Rust - Initial Impressions


Robot Crab

As part of my New Year’s resolution I’ve been studying the Rust programming language with the ultimate goal of building a bot for the game StarCraft (hence the robot Mr. Krabs, since the Rust mascot is a crab and my end goal is to build a bot with it).

I’ve been reading through the Rust Programming Language Guide and uploading my notes and intermediate projects here.

So far the language syntactically (if not semantically) is very similar to Swift. You can use if let statements to unwrap options, just like in Swift:

let config_max = Some(3u8);
if let Some(max) = config_max {
    println!("The maximum is configured to be {}", max);
} else {
    println!("The maximum is null!");

Rust also has Swift-style parameters and return type syntax, i.e.:

fn plus_one(x: i32) -> i32

Rust ranges and iterators also use the same for loop syntax as Swift:

let mut sum = 0;
for n in 1..11 {
    sum += n;

// Or

let menu = &["apple", "cake", "coffee"];

for item in menu  {
    println!("I want{}.", item);

Finally, Rust has a struct keyword like Swift:

struct Rectangle {
    width: u32,
    height: u32,

impl Rectangle {
    // Borrows immutable self w/reference; we don't want to write, just read.
    fn area(&self) -> u32 { // if you need to write, use `&mut self`
        self.width * self.height

Unlike Swift, structs are either entirely mutable or immutable. They can’t contain functions and instead must declare them in an impl block as demonstrated above.

The semantics of Rust are actually much more similar to the first programming language I learned, Ada. Memory and null safety are critical concepts in Rust and the language is designed to ensure that most memory and null-pointer exceptions are caught at compile time. Memory is managed through ownership.

There are two possible storage mechanisms:

The ownership rules in Rust are:

  1. Each value has an owner
  2. There can only be one owner at a time
  3. When an owner goes out of scope, the value is dropped

The last rule especially caught me by surprise. Unlike Kotlin or Swift, passing a heap pointer to a function will cause the variable to invalidate afterwards!

There are a number of solutions to keep the variable in scope afterwards:

  1. Make a clone first (slow).
  2. Have the fn return the pointer in a tuple at the end (ugly).
  3. Pass a reference type.

Passing a reference is like letting an inner scope “borrow” a pointer. Reference types are created by prepending the parameter type and heap variable with &. You can explicitly de-reference with *. References are immutable by default. They can be made mutable with &mut, but a mutable reference can only have one borrower. You’ll get compile error otherwise.

Here are the usage of these symbols:

i32          // a pointer
&i32         // a reference
&'a i32      // a reference with an explicit lifetime
&'a mut i32  // a mutable reference with an explicit lifetime
&'static i32 // a static explicit lifetime

These notes only scratch the surface of Rust. I haven’t even gotten to package management (cargo), closures (very similar to Swift), multithreading, or data passing. My hope is that by highlighting the similarities and differences Rust has with other high-level programming languages you can get an idea of whether or not Rust is right for you.

Until next time!

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