow_copy = data; // Just copies the vector structure, not the elements themselves

// Modifying the shallow copy affects the original
shallow_copy[0] = 10;
println!("Original: {:?}", data); // Output: [10, 2, 3]

Deep Copy

A deep copy, while not directly achievable with built-in Rust functionality for all cases, aims to create a completely independent copy of the data structure and its contents.
This involves allocating new memory for the copied data and ensuring the new copy doesn’t share ownership with the original.

The following are some approaches to achieve deep copying in Rust:

Manual Cloning

Implement the Clone trait for your custom data structures.
The clone method can perform a deep copy by recursively copying nested data.

Recursive Function

Write a function that traverses the original data structure and allocates new memory for each element, creating a completely independent copy.

Using Libraries

Some libraries like serde can be used for serialization and deserialization, which can effectively achieve deep copying in some cases.

When to Use Each

Shallow Copy

If you only need a second reference to the same data (e.g., passing data for read-only operations).
When modifying the copy doesn’t affect your application logic (be cautious!).

Deep Copy

When you need an independent copy of the data and modifications to one shouldn’t affect the other (common scenario).
When dealing with complex data structures that contain nested data.

Important points to remember

Rust’s ownership system often avoids the need for explicit deep copies by encouraging data immutability.
Consider if a shallow copy with proper borrowing or immutability can achieve your goal before resorting to deep copying.

Question 32 — Explain the concept of lifetime elision in Rust and how it simplifies borrowing in certain cases

In Rust, lifetimes are annotations that specify the lifetime of references. They ensure references point to valid data for as long as they are needed. However, writing out lifetimes for every reference can become verbose. This is where lifetime elision comes in.

Purpose of Lifetime Elision

Lifetime elision is a feature of the Rust compiler that allows it to infer lifetimes automatically in certain common scenarios.
This reduces the boilerplate code you need to write when dealing with references and simplifies borrowing.

Rules for Lifetime Elision

The compiler follows specific rules to determine lifetimes when elision is applied:

Each parameter that’s a reference gets a distinct lifetime. (e.g., fn foo<'a, 'b>(&'a i32, &'b str) -> ...)
If there’s exactly one input lifetime parameter (elided or not), that lifetime is used for all output lifetimes. (e.g., fn foo<'a>(&'a i32) -> &'a str)
If there are multiple input lifetime parameters, but one of them is &self or &mut self, the lifetime of self is used for all output lifetimes. (e.g., fn get(&self) -> &i32)

Benefits of Lifetime Elision

Reduced Boilerplate: You don’t need to explicitly write out lifetimes in many cases, making code cleaner and more readable.
Improved Maintainability: Less verbose code can be easier to understand and maintain.

Example: Simplified Borrowing with Elision

Without elision:

fn compare<'a, 'b>(x: &'a i32, y: &'b i32) -> bool {
  x == y
}

With elision (compiler infers lifetimes based on the rules):

fn compare(x: &i32, y: &i32) -> bool {
  x == y
}

Cautions with Elision

While convenient, lifetime elision can sometimes make code less explicit.
If the compiler cannot infer lifetimes due to complex scenarios, you might need to provide them manually to ensure borrow checker satisfaction.

Question 33 — Describe the difference between panic and unwind behavior in Rust error handling

Rust enforces type safety and memory management through its ownership system. However, there are situations where errors might occur during program execution. Rust provides mechanisms to handle these errors gracefully, including:

Result type (Result<T, E>) for indicating success (Ok(value)) or failure (Err(error)) with a specific error type.
Option type (Option<T>) for representing optional values that might be present (Some(value)) or absent (None).

Panic vs. Unwind

While Result and Option are preferred for handling expected errors, Rust offers two additional mechanisms for dealing with unexpected or unrecoverable situations: panic and unwind.

Panic

Initiated by calling the panic! macro or encountering a runtime failure like division by zero.
Prints an error message with a backtrace (call stack information) to the standard error stream.
Causes the program to abruptly terminate.

fn divide(x: i32, y: i32) -> i32 {
  if y == 0 {
    panic!("Division by zero!"); // Initiates panic
  }
  x / y
}

fn main() {
  let result = divide(10, 0); // This will cause panic and program termination
  println!("Result: {}", result);
}

Unwind

Occurs when a panic happens.
Rust attempts to unwind the stack, cleaning up resources allocated by functions in the call stack, leading to the panic.
This process allows the possibility of catching the panic and handling it gracefully (though not commonly used in Rust).

Choosing between panic and unwind

Use panic! for unexpected errors that halt program execution and provide debugging information.
Result and Option are preferred approaches for handling anticipated errors in a more controlled way.
Unwinding happens behind the scenes during panic and can potentially be caught for custom handling, although this is not a common use case in Rust.

Question 34 — Explain the concept of interior mutability (&mut self) in Rust methods. How does it allow modifying data within an immutable reference?

Rust’s ownership system enforces memory safety and prevents data races. Borrowing allows temporary access to a value while the original owner retains ownership. However, by default, references (&T) are immutable, meaning you can't modify the data they point to.

Interior Mutability (&mut self)

&mut self is a special reference type used in methods of structs.
It allows borrowing the struct itself mutably, granting temporary access for modification of the struct’s fields while maintaining ownership within the method.

How it Works

The &mut self parameter in a method receives a mutable reference to the struct instance itself.
This reference grants access to the struct’s fields, allowing you to modify them within the method.
Since it’s still a reference, ownership remains with the original caller.

struct Point {
  x: i32,
  y: i32,
}

impl Point {
  fn increment(&mut self) {
    self.x += 1;
    self.y += 1;
  }

  fn double(&mut self) {
    self.x *= 2;
    self.y *= 2;
  }
}

fn main() {
  let mut p = Point { x: 5, y: 10 };
  p.increment();
  println!("After increment: {:?}", p); // Output: Point { x: 6, y: 11 }
  p.double();
  println!("After double: {:?}", p); // Output: Point { x: 12, y: 22 }
}

Explanation

The increment and double methods take &mut self as an argument.
Inside the methods, we can modify the x and y fields of the struct using self.x += 1 and similar expressions.
Even though p is an immutable reference in main, the &mut self parameter allows modifying its internal state within the methods.

Important Considerations

Interior mutability should be used cautiously, as excessive mutation can lead to decreased code readability and potential issues with data races if not managed properly.
In many cases, consider using immutable references with methods that return a new modified version of the data to preserve immutability principles.

Question 35 — Describe the difference between slices (&[T]) and raw pointers (*mut T) in Rust. Why are raw pointers discouraged?

Slices (&[T])

Represent a contiguous sequence of elements of the same type T.
Essentially a “fat pointer” containing both a pointer to the first element and the length of the slice.
Safe to use within Rust’s ownership system, as they borrow memory without taking ownership.
Provide bounds checking to ensure you don’t access elements outside the valid range of the slice.

let numbers = [1, 2, 3, 4, 5];
let slice = &numbers[1..3]; // Slice from index 1 (inclusive) to 3 (exclusive)

// Accessing elements using indexing within bounds
println!("Second element: {}", slice[1]);

Raw Pointers (*mut T)

Represent raw memory addresses pointing to data of type T.
Don’t inherently contain information about the length of the data being pointed to.
Bypassing Rust’s ownership system, requiring manual memory management.
Can lead to memory-related errors like dangling pointers, buffer overflows, and double frees if not used cautiously.

use std::ptr;

let mut numbers: [i32; 5] = [1, 2, 3, 4, 5];
let raw_ptr = numbers.as_ptr() as *mut i32; // Unsafe conversion to raw pointer

// Manual check required to avoid accessing invalid memory
if raw_ptr.offset(2) < raw_ptr.add(numbers.len() as isize) {
    unsafe { *(raw_ptr.offset(2)) = 10; } // Unsafe operation
}

Why Raw Pointers are Discouraged

Memory Safety: Bypassing ownership checks can lead to memory leaks, dangling pointers, and other memory-related errors.
Undefined Behavior: Incorrect usage of raw pointers can result in undefined behavior, making debugging difficult.
Complexity: Manual memory management with raw pointers increases code complexity and error-proneness.
Alternatives: Often, slices or safe abstractions like Box<T> can achieve the desired functionality without resorting to raw pointers.

In summary

Slices are the preferred way to work with contiguous memory regions in Rust due to their safety, bounds checking, and adherence to the ownership system.
Raw pointers should be used with caution and only when absolutely necessary, due to the potential risks associated with manual memory management. When possible, consider using safer alternatives provided by Rust.

Question 36 — What are some common ways to handle concurrency with channels (mpsc) in Rust?

Concurrency with channels (mpsc — multiple-producer, single-consumer) is a powerful approach for handling tasks that can run independently in Rust. Here’s a breakdown of common usage patterns:

Producer-Consumer pattern

This is a classic pattern where one or more producers generate data and send it to a channel, while a single consumer receives and processes the data.

use std::thread;
use std::sync::mpsc::channel;

fn producer(tx: mpsc::Sender<i32>) {
  for i in 0..10 {
    tx.send(i).unwrap();
  }
}

fn consumer(rx: mpsc::Receiver<i32>) {
  for value in rx.iter() {
    println!("Received: {}", value);
  }
}

fn main() {
  let (tx, rx) = channel();
  let producer_thread = thread::spawn(move || producer(tx));
  let consumer_thread = thread::spawn(move || consumer(rx));

  producer_thread.join().unwrap();
  consumer_thread.join().unwrap();
}

Shared state updates

Channels can be used to safely update shared state between threads. The producer sends updates, and the consumer applies them to the shared state while avoiding data races.

use std::thread;
use std::sync::mpsc::channel;

struct Counter {
  value: i32,
}

impl Counter {
  fn increment(&mut self, amount: i32) {
    self.value += amount;
  }
}

fn main() {
  let (tx, rx) = channel();
  let mut counter = Counter { value: 0 };
  let increment_thread = thread::spawn(move || {
    for _ in 0..10 {
      tx.send(1).unwrap();
    }
  });

  let update_thread = thread::spawn(move || {
    for _ in rx.iter() {
      counter.increment(1);
    }
  });

  increment_thread.join().unwrap();
  update_thread.join().unwrap();

  println!("Final counter value: {}", counter.value); // Output: 10
}

Sending tasks over channels

Channels can be used to send closures or functions (tasks) to be executed by another thread. This allows for dynamic workload distribution.

use std::thread;
use std::sync::mpsc::channel;

fn worker(task: fn(i32)) {
  task(10);
}

fn main() {
  let (tx, rx) = channel();
  let worker_thread = thread::spawn(move || {
    let task = rx.recv().unwrap();
    worker(task);
  });

  let task_to_send = |x| println!("Processing: {}", x);
  tx.send(task_to_send).unwrap();

  worker_thread.join().unwrap();
}

Key Considerations

Channel Types: Choose between asynchronous (channel) and synchronous (sync_channel) channels depending on your use case. Asynchronous channels are generally preferred for performance.
Error Handling: Use Result<T, E> to handle potential errors when sending or receiving data on the channel.
Channel Closing: Once a producer is finished sending data, it should drop the sender to indicate the channel is closed. The consumer can then check for the Err variant from recv to determine the end of the stream.

Question 37 — Describe the purpose of the drop trait and its role in resource management during variable destruction

Purpose

The Drop trait allows you to define custom behavior that gets executed when a variable goes out of scope.
This behavior typically involves releasing resources associated with the variable, such as closing files, releasing network connections, or freeing memory allocated on the heap (Box<T>).

The drop Method

The Drop trait defines a single method named drop.
This method takes self by reference (&mut self) to allow potential modifications during resource cleanup.
The implementation of drop specifies the actions to be taken when the variable is about to be destroyed.

use std::fs::File;
use std::io::Write;

struct MyFile {
  file: File,
}

impl Drop for MyFile {
  fn drop(&mut self) {
    // Close the file when the MyFile instance goes out of scope
    self.file.write_all(b"Closing the file...").unwrap();
    println!("File closed!");
  }
}

fn main() {
  let my_file = MyFile { file: File::create("data.txt").unwrap() };
  // Use the file here
  my_file.file.write_all(b"Hello, world!").unwrap();
}

Explanation

The MyFile struct holds a File handle.
Implementing Drop for MyFile allows defining custom cleanup behavior in drop.
In this case, drop closes the file and writes a message before it's released.

Key points:

The Drop trait ensures automatic resource management at the end of a variable's lifetime.
It helps prevent resource leaks and unexpected program behavior.
You can implement Drop for any custom type that requires specific cleanup actions.
While automatic through Drop, consider explicitly calling std::mem::drop(variable) to force immediate resource deallocation if necessary (use cautiously).

Question 38 — Explain the concept of async/await syntax in Rust and its benefits for handling asynchronous operations

In many languages, asynchronous operations are typically handled with callbacks, promises, or futures. These approaches can lead to complex code with nested callbacks, making it difficult to reason about and maintain.

Async/await in rust

Introduced in Rust 1.39, async/await provides a cleaner and more intuitive way to write asynchronous code.
It leverages the concept of futures, which represent eventual computations that might not be ready yet.

Key components

Async functions

Defined with the async keyword before the function signature.
Can return a type that implements the Future trait, which represents the eventual result of the asynchronous operation.
Often return Result<T, E> for successful computations with a value (T) or errors (E).

Await keyword

Used within async functions to pause execution and wait for a future to become available.
The syntax resembles waiting for a synchronous expression.
The compiler automatically handles scheduling and resuming the execution when the future is ready.

Example: Making an async HTTP request

use std::net::TcpStream;
use std::io::{Read, Write};

async fn fetch_data(url: &str) -> Result<String, std::io::Error> {
  let mut stream = TcpStream::connect(url)?;
  stream.write_all(format!("GET {} HTTP/1.1\r\nHost: {}\r\n\r\n", url, url).as_bytes())?;

  let mut response = String::new();
  stream.read_to_string(&mut response)?;
  Ok(response)
}

async fn main() -> Result<(), std::io::Error> {
  let data = fetch_data("www.example.com").await?;
  println!("Fetched data: {}", data);
  Ok(())
}

Benefits of async/await

Improved readability: Code resembles synchronous style, making it easier to understand the flow of asynchronous operations.
Reduced nesting: Avoids deep nesting of callbacks, leading to cleaner and more maintainable code.
Error handling: Integrates well with Result<T, E> for handling success and errors in asynchronous operations.

Question 39 — Explain the concept of associated functions in Rust and how they differ from regular methods within a struct

Regular methods

Defined within a struct implementation block (impl).
Operate on instances (structs) of the type.
Have access to the self parameter, which refers to the specific struct instance the method is called on.
Used to define behaviors specific to that struct type.

struct Point {
  x: i32,
  y: i32,
}

impl Point {
  fn distance_from_origin(&self) -> f64 {
    (self.x.pow(2) + self.y.pow(2)).sqrt()
  }
}

Associated functions

Defined outside the struct implementation block, but associated with a type using the :: syntax.
Don’t take self as an argument.
Can be used to create new instances of the type, perform utility functions related to the type, or interact with the type in a more general way.

struct Point {
  x: i32,
  y: i32,
}

impl Point {
  fn distance_from_origin(&self) -> f64 {
    (self.x.pow(2) + self.y.pow(2)).sqrt()
  }
}

// Associated function to create a Point from coordinates
fn create_point(x: i32, y: i32) -> Point {
  Point { x, y }
}

When to use each

Use regular methods when you need to operate on a specific instance of the struct and access its fields using self.
Use associated functions when you want to define utility functions that don’t require access to a specific instance, create new instances of the type, or interact with the type in a more general way.

Additional considerations

Associated functions can also be generic, allowing them to work with different types.
Traits can leverage associated functions to define common functionality for implementing types.

Question 40 — Describe the purpose of the Copy trait and which data types implement it by default

Purpose of the copy trait

The Copy trait signifies that a value of a type can be efficiently duplicated by copying the bits without any additional allocation or deallocation.
This is in contrast to moving, where ownership is transferred and the original value might be invalidated.
Copying is typically faster than moving, especially for small data types.

Data types implementing copy by default:

Primitive types

Integers (e.g., i8, u32, usize)
Floats (e.g., f32, f64)
Booleans (bool)
Characters (char)

Tuples:

As long as all elements within the tuple implement Copy themselves.

let x = 10; // i32 implements Copy
let y = x; // Copying the value of x to y

let point1 = (1, 2); // Copying a tuple with Copy elements

fn double_value(x: i32) -> i32 {
  x * 2 // Doesn't modify the original value because of Copy
}

let doubled = double_value(x);
println!("Original: {}, Doubled: {}", x, doubled); // Output: Original: 10, Doubled: 20 (Original value remains unchanged)

Important considerations

Not all data types implement Copy. For example, strings (String) and vectors (Vec) do not, as they manage their own memory on the heap. Moving them is preferred to avoid unnecessary copying.
While copying might be faster for small Copy types, excessive copying can impact performance. Consider using references or immutable borrows when appropriate.

Thanks for reading this article! I hope this would help you in your next Rust interview.