Some Highlights About Rust
(for C/C++ developers)

betterCode() Rust 2023
Rainer Stropek | time cockpit

Ownership and Borrowing

The "Highlander"  Princple 😉:

About Highlander (film)

There can be
only one

struct Ring { inscription: &'static str, }




    










fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

Owner

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();
    println!("My powerful 💍! {}", ring.inscription);
    








}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

Gives ownership to caller

Caller is now owner

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();
    println!("My powerful 💍! {}", ring.inscription);
    


    


    


}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

fn keep(ring: Box<Ring>) {
    println!("Now the 💍 is mine! {}", ring.inscription);
}

Takes ownership (see also Drop trait 🔗)

ring goes out of scope ➡️ is freed

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();
    println!("My powerful 💍! {}", ring.inscription);
    


    


    
    keep(ring);
    println!("My powerful 💍! {}", ring.inscription);
}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

fn keep(ring: Box<Ring>) {
    println!("Now the 💍 is mine! {}", ring.inscription);
}

Ownership
moved to
callee

ring cannot be used anymore

13 |     keep(ring);
   |          ---- value moved here
14 |     println!("My powerful 💍! {}", ring.inscription);
   |                                    ^^^^^^^^^^^^^^^^ value borrowed here after move

Owner is responsible for cleaning up

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();
    println!("My powerful 💍! {}", ring.inscription);
    
    borrow(&ring);
    println!("My powerful 💍! {}", ring.inscription);
    


    
    keep(ring);
    //println!("My powerful 💍! {}", ring.inscription);
}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

fn keep(ring: Box<Ring>) {
    println!("Now the 💍 is mine! {}", ring.inscription);
}

fn borrow(ring: &Box<Ring>) {
    println!("I can read the 💍's inscription: {}", ring.inscription);
}

ring is borrowed (immutable) ➡️
no moving of ownership

ring can still be used

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();
    println!("My powerful 💍! {}", ring.inscription);
    
    borrow(&ring);
    println!("My powerful 💍! {}", ring.inscription);
    
    to_english(&mut ring);
    println!("My powerful 💍! {}", ring.inscription);
    
    keep(ring);
    //println!("My powerful 💍! {}", ring.inscription);
}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

fn keep(ring: Box<Ring>) {
    println!("Now the 💍 is mine! {}", ring.inscription);
}

fn borrow(ring: &Box<Ring>) {
    println!("I can read the 💍's inscription: {}", ring.inscription);
}

fn to_english(ring: &mut Box<Ring>) {
    ring.inscription = "One Ring to rule them all, One Ring to find them, ...";
}

Mutable borrow ➡️ method can change ring

Caller agrees to mutable borrow

Example in playground

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();

    let first_borrow = &ring;
    let second_borrow = &ring;

    println!("{} and {}", first_borrow.inscription, second_borrow.inscription);






}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

Multiple immutable borrows are ✅

struct Ring { inscription: &'static str, }

fn main() {
    let mut ring = forge();

    let first_borrow = &ring;
    let second_borrow = &ring;

    println!("{} and {}", first_borrow.inscription, second_borrow.inscription);


    let mut first_borrow = &mut ring;
    let mut second_borrow = &mut ring;
        
    first_borrow.inscription = "One ring to rule them all, one ring to find them, ...";
}

fn forge() -> Box<Ring> {
    let one_ring = Box::new(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", });
    one_ring
}

12 |     let mut first_borrow = &mut ring;
   |                            --------- first mutable borrow occurs here
13 |     let mut second_borrow = &mut ring;
   |                             ^^^^^^^^^ second mutable borrow occurs here

Only one mutable borrow at a time

It is ok to have multiple immutable borrows

Example in playground

 or multiple immutable references

Either one mutable reference 

References must always be valid

What if you need multiple references?

  • It is fine to have multiple immutable borrows
  • Use reference-counted smart pointers
    • Rc<T> 🔗
    • Arc<T> (shared ownership between threads, 🔗)
  • Use RefCell<T> 🔗 for runtime borrow checker
    • Sometimes, the compiler simply isn't smart enough
      or to conservative
  • Unsafe code
    • Last resort
    • Bypasses borrow checker completely

Lifetimes

struct Ring {
    inscription: &'static str,
}

fn get_the_one    (ring1: &   Ring, ring2: &   Ring) -> Option<&   Ring> {
    [ring1, ring2]
        .into_iter()
        .find(|ring| ring.inscription.starts_with("Ash nazg durbatulûk"))
}

68 | fn get_the_one    (ring1: &   Ring, ring2: &   Ring) -> Option<&   Ring> {
   |                           --------         --------            ^ expected named lifetime parameter

Example in playground

Photo of "The One Ring": https://en.wikipedia.org/wiki/One_Ring#/media/File:One_Ring_Blender_Render.png (CC BY-SA 4.0)

fn main() {
  let some_ring = Ring { inscription: "I whish you luck 🍀", };
  let result: Option<&Ring>;
  {
    let the_one_ring = forge();
    result = get_the_one(&the_one_ring, &some_ring);
  }

  println!("The one ring is: {}", match result { 
    Some(ring) => ring.inscription, 
    None => "No ring found", 
   });
}

some_ring

💍

the_one_ring

result

⚡

the_one_ring goes out-of-scope

Potentially accessing invalid reference

What's the Problem?

Example in playground

struct Ring {
    inscription: &'static str,
}

fn get_the_one    (ring1: &   Ring, ring2: &   Ring) -> Option<&   Ring> {
    [ring1, ring2]
        .into_iter()
        .find(|ring| ring.inscription.starts_with("Ash nazg durbatulûk"))
}
 
<'a>

'a

'a

'a

Adding lifetime
specifiers

Result must not outlive the inputs!

fn main() {
  let some_ring = Ring { inscription: "I whish you luck 🍀", };
  let result: Option<&Ring>;
  {
    let the_one_ring = forge();
    result = get_the_one(&the_one_ring, &some_ring);
  }

  println!("The one ring is: {}", match result { 
    Some(ring) => ring.inscription, 
    None => "No ring found", 
   });
}
31 |             let the_one_ring = forge();
   |                 ------------ binding `the_one_ring` declared here
32 |             result = get_the_one(&the_one_ring, &some_ring);
   |                                  ^^^^^^^^^^^^^ borrowed value does not live long enough
33 |         }
   |         - `the_one_ring` dropped here while still borrowed
struct Ring {
    inscription: &'static str,
}

fn get_the_one    (ring1: &   Ring, ring2: &   Ring) -> Option<&   Ring> {
    [ring1, ring2]
        .into_iter()
        .find(|ring| ring.inscription.starts_with("Ash nazg durbatulûk"))
}

Example in playground

struct Ring { inscription: &'static str, }









fn main() {











}

fn forge() -> Ring {
    let one_ring = Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", };
    one_ring
}

static represents the entire duration
of the program

struct Ring { inscription: &'static str, }

struct Hobbit<'a> {
  ring: &'a Ring,
}





fn main() {











}

fn forge() -> Ring {
    let one_ring = Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", };
    one_ring
}

Hobbit must not outlive the borrowed ring
➡️ needs a lifetime

struct Ring { inscription: &'static str, }

struct Hobbit<'a> {
  ring: &'a Ring,
}

struct RingCollection<'a> {
  rings: Vec<&'a Ring>,
}

fn main() {











}

fn forge() -> Ring {
    let one_ring = Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...", };
    one_ring
}

Collection must not outlive the borrowed rings
➡️ needs a lifetime

struct Ring { inscription: &'static str, }

struct Hobbit<'a> {
  ring: &'a Ring,
}

struct RingCollection<'a> {
  rings: Vec<&'a Ring>,
}

fn main() {
    let frodo: Hobbit;
    let saurons_rings: RingCollection;

    {
        let the_one_ring = forge();

        frodo = Hobbit { ring: &the_one_ring, };
        saurons_rings = RingCollection { rings: vec![&the_one_ring], };
    }

    println!("Sauron has {} rings", saurons_rings.rings.len());
    println!("Frodo has the ring {}", frodo.ring.inscription);
}

fn forge() -> Ring {
    let one_ring = Ring { inscription: "...", };
    one_ring
}

16 |         let the_one_ring = forge();
   |             ------------ binding `the_one_ring` declared here
17 |
18 |         frodo = Hobbit { ring: &the_one_ring, };
   |                                ^^^^^^^^^^^^^ borrowed value does not live long enough
19 |         saurons_rings = RingCollection { rings: vec![&the_one_ring], };
20 |     }
   |     - `the_one_ring` dropped here while still borrowed
...
23 |     println!("Frodo has the ring {}", frodo.ring.inscription);
   |                                       ---------------------- borrow later used here

the_one_ring goes out of scope

Example in playground

the_one_ring

frodo

saurons_rings

⚡

Parallel Programming
and Ownership

struct Ring {
	inscription: String,
}

fn forge(rings: &mut Vec<Ring>) {
    rings.push(Ring { inscription: "Narya".to_string() });
    rings.push(Ring { inscription: "Nenya".to_string() });
    rings.push(Ring { inscription: "Vilya".to_string() });
    rings.push(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...".to_string() });
}

Producer

Vector owns rings

struct Ring {
	inscription: String,
}

fn forge(rings: &mut Vec<Ring>) {
    rings.push(Ring { inscription: "Narya".to_string() });
    rings.push(Ring { inscription: "Nenya".to_string() });
    rings.push(Ring { inscription: "Vilya".to_string() });
    rings.push(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...".to_string() });
}

fn distribute(rings: &[Ring]) -> Vec<(&Ring, String)> {
    let mut result = Vec::new();
    for ring in rings {
        result.push((ring, match ring.inscription.as_str() {
            "Narya" => "Gandalf".to_string(),
            "Nenya" => "Galadriel".to_string(),
            "Vilya" => "Elrond".to_string(),
            _ => "Frodo".to_string(),
        }));
    }
    result
}

Consumes rings and produces annotated ones

struct Ring {
	inscription: String,
}

fn forge(rings: &mut Vec<Ring>) {
    rings.push(Ring { inscription: "Narya".to_string() });
    rings.push(Ring { inscription: "Nenya".to_string() });
    rings.push(Ring { inscription: "Vilya".to_string() });
    rings.push(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...".to_string() });
}

fn distribute(rings: &[Ring]) -> Vec<(&Ring, String)> {
    let mut result = Vec::new();
    for ring in rings {
        result.push((ring, match ring.inscription.as_str() {
            "Narya" => "Gandalf".to_string(),
            "Nenya" => "Galadriel".to_string(),
            "Vilya" => "Elrond".to_string(),
            _ => "Frodo".to_string(),
        }));
    }
    result
}

fn count_rings(rings: &[(&Ring, String)], receiver: &'static str) -> usize {
    rings.iter().filter(|(_, name)| name == receiver).count()
}

Consumes annotated rings

fn main() {
    let mut rings = Vec::new();

    forge(&mut rings);
    
    let distributed_rings = distribute(&rings);

    const RECEIVER: &'static str = "Gandalf";
    let number_of_rings = count_rings(&distributed_rings, RECEIVER);
    println!("{RECEIVER} has {number_of_rings} ring(s).");

}

Producer

Producer/consumer

Consumer

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

struct Ring { inscription: String, }

fn forge(tx: mpsc::Sender<Ring>) {

    tx.send(Ring { inscription: "Narya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Nenya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Vilya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...".to_string() }).unwrap();
}

Send takes ownership of ring

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

struct Ring { inscription: String, }

fn forge(tx: mpsc::Sender<Ring>) {
    
    tx.send(Ring { inscription: "Narya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Nenya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Vilya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...".to_string() }).unwrap();
}

fn distribute(rx: mpsc::Receiver<Ring>, tx: mpsc::Sender<(Ring, String)>) {


    for received_ring in rx {
        let receiver = match received_ring.inscription.as_str() {
            "Narya" => "Gandalf".to_string(),
            "Nenya" => "Galadriel".to_string(),
            "Vilya" => "Elrond".to_string(),
            _ => "Frodo".to_string(),
        };
        tx.send((received_ring, receiver)).unwrap();
    }
}

Ring has been moved in memory,
but not string content

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

struct Ring { inscription: String, }

fn forge(tx: mpsc::Sender<Ring>) {
    // Note that send takes ownership of Ring.
    tx.send(Ring { inscription: "Narya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Nenya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Vilya".to_string() }).unwrap();
    tx.send(Ring { inscription: "Ash nazg durbatulûk, ash nazg gimbatul, ...".to_string() }).unwrap();
}

fn distribute(rx: mpsc::Receiver<Ring>, tx: mpsc::Sender<(Ring, String)>) {


    for received_ring in rx {
        let receiver = match received_ring.inscription.as_str() {
            "Narya" => "Gandalf".to_string(),
            "Nenya" => "Galadriel".to_string(),
            "Vilya" => "Elrond".to_string(),
            _ => "Frodo".to_string(),
        };
        tx.send((received_ring, receiver)).unwrap();
    }
}

fn count_rings(rx: mpsc::Receiver<(Ring, String)>, receiver: &'static str) -> usize {
    rx.iter().filter(|(_, name)| name == receiver).count()
}

Consumer

fn main() {
    let (tx_forged, rx_forged) = mpsc::channel();
    let (tx_distributed, rx_distributed) = mpsc::channel();


    thread::spawn(move || forge(tx_forged));
    
    thread::spawn(move || distribute(rx_forged, tx_distributed));

    const RECEIVER: &'static str = "Gandalf";
    let number_of_rings = count_rings(rx_distributed, RECEIVER);
    println!("{RECEIVER} has {number_of_rings} ring(s).");
}

Example in playground

Ownership of sender tx_foged is moved to the thread

Producer:

tx_forged

Consumer/
Producer:

rx_forged
tx_distributed

Consumer:

rx_distributed

Error Handling

Error Handling in Rust

  • Rust does not have exceptions
  • Rust does not have null pointers
    • At least that's true for safe Rust
  • Rust does not have HRESULTs

Rust has a superpower 🦸‍♀️
Enums 

use serde::Deserialize;

#[derive(Debug, Deserialize)]
struct Ring {
    #[allow(dead_code)]
    inscription: String,
    #[allow(dead_code)]
    owner: String,
}

const RING_URL: &str = "https://cddataexchange.blob.core.windows.net/data-exchange/ring.json";

async fn get_rings_rookie() -> Vec<Ring> {
    let rings: Vec<Ring> =
        reqwest::get(RING_URL)
            .await
            .unwrap()
            .json()
            .await
            .unwrap();
    rings
}

#[tokio::main]
async fn main() {
    println!("{:?}", get_rings_rookie().await);
}

Methods that can fail return Result 🔗.
unwrap will panic in case of an error

async fn get_rings_with_panic() -> Vec<Ring> {
    let response = reqwest::get(RING_URL).await;
    if response.is_err() {
        panic!();
    }

    let response = response.unwrap();
    let rings = response.json::<Vec<Ring>>().await;
    if rings.is_err() {
        panic!();
    }

    rings.unwrap()
}

#[tokio::main]
async fn main() {
    println!("{:?}", get_rings_with_panic().await);
}

In case of a bug, we can panic

  • Anticipated runtime error: Handle Result gracefully
  • Unanticipated bug: panic
#[derive(Debug)]
struct RingError;

async fn get_rings_with_error() -> Result<Vec<Ring>, RingError> {
    let response = reqwest::get(RING_URL).await;
    if response.is_err() {
        return Err(RingError);
    }

    let response = response.unwrap();
    let rings = response.json::<Vec<Ring>>().await;
    if rings.is_err() {
        return Err(RingError);
    }

    Ok(rings.unwrap())
}

#[tokio::main]
async fn main() {
    match get_rings_with_error().await {
        Ok(rings) => println!("{:?}", rings),
        Err(_) => println!("Error while retrieving rings"),
    };
}

Error handling with if or match

#[derive(Debug, Deserialize)]
struct Ring {
    #[allow(dead_code)]
    inscription: String,
    #[allow(dead_code)]
    owner: String,
}

#[derive(Debug)]
struct RingError;

impl From<reqwest::Error> for RingError {
    fn from(_: reqwest::Error) -> Self {
        RingError
    }
}

Conversion into error type

#[derive(Debug, Deserialize)]
struct Ring {
    #[allow(dead_code)]
    inscription: String,
    #[allow(dead_code)]
    owner: String,
}

#[derive(Debug)]
struct RingError;

impl From<reqwest::Error> for RingError {
    fn from(_: reqwest::Error) -> Self {
        RingError
    }
}

async fn get_rings_with_better_error() -> Result<Vec<Ring>, RingError> {
    Ok(reqwest::get(RING_URL)
        .await?
        .json()
        .await?)
}

#[tokio::main]
async fn main() {
    match get_rings_with_better_error().await {
        Ok(rings) => println!("{:?}", rings),
        Err(_) => println!("Error while retrieving rings"),
    };
}

Error propagation

Example in playground

⚠️ Two Very Useful Crates ⚠️

Structs and Traits

struct Ring(String);

impl Ring {
    fn new(inscription: &str) -> Self {
        Ring(inscription.to_string())
    }

    fn is_the_one(&self) -> bool {
        self.0.starts_with("Ash nazg durbatulûk, ash nazg gimbatul")
    }
}

Tuple struct

Methods

Instance member

struct Ring(String);

impl Ring {
    fn new(inscription: &str) -> Self {
        Ring(inscription.to_string())
    }

    fn is_the_one(&self) -> bool {
        self.0.starts_with("Ash nazg durbatulûk, ash nazg gimbatul")
    }
}

fn main() {
    let ring = Ring::new("Ash nazg durbatulûk, ash nazg gimbatul, ...");
    if ring.is_the_one() {
        println!("One ring to rule them all");
    } else {
        println!("Not the one");
    }

    let ring = Ring::new("Three Rings for the Elven-kings under the sky, ...");
    if ring.is_the_one() {
        println!("One ring to rule them all");
    } else {
        println!("Not the one");
    }
}

Example in playground

struct Ring(String);

impl Ring { ... }

trait IsTheOne {
    fn is_the_one(&self) -> bool;
}

impl IsTheOne for Ring {
    fn is_the_one(&self) -> bool {
        self.0.starts_with("Ash nazg durbatulûk, ash nazg gimbatul")
    }
}

Trait, somewhat like an interface, but so much more...

Trait implementation

struct Ring(String);

impl Ring { ... }

trait IsTheOne {
    fn is_the_one(&self) -> bool;
}

impl IsTheOne for Ring {
    fn is_the_one(&self) -> bool {
        self.0.starts_with("Ash nazg durbatulûk, ash nazg gimbatul")
    }
}

fn get_a_ring() -> impl IsTheOne {
    let mut rng = rand::thread_rng();
    if rng.gen() {
        Ring::new("Ash nazg durbatulûk, ash nazg gimbatul, ...")
    } else {
        Ring::new("Three Rings for the Elven-kings under the sky, ...")
    }
}

fn main() {
    let ring = get_a_ring();
    if ring.is_the_one() {
        println!("One ring to rule them all");
    } else {
        println!("Not the one");
    }
}

All return values must have same type

Example in playground

use rand::Rng;

struct Ring(String);

trait IsTheOne {
    fn is_the_one(&self) -> bool;
}

struct SignificantOther(String);

impl SignificantOther {
    fn new(name: &str) -> Self {
        SignificantOther(name.to_string())
    }
}

impl IsTheOne for SignificantOther {
    fn is_the_one(&self) -> bool {
        self.0 == "Karin"
    }
}

Another struct that implements IsTheOne

use rand::Rng;

struct Ring(String);

trait IsTheOne {
    fn is_the_one(&self) -> bool;
}

struct SignificantOther(String);

impl SignificantOther {
    fn new(name: &str) -> Self {
        SignificantOther(name.to_string())
    }
}

impl IsTheOne for SignificantOther {
    fn is_the_one(&self) -> bool {
        self.0 == "Karin"
    }
}

fn get_something_precious() -> Box<dyn IsTheOne> {
    let mut rng = rand::thread_rng();
    if rng.gen() {
        Box::new(Ring::new("Ash nazg durbatulûk, ash nazg gimbatul, ..."))
    } else {
        Box::new(SignificantOther::new("Karin"))
    }
}

Trait object

Return values do not need to have same type

fn main() {
    let precious = get_something_precious();
    
    if precious.is_the_one() {
        println!("I found someone or something really precious");
    } else {
        println!("Not the one");
    }

    let precious_things = vec![
        Box::new(Ring::new("Ash nazg durbatulûk, ash nazg gimbatul, ...")) as Box<dyn IsTheOne>,
        Box::new(SignificantOther::new("Karin")),
    ];
    for precious in precious_things {
        if precious.is_the_one() {
            println!("I found someone or something really precious");
        } else {
            println!("Not the one");
        }
    }
}

Dynamic method dispatching

Example in playground

There is so much more to say and learn... 🔗

Q&A

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some_highlights_about_rust

By Rainer Stropek

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