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Introduction

Rust is a new language that already has good textbooks. However, sometimes the textbooks are difficult to understand because they are written for native English speakers. Many companies and people in other countries now want to learn Rust, and could learn faster with a textbook that uses easy English. This textbook is for these companies and people to learn Rust without difficult English.

Rust Playground

Maybe you don't want to install Rust yet, and that's fine. You can just go to https://play.rust-lang.org/ and start writing Rust. You can write your code and click Run to see the results fast.

Here are some tips when using the Rust Playground:

Run your code with Run
Change Debug to Release if you want your code to be faster. Debug: compiles faster, runs slower, contains debug information. Release: compiles slower, runs much faster, removes debug information.
Click on Share to get a url. You can use that to share your code if you want help.
Tools: Rustfmt will format your code nicely.
Tools: Clippy will give you extra information about how to make your code better.
Config: here you can change your theme to dark mode, and many other configurations.

If you want to install Rust, go here https://www.rust-lang.org/tools/install and follow the instructions. Usually you will use rustup to install and update Rust.

Primitive types

Rust has simple types that are called primitive types. We will start with integers. Integers are whole numbers with no decimal point. There are two types of integers:

Signed integers,
Unsigned integers.

Signs are + and -, so signed integers can be positive or negative (e.g. +8, -8). But unsigned integers can only be positive, because they do not have a sign.

The signed integers are: i8, i16, i32, i64, i128, and isize. The unsigned integers are: u8, u16, u64, u128, and usize.

The number after the i or the u means the number of bits for the number, so numbers with more bits can be larger. 8 bits = one byte, so i8 is one byte, i64 is 8 bytes, and so on. Number types with larger sizes can hold larger numbers. For example, a u8 can hold up to 255, but a u16 can hold up to 65535. And a u128 can hold up to 340282366920938463463374607431768211455.

So what is isize and usize? This means the number of bits on your type of computer. (This is called the architecture of your computer) So isize and usize on a 32-bit computer is like i32 and u32, and isize and usize on a 64-bit computer is like i64 and u64.

There are many uses for the different types of integers. One use is computer performance: a smaller number of bytes is faster to process. But here are some other uses:

Characters in Rust are called char. u8 numbers only go up to 255, and Unicode and ASCII are the same for these numbers. This means that Rust can safely cast a u8 into a char, using as. (Cast u8 as char means "pretend u8 is a char")

Casting with as is useful because Rust always needs to know the type of the integer. For example, this will not compile:

fn main() {
    let my_number = 100; // We didn't write a type of integer, 
                         // so Rust chooses i32. Rust always
			 // chooses i32 for integers if you don't
			 // tell it to use a different type
    println!("{}", my_number as char);

}

Here is the reason:

error[E0604]: only `u8` can be cast as `char`, not `i32`
 --> src\main.rs:3:20
  |
3 |     println!("{}", my_number as char);
  |                    ^^^^^^^^^^^^^^^^^

One easy way to fix this is with as. First we use as to make my_number a u8, then one more as to make it a char. Now it will compile:

fn main() {
    let my_number = 100;
    println!("{}", my_number as u8 as char);
}

Chars

A char is one character. For a char, use '' instead of "".

All chars are 4 bytes. They are 4 bytes because some characters in a string are more than one byte. For example:

fn main() {
    let slice = "Hello!";
    println!("Slice is {:?} bytes.", std::mem::size_of_val(slice));
    let slice2 = "안녕!"; // Korean for "hi"
    println!("Slice2 is {:?} bytes.", std::mem::size_of_val(slice2));
}

slice is six characters in length and six bytes, but slice2 is three characters in length and seven bytes. char needs to fit any character in any language, so it is 4 bytes long.

Type inference

Type inference means that if you don't tell the compiler the type, but it can decide by itself, it will decide. The compiler always needs to know the type of the variables, but you don’t always need to tell it. For example, for let my_number = 8, my_number will be an i32 (the compiler chooses i32 for integers if you don't tell it). But if you say let my_number: u8 = 8, it will make my_number a u8, because you told it u8.

Sometimes you need to tell the compiler, for two reasons:

You are doing something very complex and the compiler doesn't know the type you want,
You want a different type (for example, you want an i128, not an i32)

To specify a type, add a colon after the variable name.

fn main() {
    let small_number: u8 = 10;
}

For numbers, you can specify a type after the number.

fn main() {
    let small_number = 10u8;
}

You can also add _ if you want to make the number easy to read.

fn main() {
    let small_number = 10_u8;
    let big_number = 100_000_000_i32;
}

The _ does not change the number. It is only to make it easy for you to read. And it doesn't matter how many _ you use:

fn main() {
    let number = 0________u8;
    let number2 = 1___6______2____4______i32;
    println!("{}, {}", number, number2);
}

This prints 0, 1624.

Floats

Floats are numbers with decimal points. 5.5 is a float, and 6 is an integer. 5.0 is also a float, and even 5. is a float.

fn main() {
    let my_float = 5.; // Rust sees . and knows that it is a float
}

But the types are not called float, they are called f32 and f64. It is the same as integers: the number after f shows the number of bits. If you don't write the type, Rust will choose f64.

Of course, only floats of the same type can be used together.

fn main() {
    let my_float: f64 = 5.0; // This is an f64
    let my_other_float: f32 = 8.5; // This is an f32

    let third_float = my_float + my_other_float;
}

When you try to run this, Rust will say:

error[E0308]: mismatched types
 --> src\main.rs:5:34
  |
5 |     let third_float = my_float + my_other_float;
  |                                  ^^^^^^^^^^^^^^ expected `f64`, found `f32`

The compiler often writes '''expected (type), found (type)''' when you use the wrong type. This means:

let my_float: f64 = 5.0; // The compiler sees an f64
let my_other_float: f32 = 8.5; // The compiler sees an f32
let third_float = my_float + // The compiler sees a new variable. It is going to be f64 plus another f64. Now it expects an f64...
let third_float = my_float + my_other_float; // But it found an f32.

So when you see "expected (type), found (type)", you must find why the compiler expected a different type.

Of course, with simple numbers it is easy to fix. You can cast the f32 to an f64:

fn main() {
    let my_float: f64 = 5.0;
    let my_other_float: f32 = 8.5;

    let third_float = my_float + my_other_float as f64; // my_other_float as f64 = use it like an f64
}

Or even more simply, remove the type declarations:

fn main() {
    let my_float = 5.0; // Rust will choose f64
    let my_other_float = 8.5; // Here again it will choose f64

    let third_float = my_float + my_other_float;
}

The Rust compiler is smart and will not choose f64 if you need f32:

fn main() {
    let my_float: f32 = 5.0;
    let my_other_float = 8.5; // Rust will choose f32, because it knows you need to add it to an f32

    let third_float = my_float + my_other_float;
}

Printing hello, world!

A new Rust program always starts with this:

fn main() {
    println!("Hello, world!");
}

fn means function,
main is the function that starts the program,
() means that we didn't give the function anything to start.

{} is a code block.

println! is a macro that prints to the console. A macro is like a function, but more complicated. Macros have a ! after them. We will learn about making macros later. For now, please remember that ! means that it is a macro.

To learn about the ;, we will create another function. First, in main we will print a number 8:

fn main() {
    println!("Hello, world number {}!", 8);
}

The {} in println! means "put the variable inside here". This prints Hello world number 8.

We can put more in:

fn main() {
    println!("Hello, worlds number {} and {}!", 8, 9);
}

This prints Hello, worlds number 8 and 9!.

Now let's create the function.

fn main() {
    println!("Hello, world number {}", number());
}

fn number() -> i32 {
    8
}

This also prints Hello, world number 8!. When Rust looks at number() it sees a function. This function:

Does not take anything (because it has ())
Returns an i32. The -> (called a "skinny arrow") shows what the function returns.

Inside the function is just 8. Because there is no ;, this is the value it returns. If it had a ;, it would not return anything. Rust will not compile if it has a ;:

fn main() {
    println!("Hello, world number {}", number());
}

fn number() -> i32 {
    8;
}

5 | fn number() -> i32 {
  |    ------      ^^^ expected `i32`, found `()`
  |    |
  |    implicitly returns `()` as its body has no tail or `return` expression
6 |     8;
  |      - help: consider removing this semicolon

This means "you told me that number() returns an i32, but you added a ; so it doesn't return anything". So the compiler suggests removing the semicolon.

When you want to give variables to a function, put them inside the (). You have to give them name and write the type.

fn main() {
    multiply(8, 9); // We can give the numbers directly
    let some_number = 10; // Or we can declare two variables
    let some_other_number = 2;
    multiply(some_number, some_other_number); // and put them in the function
}

fn multiply(number_one: i32, number_two: i32) { // Two i32s will enter the function. We will call them number_one and number_two.
    let result = number_one * number_two;
    println!("{} times {} is {}", number_one, number_two, result);
}

Of course, we can also return an i32:

fn main() {
    let multiply_result = multiply(8, 9); // We used multiply() to print and to give the result to multiply_result
}

fn multiply(number_one: i32, number_two: i32) -> i32 {
    let result = number_one * number_two;
    println!("{} times {} is {}", number_one, number_two, result);
    result
}

Declaring variables and code blocks

Use let to declare a variable (declare a variable = tell Rust to make a variable).

fn main() {
    let my_number = 8;
    println!("Hello, number {}", my_number);
}

Variables start and end inside a code block {}. In this example, my_number ends before we call println!, because it is inside its own code block.

fn main() {
    {
        let my_number = 8; // my_number starts here
                           // my_number ends here!
    }
    
    println!("Hello, number {}", my_number); // there is no my_number and
                                             // println!() can't find it
}

You can use a code block to return a value:

fn main() {
    let my_number = {
	let second_number = 8;
        second_number + 9 // No semicolon, so the code block returns 8 + 9
    };

    println!("My number is: {}", my_number);
}

If you add a semicolon inside the block, it will return () (nothing):

fn main() {
    let my_number = {
	let second_number = 8; // declare second_number,
        second_number + 9; // add 9 to second_number
		           // but we didn't return it!
                           // second_number dies now
    };

    println!("My number is: {:?}", my_number);
}

So why did we write {:?} and not {}? We will talk about that now.

Display and debug

Simple variables in Rust can be printed with {} inside println!(). But some variables can't, and you need to debug print. Debug print is printing for the programmer, because it usually shows more information.

How do you know if you need {:?} and not {}? The compiler will tell you. For example:

fn main() {
    let doesnt_print = ();
    println!("This will not print: {}", doesnt_print);
}

When we run this, the compiler says:

error[E0277]: `()` doesn't implement `std::fmt::Display`
 --> src\main.rs:3:41
  |
3 |     println!("This will not print: {}", doesnt_print);
  |                                         ^^^^^^^^^^^^ `()` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `()`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: required by `std::fmt::Display::fmt`
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

This is a lot of information. But the important part is: you may be able to use `{:?}` (or {:#?} for pretty-print) instead. This means that you can try {:?}, and also {:#?} ({:#?} prints with different formatting).

So Display means printing with {}, and Debug means printing with {:?}.

One more thing: you can also use print!() without ln if you don't want a new line.

fn main() {
    print!("This will not print a new line");
    println!(" so this will be on the same line");
}

This prints This will not print a new line so this will be on the same line.

Smallest and largest numbers

If you want to see the smallest and biggest numbers, you can use MIN and MAX.

fn main() {
    println!("The smallest i8 is {} and the biggest i8 is {}.", std::i8::MIN, std::i8::MAX);
    println!("The smallest u8 is {} and the biggest u8 is {}.", std::u8::MIN, std::u8::MAX);
    println!("The smallest i16 is {} and the biggest i16 is {}.", std::i16::MIN, std::i16::MAX);
    println!("The smallest u16 is {} and the biggest u16 is {}.", std::u16::MIN, std::u16::MAX);
    println!("The smallest i32 is {} and the biggest i32 is {}.", std::i32::MIN, std::i32::MAX);
    println!("The smallest u32 is {} and the biggest u32 is {}.", std::u32::MIN, std::u32::MAX);
    println!("The smallest i64 is {} and the biggest i64 is {}.", std::i64::MIN, std::i64::MAX);
    println!("The smallest u64 is {} and the biggest u64 is {}.", std::u64::MIN, std::u64::MAX);
    println!("The smallest i128 is {} and the biggest i128 is {}.", std::i128::MIN, std::i128::MAX);
    println!("The smallest u128 is {} and the biggest u128 is {}.", std::u128::MIN, std::u128::MAX);

}

This will print:

The smallest i8 is -128 and the biggest i8 is 127.
The smallest u8 is 0 and the biggest u8 is 255.
The smallest i16 is -32768 and the biggest i16 is 32767.
The smallest u16 is 0 and the biggest u16 is 65535.
The smallest i32 is -2147483648 and the biggest i32 is 2147483647.
The smallest u32 is 0 and the biggest u32 is 4294967295.
The smallest i64 is -9223372036854775808 and the biggest i64 is 9223372036854775807.
The smallest u64 is 0 and the biggest u64 is 18446744073709551615.
The smallest i128 is -170141183460469231731687303715884105728 and the biggest i128 is 170141183460469231731687303715884105727.  
The smallest u128 is 0 and the biggest u128 is 340282366920938463463374607431768211455.

Mutability (changing)

When you declare a variable with let, it is immutable (cannot be changed).

This will not work:

fn main() {
    let my_number = 8;
    my_number = 10;
}

The compiler says: error[E0384]: cannot assign twice to immutable variable `my_number. This is because variables are immutable if you only write let.

To change a variable, add mut:

fn main() {
    let mut my_number = 8;
    my_number = 10;
}

Now there is no problem.

However, you cannot change the type even with mut. This will not work:

fn main() {
    let mut my_variable = 8;
    my_variable = "Hello, world!";
}

You will see the same "expected" message from the compiler: expected integer, found `&str````. &str``` is a string type that we will learn soon.

Shadowing

Shadowing means using let to declare a new variable with the same name as another variable. It looks like mutability, but it is completely different. Shadowing looks like this:

fn main() {
    let my_number = 8; // This is an i32
    println!("{}", my_number); // prints 8
    let my_number = 9.2; // This is an f64. It is not my_number - it is completely different!
    println!("{}", my_number) // Prints 9.2
}

Here we say that we "shadowed" my_number with a new "let binding".

So is the first my_number destroyed? No, but when we call my_number we now get my_number the f64. And because they are in the same scope block, we can't see the first my_number anymore.

But if they are in different blocks, we can see both. For example:

So when you shadow a variable, you don't destroy it. You block it.

fn main() {
    let my_number = 8; // This is an i32
    println!("{}", my_number); // prints 8
    {
        let my_number = 9.2; // This is an f64. It is not my_number - it is completely different!
        println!("{}", my_number) // Prints 9.2
                                  // But the shadowed my_number only lives until here.
                                  // The first my_number is still alive!
    }
    println!("{}", my_number); // prints 8
}

So what is the advantage of shadowing? Shadowing is good when you need to change a variable a lot.

fn main() {
    let final_number = {
        let y = 10;
        let x = 9; // x starts at 9
        let x = times_two(x); // shadow with new x: 18
        let x = x + y; // shadow with new x: 28
        x // return x: final_number is now the value of x
    };
    println!("The number is now: {}", final_number)
}

fn times_two(number: i32) -> i32 {
    number * 2
}

Without shadowing you would have to think of different names, even though you don't care about x:

fn main() {
	// Pretending we are using Rust without shadowing
    let final_number = {
        let y = 10;
        let x = 9; // x starts at 9
        let x_twice = times_two(x); // second name for x
        let x_twice_and_y = x + y; // third name for x
        x_twice_and_y // too bad we didn't have shadowing - we could have just used x
    };
    println!("The number is now: {}", final_number)
}

fn times_two(number: i32) -> i32 {
    number * 2
}

The stack, the heap, and pointers

The stack, the heap, and pointers are very important in Rust.

The stack and the heap are two places to keep memory. The important differences are:

The stack is very fast, but the heap is not so fast.
The stack needs to know the size of a variable at compile time. So simple variables like i32 go on the stack, because we know their exact size.
Some types don't know the size at compile time. But the stack needs to know the exact size. What do you do? You put the data in the heap, because the heap can have any size of data. And to find it, a pointer goes on the stack, because we always know the size of a pointer.

A pointer is like a table of contents in a book.

MY BOOK

Chapter                        Page
Chapter 1: My life              1
Chapter 2: My cat               15
Chapter 3: My job               23
Chapter 4: My family            30
Chapter 5: Future plans         43

So this is like five pointers. Where is the chapter "My life"? It's on page 1 (it points to page 1). Where is the chapter "My job?" It's on page 23.

A pointer in Rust is usually called a reference. A reference means you borrow the value, but you don't own it. In Rust, references have a &. So let my_variable = 8 makes a regular variable, but let my_reference = &my_variable makes a reference. This means that my_reference is only looking at the data of my_variable. my_variable still owns its data.

More about printing

We know that println! can print with {} (for Display) and {:?} (for Debug), plus {:#?} for pretty printing. But there are many other ways to print.

For example, if you have a reference, you can use {:p} to print the pointer address.

fn main() {
    let number = 9;
    let number_ref = &number;
    println!("{:p}", number_ref);
}

This prints 0xe2bc0ffcfc or some other address (it can be different every time).

Or you can print binary, hexadecimal and octal:

fn main() {
    let number = 555;
    println!("Binary: {:b}, hexadecimal: {:x}, octal: {:o}", number, number, number);
}

Or you can add numbers to change the order:

fn main() {
    let father_name = "Vlad";
    let son_name = "Adrian Fahrenheit";
    let family_name = "Țepeș";
    println!("This is {1} {2}, son of {0} {2}.", father_name, son_name, family_name);
}

father_name is in position 0, son_name is in position 1, and family_name is in position 2. So it prints This is Adrian Fahrenheit Țepeș, son of Vlad Țepeș.

Maybe you have a very complex string to print and want to add names to the {}. You can do that:

fn main() {
    println!("{city1} is in {country} and {city2} is also in {country},
but {city3} is not in {country}.", city1 = "Seoul", city2 = "Busan", city3 = "Tokyo", country = "Korea");
}

Very complex printing is not used too much in Rust. But here is how to do it.

{variable:padding alignment minimum.maximum}

Do you want a variable name? (Then add a :)
Do you want a padding character?
What alignment (left / middle / right) or the padding?
Do you want a minimum length? (just write a number)
Do you want a maximum length? (write a number with a . in front)

For example, if I want to write "a" with five ㅎ characters on the left and five ㅎ characters on the right:

fn main() {
    let letter = "a";
    println!("{:ㅎ^11}", letter);
}

This prints ㅎㅎㅎㅎㅎaㅎㅎㅎㅎㅎ. Let's look at 1) to 5) for this.

Do you want a variable name? {:ㅎ^11} No variable name: it comes before :.
Do you want a padding character? {:ㅎ^11} Yes. ㅎ comes after the : and has a ^. < means padding with the character on the left, > means on the right, and ^ means in the middle.
Do you want a minimum length? {:ㅎ^11} Yes: there is an 11 after.
Do you want a maximum length? {:ㅎ^11} No: there is no number with a . before.

Here is an example of many types of formatting.

fn main() {
    let title = "TODAY'S NEWS";
    println!("{:-^30}", title); // no variable name, pad with -, put in centre, 30 characters long
    let bar = "|";
    println!("{: <15}{: >15}", bar, bar); // no variable name, pad with space, 15 characters each, one to the left, one to the right
    let a = "SEOUL";
    let b = "TOKYO";
    println!("{city1:-<15}{city2:->15}", city1 = a, city2 = b); // variable names city1 and city2, pad with -, one to the left, one to the right
}

It prints:

---------TODAY'S NEWS---------
|                            |
SEOUL--------------------TOKYO

Strings

Rust has two main types of strings: String and &str. What is the difference?

&str is a simple string. When you write let my_variable = "Hello, world!", you create a &str. A &str is very fast.
String is a more complicated string. It is a bit slower, and has more functions. A String is a pointer, with data on the heap.

Both &str and String are UTF-8. For example, you can write:

fn main() {
    let name = "서태지"; // This is a Korean name. No problem, because a &str is UTF-8.
    let other_name = String::from("Adrian Fahrenheit Țepeș"); // Ț and ș are no problem in UTF-8.
}

You can even write emojis thanks to UTF-8.

fn main() {
    let name = "😂";
    println!("My name is actually {}", name);   
}

So why do we need a & in front of str, but not for String?

str is a dynamically sized type (dynamically sized = the size can be different). For example, the names "서태지" and "Adrian Fahrenheit Țepeș" are not the same size on the stack:

fn main() {

    println!("A String is always {:?} bytes. It is Sized.", std::mem::size_of::<String>()); // std::mem::size_of::<Type>() gives you the size in bytes of a type
    println!("And an i8 is always {:?} bytes. It is Sized.", std::mem::size_of::<i8>());
    println!("And an f64 is always {:?} bytes. It is Sized.", std::mem::size_of::<f64>());
    println!("But a &str? It can be anything. '서태지' is {:?} bytes. It is not Sized.", std::mem::size_of_val("서태지")); // std::mem::size_of_val() gives you the size in bytes of a variable
    println!("And 'Adrian Fahrenheit Țepeș' is {:?} bytes. It is not Sized.", std::mem::size_of_val("Adrian Fahrenheit Țepeș"));
    
}

That is why we need a &, because & makes a pointer, and Rust knows the size of the pointer. So the pointer goes on the stack. If we wrote str, Rust wouldn't know what to do because it doesn't know the size.

More on references

References are very important in Rust. Rust uses references to make sure that all memory access is safe. We know that we use & to create a reference:

fn main() {
    let country = String::from("Austria");
    let ref_one = &country;
    let ref_two = &country;

    println!("{}", ref_one);
}

country is a String. We created two references to country. They have the type &String: a "reference to a String". We could create one hundred references to country and it would be no problem.

But this is a problem:

fn main() {
    let country = return_str();
}

fn return_str() -> &str {
    let country = String::from("Austria");
    let country_ref = &country;
    country_ref
}

The function return_str() creates a String, then it creates a reference to the string. Then it tries to return the reference. But country only lives inside the function. So after the function is over, country_ref is referring to memory that is already gone. Rust prevents us from making a mistake with memory.

Mutable references

If you want to use a reference to change data, you can use a mutable reference. For a mutable reference, you write &mut.

fn main() {
    let mut my_number = 8; // don't forget to write mut here!
    let num_ref = &mut my_number;
}

So what are the two types? my_number is an i32, and num_ref is &mut i32 (a "mutable reference to an i32).

So let's use it to add 10 to my_number. But you can't write num_ref += 10, because num_ref is not the i32 value. To reach the value, we use *. * means "I don't want the reference, I want the value behind the reference". In other words, one * erases one &.

fn main() {
    let mut my_number = 8; 
    let num_ref = &mut my_number;
    *num_ref += 10; // Use * to change the i32 value.
    println!("{}", my_number);
}

Because using & is called "referencing", using * is called "dereferencing".

Rust has rules for mutable and immutable references.

Rule 1: If you have only immutable references, you can have as many as you want. Rule 2: If you have a mutable references, you can only have one. You can't have an immutable reference and a mutable reference.

This is because mutable references can change the data. You could get problems if you change the data when other references are reading it.

A good way to understand is to think of a Powerpoint presentation.

Situation one is about only one mutable reference.

Situation one: An employee is writing a Powerpoint presentation. He wants his manager to help him. The employee gives his login information to his manager, and asks him to help by making edits. Now the manager has a "mutable reference" to the employee's presentation. This is fine, because nobody else is looking at the presentation.

Situation two is about only immutable references.

Situation two: The employee is giving the presentation to 100 people. All 100 people can now see the employee's data. This is fine, because nobody can change the data.

Situation three is the problem situation.

Situation three: The Employee his manager his login information. Then the employee went to give the presentation to 100 people. This is not fine, because the manager can log in and do anything. Maybe his manager will log into the computer and type an email to his mother? Now the 100 people have to watch the manager write an email to his mother instead of the presentation. That's not what they expected to see.

Here is an example of a mutable borrow with an immutable borrow:

fn main() {
    let mut number = 10;
    let number_ref = &number; 
    let number_change = &mut number; 
    *number_change += 10;
    println!("{}", number_ref); 
}

The compiler prints a helpful message to show us the problem.

error[E0502]: cannot borrow `number` as mutable because it is also borrowed as immutable
 --> src\main.rs:4:25
  |
3 |     let number_ref = &number;
  |                      ------- immutable borrow occurs here
4 |     let number_change = &mut number;
  |                         ^^^^^^^^^^^ mutable borrow occurs here
5 |     *number_change += 10;
6 |     println!("{}", number_ref);
  |                    ---------- immutable borrow later used here

However, this code will work. Why?

fn main() {
    let mut number = 10;
    let number_change = &mut number; // create a mutable reference
    *number_change += 10; // use mutable reference to add 10
    let number_ref = &number; // create an immutable reference
    println!("{}", number_ref); // print the immutable reference
}

The compiler knows that we used number_change to change number, but didn't use it again. So here there is no problem. We are not using immutable and mutable references together.

Shadowing again

Remember when we said that shadowing doesn't destroy a value but blocks it? Now we can use references to see this.

fn main() {
    let country = String::from("Austria");
    let country_ref = &country;
    let country = 8;
    println!("{}, {}", country_ref, country);

}

Does this print Austria, 8 or 8, 8? It prints Austria, 8. First we declare a String called country. Then we create a reference country_ref to this string. Then we shadow country with 8, which is an i32. But the first country was not destroyed, so country_ref still says "Austria", not "8".

Giving references to functions

References are very useful for functions. The rule in Rust on variables is: a variable can only have one owner.

This code will not work:

fn main() {
    let country = String::from("Austria");
    print_country(country); // We print "Austria"
    print_country(country); // That was fun, let's do it again!
}

fn print_country(country_name: String) {
    println!("{}", country_name);
}

It does not work because country is destroyed. Here's how:

Step 1: We create the String country. country is the owner.
Step 2: We give country to print_country. print_country doesn't have an ->, so it doesn't return anything. After print_country finishes, our String is now dead.
Step 3: We try to give country to print_country, but we already did that. We don't have country to give anymore.

We can fix this by adding &.

fn main() {
    let country = String::from("Austria");
    print_country(&country); // We print "Austria"
    print_country(&country); // That was fun, let's do it again!
}

fn print_country(country_name: &String) {
    println!("{}", country_name);
}

Now print_country() is a function that takes a reference to a String: a &String. Also, we give it a reference to country by writing &country. This says "you can look at it, but I will keep it".

Here is an example of a function that uses a mutable variable.

fn main() {
    let mut country = String::from("Austria");
    add_hungary(&mut country);
}

fn add_hungary(country_name: &mut String) {
    country_name.push_str("-Hungary"); // push_str() adds a &str to a String
    println!("Now it says: {}", country_name);
}

So to conclude:

fn function_name(variable: String) takes a String and owns it. If it doesn't return anything, then the variable dies after the function is done.
fn function_name(variable: &String) borrows a String and can look at it
fn function_name(variable: &mut String) borrows a String and can change it

Here is an example that looks like a mutable reference, but it is different.

fn main() {
    let country = String::from("Austria"); // country is not mutable
    adds_hungary(country);
}

fn adds_hungary(mut country: String) { // but adds_hungary takes the string and it is mutable!
    country.push_str("-Hungary");
    println!("{}", country);
}

How is this possible? It is because mut hungary is not a reference: adds_hungary owns country now. (Remember, it takes String and not &String). adds_hungary is the full owner, so it can take country as mutable.

Copy types

Some types in Rust are very simple. They are called copy types. These simple types are all on the stack, and the compiler knows their size. That means that they are very easy to copy, so the compiler always copies when you send it to a function. So you don't need to worry about ownership.

These simple types include: integers, floats, booleans (true and false), and char.

How do you know if a type implements copy? (implements = can use) You can check the documentation. For example, here is the documentation for char:

https://doc.rust-lang.org/std/primitive.char.html

On the left you can see Trait Implementations. You can see for example Copy, Debug, and Display. So you know that a char:

is copied when you send it to a function (Copy)
can use {} to print (Display)
can use {:?} to print (Debug)

fn main() {
    let my_number = 8;
    prints_number(my_number); // Prints 8. prints_number gets a copy of my_number
    prints_number(my_number); // Prints 8 again.
                              // No problem, because my_number is copy type!
}

prints_number(number: i32) { // No return with ->
                             // If number was not copy type, it would take it
                             // and we couldn't use it again
    println!("{}", number);
}

But if you look at the documentation for String, it is not copy type.

https://doc.rust-lang.org/std/string/struct.String.html

On the left in Trait Implementations you can look in alphabetical order. A, B, C... there is no Copy in C. But there is Clone. Clone is similar to Copy, but needs more memory.

In this example, prints_country() prints the country name, a String. We want to print it two times, but we can't:

fn main() {
    let country = String::from("Kiribati");
    prints_country(country);
    prints_country(country);
}

fn prints_country(country_name: String) {
    println!("{}", country_name);
}

But now we understand the message.

error[E0382]: use of moved value: `country`
 --> src\main.rs:4:20
  |
2 |     let country = String::from("Kiribati");
  |         ------- move occurs because `country` has type `std::string::String`, which does not implement the `Copy` trait
3 |     prints_country(country);
  |                    ------- value moved here
4 |     prints_country(country);
  |                    ^^^^^^^ value used here after move

The important part is which does not implement the `Copy` trait. But in the documentation we saw that String implements the Clone trait. So we can add .clone(). This creates a clone, and we send the clone to the function. country is still alive, so we can use it.

fn main() {
    let country = String::from("Kiribati");
    prints_country(country.clone()); // make a clone and give it to the function
    prints_country(country);
}

fn prints_country(country_name: String) {
    println!("{}", country_name);
}

Of course, if the String is very large, .clone() can use a lot of memory. For example, one String can be a whole book, and every time we call .clone() it will copy the book. So using & for a reference is faster, if you can.

fn main() {
    let mut country = String::from("Kiribati"); // country is mutable
    let country_ref = &country; // country_ref needs a reference
    changes_country(&mut country); // changes_country needs a &mut ref
    println!("{}", country_ref); // immutable and mutable borrow together

}

fn prints_country(country_name: String) {
    println!("{}", country_name);
}

fn changes_country(country_name: &mut String) {
    country_name.push_str(" is a country");
    println!("{}", country_name);
}

Maybe we can change the order, but we can also just .clone().

fn main() {
    let mut country = String::from("Kiribati"); // country is mutable
    let country_ref = &country; // country_ref needs a reference
    changes_country(&mut country.clone()); // give changes_country a clone instead
    println!("{}", country_ref); // now the code works

}

fn prints_country(country_name: String) {
    println!("{}", country_name);
}

fn changes_country(country_name: &mut String) {
    country_name.push_str(" is a country");
    println!("{}", country_name);
}

Collection types

Here are some types for making a collection.

Arrays

An array is data inside square brackets: []. Arrays:

must not change their size,
must only contain the same type.

They are very fast, however.

The type of an array is: [type; number]. For example, the type of ["One", "Two"] is [&std; 2]. This means that even these two arrays have different types:

let array1 = ["One", "Two"];
let array["One", "Two", "Five"];

A good tip: to know the type of a variable, you can "ask" the compiler by giving it bad instructions. For example:

fn main() {
    let seasons = ["Spring", "Summer", "Autumn", "Winter"];
    let seasons2 = ["Spring", "Summer", "Fall", "Autumn", "Winter"];
    let () = seasons; // This will make an error
    let () = seasons2; // This will also make an error
}

The compiler says "seasons isn't type () and seasons2 isn't type () either!" as you can see:

error[E0308]: mismatched types
 --> src\main.rs:4:9
  |
4 |     let () = seasons;     
  |         ^^   ------- this expression has type `[&str; 4]`
  |         |
  |         expected array `[&str; 4]`, found `()`

error[E0308]: mismatched types
 --> src\main.rs:5:9
  |
5 |     let () = seasons2;
  |         ^^   -------- this expression has type `[&str; 5]`
  |         |
  |         expected array `[&str; 5]`, found `()`

If you want an array with all the same value, you can declare it like this:

fn main() {
    let my_array = ["a"; 20];
    println!("{:?}", my_array);
}

This prints ["a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a", "a"].

This method is used a lot to create buffers. For example, let mut buffer = [0; 640] creates an array of 640 zeroes. Then we can change zero to other numbers in order to add data.

You can index (find) entries in an array with []. The first entry is [0], the second is [1], and so on.

fn main() { 
    let my_numbers = [0, 10, -20];
    println!("{}", my_numbers[1]); // prints 10
}

You can get a slice (a piece) of an array. First you need a &, because the compiler doesn't know the size. Then you can use .. to show the range.

For example, let's use this array: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].

fn main() {
    let array_of_ten = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    let three_to_five = &array_of_ten[2..5];
    let start_at_two = &array_of_ten[1..];
    let end_at_five = &array_of_ten[..5];
    let everything = &array_of_ten[..];

    println!("Three to five: {:?},
start at two: {:?}
end at five: {:?}
everything: {:?}", three_to_five, start_at_two, end_at_five, everything);
}

Remember that:

Index numbers start at 0 (not 1)
Index ranges are exclusive (they do not include the last number)

So [0..2] means the first index and the second index (0 and 1). Or you can call it the "zeroth and first" index.

Vectors

In the same way that we have &str and String, we have arrays and vectors. Arrays are faster with less functionality, and vectors are slower with more functionality. The type is written Vec.

There are two main ways to declare a vector. One is like with String using new:

fn main() {
    let name1 = String::from("Windy");
    let name2 = String::from("Gomesy");
    
    let mut my_vec = Vec::new();
    // If we run the program now, the compiler will give an error.
    // It doesn't know the type of vec.

    my_vec.push(name1); // Now it knows: it's Vec<String>
    my_vec.push(name2);

}

Or you can just declare the type.

fn main() {
    let mut my_vec: Vec<String> = Vec::new(); // The compiler knows the type
                                              // so there is no error.
}

You can see that items in vectors must have the same type.

Another easy way to create a vector is with the vec! macro. It looks like an array declaration, but has vec! in front of it.

fn main() {
    let mut my_vec = vec![8, 10, 10];
}

The type is Vec<i32>. You call it a "vec of i32s". And a Vec is a "vec of strings". And a Vec<Vec> is a "vec of a vec of strings".

You can slice a vector too, just like in an array.

fn main() {
    let vec_of_ten = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
	// Everything is the same except we added vec!
    let three_to_five = &vec_of_ten[2..5];
    let start_at_two = &vec_of_ten[1..];
    let end_at_five = &vec_of_ten[..5];
    let everything = &vec_of_ten[..];

    println!("Three to five: {:?},
start at two: {:?}
end at five: {:?}
everything: {:?}", three_to_five, start_at_two, end_at_five, everything);
}

Because a vec is slower than an array, we can use some methods to make it faster. A vec has a capacity, which means the space given to the vector. If you add more to a vector than its capacity, it will make its capacity double and copy the items into the new space. This is called reallocation.

For example:

fn main() {
    let mut num_vec = Vec::new();
    num_vec.push('a'); // add one character
    println!("{}", num_vec.capacity()); // prints 1
    num_vec.push('a'); // add one more
    println!("{}", num_vec.capacity()); // prints 2
    num_vec.push('a'); // add one more
    println!("{}", num_vec.capacity()); // prints 4. It has three elements, but capacity is 4
    num_vec.push('a'); // add one more
    num_vec.push('a'); // add one more // Now we have 5 elements
    println!("{}", num_vec.capacity()); // Now capacity is 8
}

So this vector has three reallocations: 1 to 2, 2 to 4, and 4 to 8. We can make it faster:

fn main() {
    let mut num_vec = Vec::with_capacity(8); // Give it capacity 8
    num_vec.push('a'); // add one character
    println!("{}", num_vec.capacity()); // prints 8
    num_vec.push('a'); // add one more
    println!("{}", num_vec.capacity()); // prints 8
    num_vec.push('a'); // add one more
    println!("{}", num_vec.capacity()); // prints 8.
    num_vec.push('a'); // add one more
    num_vec.push('a'); // add one more // Now we have 5 elements
    println!("{}", num_vec.capacity()); // Still 8
}

This vector has 0 reallocations, which is better. So if you think you know how many elements you need, you can use Vec::with_capacity() to make it faster.

Tuples

Tuples in Rust use (). We have seen many empty tuples already. fn do_something() {} has an empty tuple. Also, when you don't return anything in a function, you actually return an empty tuple.

fn main() {
  
}

fn just_prints() {
    println!("I am printing"); // Adding ; means we return an empty tuple
}

But tuples can hold many things, and can hold different types too. To access the items inside of a tuple, don't use [], use ..

fn main() {
    let mut new_vec = Vec::new();
    new_vec.push('a');
    let random_tuple = ("Here is a name", 8, new_vec, 'b', [8, 9, 10], 7.7);
    println!(
        "Inside the tuple is: First item: {:?} 
Second item: {:?} 
Third item: {:?} 
Fourth item: {:?} 
Fifth item: {:?} 
Sixth item: {:?}",
        random_tuple.0,
        random_tuple.1,
        random_tuple.2,
        random_tuple.3,
        random_tuple.4,
        random_tuple.5,
    )
}

You can use a tuple to create multiple variables.

fn main() {
    let str_vec = vec!["one", "two", "three"];

    let (a, b, c) = (str_vec[0], str_vec[1], str_vec[2]);
    println!("{:?}", b);
}

There are many more collection types, and many more ways to use arrays, vecs, and tuples. We will learn more about them. But first we will learn control flow.

Control flow

Control flow means telling your code what to do in different situations. The simplest control flow is if.

fn main() {
    let my_number = 5;
    if my_number == 7 {
        println!("It's seven");
    }
}

Please note that we wrote if my_number == 7 and not if (my_number == 7). You don't need () with if in Rust.

else if and else gives you more control:

fn main() {
    let my_number = 5;
    if my_number == 7 {
        println!("It's seven");
    } else if my_number == 6 {
        println!("It's six")
    } else {
        println!("It's a different number")
    }
}

You can add more conditions with && (and) and || (or).

Too much if, else, and else if can be difficult to read. You can use match instead. But you must match for every possible result. For example, this will not work:

fn main() {
    let my_number: u8 = 5;
    match my_number {
        0 => println!("it's zero"),
        1 => println!("it's one"),
        2 => println!("it's two"),
    }
}

The compiler says:

error[E0004]: non-exhaustive patterns: `3u8..=std::u8::MAX` not covered
 --> src\main.rs:3:11
  |
3 |     match my_number {
  |           ^^^^^^^^^ pattern `3u8..=std::u8::MAX` not covered

This means "you told me about 0 to 2, but u8s can go up to 255. What about 3? What about 4? What about 5?" And so on. So you can add _ which means "anything else".

fn main() {
    let my_number: u8 = 5;
    match my_number {
        0 => println!("it's zero"),
        1 => println!("it's one"),
        2 => println!("it's two"),
        _ => println!("It's some other number"),
    }
}

Remember this for match:

You write match and then make a {} code block.
Write the pattern on the left and use a => fat arrow to say what to do when it matches.
Each line is called an "arm".
Put a comma between the arms (not a semicolon).

You can declare a value with a match:

fn main() {
    let my_number = 5;
    let second_number = match my_number {
        0 => 0,
        5 => 10,
        _ => 2,
    };
}

Do you see the semicolon at the end? That is because, after the match is over, we actually told the compiler this: let second_number = 10;

You can match on more complicated things too. You use a tuple to do it.

fn main() {
    let sky = "cloudy";
    let temperature = "warm";

    match (sky, temperature) {
        ("cloudy", "cold") => println!("It's dark and unpleasant today"),
        ("clear", "warm") => println!("It's a nice day"),
        ("cloudy", "warm") => println!("It's dark but not bad"),
        _ => println!("Not sure what the weather is."),
    }
}

You can even put if inside of match.

fn main() {
    let children = 5;
    let married = true;

    match (children, married) {
        (children, married) if married == false => println!("Not married with {} children", children),
        (children, married) if children == 0 && married == true => println!("Married but no children"),
        _ => println!("Married? {}. Number of children: {}.", married, children),
    }
}

Structs

With structs, you can create your own type. Structs are created with the keyword struct. The name of a struct should be in UpperCamelCase (capital letter for each word, no spaces).

There are three types of structs. One is a "unit struct". Unit means "doesn't have anything".

struct FileDirectory;

The next is a tuple struct, or an unnamed struct. It is "unnamed" because you only need to write the types, not the variable names. Tuple structs are good when you need a simple struct and don't need to remember names.

struct Colour(u8, u8, u8);

fn main() {
    let my_colour = Colour(50, 0, 50); // Make a colour out of RGB (red, green, blue)
    println!("The second part of the colour is: {}", my_colour.1);
}

The third type is the named struct. This is probably the most common struct. In this struct you declare variable names and types inside a {}code block.

struct Colour(u8, u8, u8); // Declare the same Colour tuple struct

struct SizeAndColour {
    size: u32,
    colour: Colour, // And we put it in our new named struct
}

fn main() {
    let my_colour = Colour(50, 0, 50);
    
    let size_and_colour = SizeAndColour {
        size: 150,
        colour: my_colour
    };
}

Let's create a Country struct to give an example. The Country struct has the fields population, capital, and leader_name.

struct Country {
    population: u32,
    capital: String,
    leader_name: String
}

fn main() {
    let population = 500_000;
    let capital = String::from("Elist");
    let leader_name = String::from("Batu Khasikov");

    let kalmykia = Country {
        population: population,
        capital: capital,
        leader_name: leader_name,
    };

}

Did you notice that we wrote the same thing twice? Actually, you don't need to do that. If the field name and variable name are the same, you don't have to write it twice.

struct Country {
    population: u32,
    capital: String,
    leader_name: String
}

fn main() {
    let population = 500_000;
    let capital = String::from("Elist");
    let leader_name = String::from("Batu Khasikov");

    let kalmykia = Country {
        population,
        capital,
        leader_name,
    };

}

Enums

An enum is short for enumerations. They look similar to a struct, but are different. Here is the difference:

Use a struct when you want one thing AND another thing.
Use an enum when you want one thing OR another thing.

So structs are for many things together, while enums are for many choices together.

To declare an enum, write enum and use a code block with the options, separated by commas. We will create an enum called SkyState:

enum ThingsInTheSky {
    Sun,
    Stars,
}

This is an enum because you can either see the sun, or the stars: you have to choose one. These are called variants.

// create the enum with two choices
enum ThingsInTheSky {
    Sun,
    Stars,
}

// With this function we can use an i32 to create ThingsInTheSky.
fn create_skystate(time: i32) -> ThingsInTheSky {
    match time {
        6..=18 => ThingsInTheSky::Sun, // Between 6 and 18 hours we can see the sun
        _ => ThingsInTheSky::Stars, // Otherwise, we can see stars
    }
}

// With this function we can match against the two choices in ThingsInTheSky.
fn check_skystate(state: &ThingsInTheSky) {
    match state {
        ThingsInTheSky::Sun => println!("I can see the sun!"),
        ThingsInTheSky::Stars => println!("I can see the stars!")
    }
}

fn main() {
    
    let time = 8; // it's 8 o'clock
    let skystate = create_skystate(time); // create_skystate returns a ThingsInTheSky
    check_skystate(&skystate); // Give it a reference so it can read the varible skystate

}

You can add data to an enum too.

enum ThingsInTheSky {
    Sun(String), // Now each variant has a string
    Stars(String),
}

fn create_skystate(time: i32) -> ThingsInTheSky {
    match time {
        6..=18 => ThingsInTheSky::Sun(String::from("I can see the sun!")), // Write the strings here
        _ => ThingsInTheSky::Stars(String::from("I can see the stars!")),
    }
}

fn check_skystate(state: &ThingsInTheSky) {
    match state {
        ThingsInTheSky::Sun(description) => println!("{}", description), // Give the string the name description so we can use it
        ThingsInTheSky::Stars(n) => println!("{}", n), // Or you can name it n. Or anything else - it doesn't matter
    }
}

fn main() {
    
    let time = 8; // it's 8 o'clock
    let skystate = create_skystate(time); // create_skystate returns a ThingsInTheSky
    check_skystate(&skystate); // Give it a reference so it can read the varible skystate

}

Loops

With loops you can tell Rust to continue something until you want it to stop. With loop you can start a loop that does not stop, unless you tell it when to break.

fn main() { // This program will never stop
    loop {
 	       
    }
}

So let's tell the compiler when it can break.

fn main() {
    let mut counter = 0; // set a counter to 0
    loop {
        counter +=1; // increase the counter by 1
        println!("The counter is now: {}", counter);
        if counter == 5 { // stop when counter == 5
            break;
        }
    }
}

This will print:

The counter is now: 1
The counter is now: 2
The counter is now: 3
The counter is now: 4
The counter is now: 5

If you have a loop inside of a loop, you can give them names. With names, you can tell Rust which loop to break out of. Use ' (called a "tick") and a : to give it a name:

fn main() {
    let mut counter = 0;
    let mut counter2 = 0;
    println!("Now entering the first loop.");

    'first_loop: loop { // Give the first loop a name
        counter +=1;
        println!("The counter is now: {}", counter);
        if counter > 9 { // Starts a second loop inside this loop
            println!("Now entering the second loop.");

            'second_loop: loop { // now we are inside `second_loop
                println!("The second counter is now: {}", counter2);
                counter2 +=1;
                if counter2 == 3 {
                    break 'first_loop; // Break out of `first_loop so we can exit the program
                }
            }
        }
    }
}

A while loop is a loop that continues while something is still true. Each loop, Rust will check if it is still true. If it becomes false, Rust will stop the loop.

fn main() {
    let mut counter = 0;

    while counter < 5 {
        counter +=1;
        println!("The counter is now: {}", counter);
    }
}

A for loop lets you tell Rust what to do each time. But in a for loop, the loop stops after a certain number of times. for loops use ranges very often. You use .. and ..= to create a range.

.. creates an exclusive range: 0..3 creates 0, 1, 2.
..= creates an inclusive range: 0..=3 = 0, 1, 2, 3.

fn main() {
    for number in 0..3 {
        println!("The number is: {}", number);
    }

    for number in 0..=3 {
        println!("The next number is: {}", number);
    }
}

Also notice that number becomes the variable name for 0..3. We can then use that name in println!.

If you don't need a variable name, use _.

fn main() {
    for _ in 0..3 {
        println!("Printing the same thing three times");
    }
}

Actually, if you give a variable name and don't use it, Rust will tell you:

fn main() {
    for number in 0..3 {
        println!("Printing the same thing three times");
    }
}

This is not an error, but Rust will remind you that you didn't use number:

warning: unused variable: `number`
 --> src\main.rs:2:9
  |
2 |     for number in 0..3 {
  |         ^^^^^^ help: if this is intentional, prefix it with an underscore: `_number`

Rust also suggests _number. Putting _ in front of a variable name means "maybe I will use it later". But using just _ means "I don't care about this variable at all".

Implementing structs and enums

To call functions on a struct or an enum, use an impl block. These functions are called methods. There are two kinds of methods in an impl block.

Regular methods: these take self (or &self or &mut self). Regular methods use a .. .clone() is a regular method.
Associated methods (or "static" methods): these do not take self. They are written differently, using ::. String::from() is an associated method. You usually use associated methods to create new variables.

In our example we are going to create animals and print them. For a new struct or enum, you need to give it Debug if you want to use {:?} to print. If you write #[derive(Debug)] above the struct or enum then you can print it with {:?}.

#[derive(Debug)]
struct Animal {
    age: u8,
    animal_type: AnimalType,
}

impl Animal {
    fn new() -> Self {
        // Self means AnimalType.
        //You can also write AnimalType instead of Self

        Self {
            // When we write Animal::new(), we always get a cat that is 10 years old
            age: 10,
            animal_type: AnimalType::Cat,
        }
    }

    fn change_to_dog(&mut self) {
        // use .change_to_dog() to change the cat to a dog
        // with &mut self we can change it
        println!("Changing animal to dog!");
        self.animal_type = AnimalType::Dog;
    }

    fn change_to_cat(&mut self) {
        // use .change_to_dog() to change the cat to a dog
        // with &mut self we can change it
        println!("Changing animal to cat!");
        self.animal_type = AnimalType::Cat;
    }

    fn check_type(&self) {
        // we want to read self
        match self.animal_type {
            AnimalType::Dog => println!("The animal is a dog"),
            AnimalType::Cat => println!("The animal is a cat"),
        }
    }
}

#[derive(Debug)]
enum AnimalType {
    Cat,
    Dog,
}

fn main() {
    let mut new_animal = Animal::new(); // Associated method to create a new animal
                                        // It is a cat, 10 years old
    new_animal.check_type();
    new_animal.change_to_dog();
    new_animal.check_type();
    new_animal.change_to_cat();
    new_animal.check_type();
}

This prints:

The animal is a cat
Changing animal to dog!
The animal is a dog
Changing animal to cat!
The animal is a cat

Self

Remember that Self (the type Self) and self (the variable self) are abbreviations. (abbreviation = short way to write)

So in our code, Self = AnimalType. Also, fn change_to_dog(&mut self) means fn change_to_dog(&mut AnimalType)

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