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Jan 27, 2020 clock icon 27 min #rust

A half-hour to learn Rust

👋 This page was last updated ~5 years ago. Just so you know.

In order to increase fluency in a programming language, one has to read a lot of it.

But how can you read a lot of it if you don't know what it means?

In this article, instead of focusing on one or two concepts, I'll try to go through as many Rust snippets as I can, and explain what the keywords and symbols they contain mean.

Ready? Go!

Variable bindings

The let keyword

let introduces a variable binding:

let x; // declare "x"
x = 42; // assign 42 to "x"

This can also be written as a single line:

let x = 42;

Type annotation

You can specify the variable's type explicitly with :, that's a type annotation:

let x: i32; // `i32` is a signed 32-bit integer
x = 42;

// there's i8, i16, i32, i64, i128
//    also u8, u16, u32, u64, u128 for unsigned

This can also be written as a single line:

let x: i32 = 42;

Uninitialized variables

If you declare a name and initialize it later, the compiler will prevent you from using it before it's initialized.

let x;
foobar(x); // error: borrow of possibly-uninitialized variable: `x`
x = 42;

However, doing this is completely fine:

let x;
x = 42;
foobar(x); // the type of `x` will be inferred from here

Throwing values away

The underscore _ is a special name - or rather, a "lack of name". It basically means to throw away something:

// this does *nothing* because 42 is a constant
let _ = 42;

// this calls `get_thing` but throws away its result
let _ = get_thing();

Names that start with an underscore are regular names, it's just that the compiler won't warn about them being unused:

// we may use `_x` eventually, but our code is a work-in-progress
// and we just wanted to get rid of a compiler warning for now.
let _x = 42;

Shadowing bindings

Separate bindings with the same name can be introduced - you can shadow a variable binding:

let x = 13;
let x = x + 3;
// using `x` after that line only refers to the second `x`,
//
// although the first `x` still exists (it'll be dropped
// when going out of scope), you can no longer refer to it.

Tuples

Rust has tuples, which you can think of as "fixed-length collections of values of different types".

let pair = ('a', 17);
pair.0; // this is 'a'
pair.1; // this is 17

If we really wanted to annotate the type of pair, we would write:

let pair: (char, i32) = ('a', 17);

Destructuring tuples

Tuples can be destructured when doing an assignment, which means they're broken down into their individual fields:

let (some_char, some_int) = ('a', 17);
// now, `some_char` is 'a', and `some_int` is 17

This is especially useful when a function returns a tuple:

let (left, right) = slice.split_at(middle);

Of course, when destructuring a tuple, _ can be used to throw away part of it:

let (_, right) = slice.split_at(middle);

Statements

The semi-colon marks the end of a statement:

let x = 3;
let y = 5;
let z = y + x;

Which means statements can span multiple lines:

let x = vec![1, 2, 3, 4, 5, 6, 7, 8]
    .iter()
    .map(|x| x + 3)
    .fold(0, |x, y| x + y);

(We'll go over what those actually mean later).

Functions

fn declares a function.

Here's a void function:

fn greet() {
    println!("Hi there!");
}

And here's a function that returns a 32-bit signed integer. The arrow indicates its return type:

fn fair_dice_roll() -> i32 {
    4
}

Blocks

A pair of brackets declares a block, which has its own scope:

// This prints "in", then "out"
fn main() {
    let x = "out";
    {
        // this is a different `x`
        let x = "in";
        println!("{}", x);
    }
    println!("{}", x);
}

Blocks are expressions

Blocks are also expressions, which mean they evaluate to a value.

// this:
let x = 42;

// is equivalent to this:
let x = { 42 };

Inside a block, there can be multiple statements:

let x = {
    let y = 1; // first statement
    let z = 2; // second statement
    y + z // this is the *tail* - what the whole block will evaluate to
};

Implicit return

And that's why "omitting the semicolon at the end of a function" is the same as returning, ie. these are equivalent:

fn fair_dice_roll() -> i32 {
    return 4;
}

fn fair_dice_roll() -> i32 {
    4
}

Everything is an expression

if conditionals are also expressions:

fn fair_dice_roll() -> i32 {
    if feeling_lucky {
        6
    } else {
        4
    }
}

A match is also an expression:

fn fair_dice_roll() -> i32 {
    match feeling_lucky {
        true => 6,
        false => 4,
    }
}

Field access and method calling

Dots are typically used to access fields of a value:

let a = (10, 20);
a.0; // this is 10

let amos = get_some_struct();
amos.nickname; // this is "fasterthanlime"

Or call a method on a value:

let nick = "fasterthanlime";
nick.len(); // this is 14

Modules, use syntax

The double-colon, ::, is similar but it operates on namespaces.

In this example, std is a crate (~ a library), cmp is a module (~ a source file), and min is a function:

let least = std::cmp::min(3, 8); // this is 3

use directives can be used to "bring in scope" names from other namespace:

use std::cmp::min;

let least = min(7, 1); // this is 1

Within use directives, curly brackets have another meaning: they're "globs". If we want to import both min and max, we can do any of these:

// this works:
use std::cmp::min;
use std::cmp::max;

// this also works:
use std::cmp::{min, max};

// this also works!
use std::{cmp::min, cmp::max};

A wildcard (*) lets you import every symbol from a namespace:

// this brings `min` and `max` in scope, and many other things
use std::cmp::*;

Types are namespaces too

Types are namespaces too, and methods can be called as regular functions:

let x = "amos".len(); // this is 4
let x = str::len("amos"); // this is also 4

The libstd prelude

str is a primitive type, but many non-primitive types are also in scope by default.

// `Vec` is a regular struct, not a primitive type
let v = Vec::new();

// this is exactly the same code, but with the *full* path to `Vec`
let v = std::vec::Vec::new();

This works because Rust inserts this at the beginning of every module:

use std::prelude::v1::*;

(Which in turns re-exports a lot of symbols, like Vec, String, Option and Result).

Structs

Structs are declared with the struct keyword:

struct Vec2 {
    x: f64, // 64-bit floating point, aka "double precision"
    y: f64,
}

They can be initialized using struct literals:

let v1 = Vec2 { x: 1.0, y: 3.0 };
let v2 = Vec2 { y: 2.0, x: 4.0 };
// the order does not matter, only the names do

Struct update syntax

There is a shortcut for initializing the rest of the fields from another struct:

let v3 = Vec2 {
    x: 14.0,
    ..v2
};

This is called "struct update syntax", can only happen in last position, and cannot be followed by a comma.

Note that the rest of the fields can mean all the fields:

let v4 = Vec2 { ..v3 };

Destructuring structs

Structs, like tuples, can be destructured.

Just like this is a valid let pattern:

let (left, right) = slice.split_at(middle);

So is this:

let v = Vec2 { x: 3.0, y: 6.0 };
let Vec2 { x, y } = v;
// `x` is now 3.0, `y` is now `6.0`

And this:

let Vec2 { x, .. } = v;
// this throws away `v.y`

Patterns and destructuring

Destructuring with if let

let patterns can be used as conditions in if:

struct Number {
    odd: bool,
    value: i32,
}

fn main() {
    let one = Number { odd: true, value: 1 };
    let two = Number { odd: false, value: 2 };
    print_number(one);
    print_number(two);
}

fn print_number(n: Number) {
    if let Number { odd: true, value } = n {
        println!("Odd number: {}", value);
    } else if let Number { odd: false, value } = n {
        println!("Even number: {}", value);
    }
}

// this prints:
// Odd number: 1
// Even number: 2

Match arms are patterns

match arms are also patterns, just like if let:

fn print_number(n: Number) {
    match n {
        Number { odd: true, value } => println!("Odd number: {}", value),
        Number { odd: false, value } => println!("Even number: {}", value),
    }
}

// this prints the same as before

Exhaustive matches

A match has to be exhaustive: at least one arm needs to match.

fn print_number(n: Number) {
    match n {
        Number { value: 1, .. } => println!("One"),
        Number { value: 2, .. } => println!("Two"),
        Number { value, .. } => println!("{}", value),
        // if that last arm didn't exist, we would get a compile-time error
    }
}

If that's hard, _ can be used as a "catch-all" pattern:

fn print_number(n: Number) {
    match n.value {
        1 => println!("One"),
        2 => println!("Two"),
        _ => println!("{}", n.value),
    }
}

Methods

You can declare methods on your own types:

struct Number {
    odd: bool,
    value: i32,
}

impl Number {
    fn is_strictly_positive(self) -> bool {
        self.value > 0
    }
}

And use them like usual:

fn main() {
    let minus_two = Number {
        odd: false,
        value: -2,
    };
    println!("positive? {}", minus_two.is_strictly_positive());
    // this prints "positive? false"
}

Immutability

Variable bindings are immutable by default, which means their interior can't be mutated:

fn main() {
    let n = Number {
        odd: true,
        value: 17,
    };
    n.odd = false; // error: cannot assign to `n.odd`,
                   // as `n` is not declared to be mutable
}

And also that they cannot be assigned to:

fn main() {
    let n = Number {
        odd: true,
        value: 17,
    };
    n = Number {
        odd: false,
        value: 22,
    }; // error: cannot assign twice to immutable variable `n`
}

mut makes a variable binding mutable:

fn main() {
    let mut n = Number {
        odd: true,
        value: 17,
    }
    n.value = 19; // all good
}

Traits

Traits are something multiple types can have in common:

trait Signed {
    fn is_strictly_negative(self) -> bool;
}

Orphan rules

You can implement:

  • one of your traits on anyone's type
  • anyone's trait on one of your types
  • but not a foreign trait on a foreign type

These are called the "orphan rules".

Here's an implementation of our trait on our type:

impl Signed for Number {
    fn is_strictly_negative(self) -> bool {
        self.value < 0
    }
}

fn main() {
    let n = Number { odd: false, value: -44 };
    println!("{}", n.is_strictly_negative()); // prints "true"
}

Our trait on a foreign type (a primitive type, even):

impl Signed for i32 {
    fn is_strictly_negative(self) -> bool {
        self < 0
    }
}

fn main() {
    let n: i32 = -44;
    println!("{}", n.is_strictly_negative()); // prints "true"
}

A foreign trait on our type:

// the `Neg` trait is used to overload `-`, the
// unary minus operator.
impl std::ops::Neg for Number {
    type Output = Number;

    fn neg(self) -> Number {
        Number {
            value: -self.value,
            odd: self.odd,
        }        
    }
}

fn main() {
    let n = Number { odd: true, value: 987 };
    let m = -n; // this is only possible because we implemented `Neg`
    println!("{}", m.value); // prints "-987"
}

The Self type

An impl block is always for a type, so, inside that block, Self means that type:

impl std::ops::Neg for Number {
    type Output = Self;

    fn neg(self) -> Self {
        Self {
            value: -self.value,
            odd: self.odd,
        }        
    }
}

Marker traits

Some traits are markers - they don't say that a type implements some methods, they say that certain things can be done with a type.

For example, i32 implements trait Copy (in short, i32 is Copy), so this works:

fn main() {
    let a: i32 = 15;
    let b = a; // `a` is copied
    let c = a; // `a` is copied again
}

And this also works:

fn print_i32(x: i32) {
    println!("x = {}", x);
}

fn main() {
    let a: i32 = 15;
    print_i32(a); // `a` is copied
    print_i32(a); // `a` is copied again
}

But the Number struct is not Copy, so this doesn't work:

fn main() {
    let n = Number { odd: true, value: 51 };
    let m = n; // `n` is moved into `m`
    let o = n; // error: use of moved value: `n`
}

And neither does this:

fn print_number(n: Number) {
    println!("{} number {}", if n.odd { "odd" } else { "even" }, n.value);
}

fn main() {
    let n = Number { odd: true, value: 51 };
    print_number(n); // `n` is moved
    print_number(n); // error: use of moved value: `n`
}

But it works if print_number takes an immutable reference instead:

fn print_number(n: &Number) {
    println!("{} number {}", if n.odd { "odd" } else { "even" }, n.value);
}

fn main() {
    let n = Number { odd: true, value: 51 };
    print_number(&n); // `n` is borrowed for the time of the call
    print_number(&n); // `n` is borrowed again
}

It also works if a function takes a mutable reference - but only if our variable binding is also mut.

fn invert(n: &mut Number) {
    n.value = -n.value;
}

fn print_number(n: &Number) {
    println!("{} number {}", if n.odd { "odd" } else { "even" }, n.value);
}

fn main() {
    // this time, `n` is mutable
    let mut n = Number { odd: true, value: 51 };
    print_number(&n);
    invert(&mut n); // `n is borrowed mutably - everything is explicit
    print_number(&n);
}

Trait method receivers

Trait methods can also take self by reference or mutable reference:

impl std::clone::Clone for Number {
    fn clone(&self) -> Self {
        Self { ..*self }
    }
}

When invoking trait methods, the receiver is borrowed implicitly:

fn main() {
    let n = Number { odd: true, value: 51 };
    let mut m = n.clone();
    m.value += 100;
    
    print_number(&n);
    print_number(&m);
}

To highlight this: these are equivalent:

let m = n.clone();

let m = std::clone::Clone::clone(&n);

Marker traits like Copy have no methods:

// note: `Copy` requires that `Clone` is implemented too
impl std::clone::Clone for Number {
    fn clone(&self) -> Self {
        Self { ..*self }
    }
}

impl std::marker::Copy for Number {}

Now, Clone can still be used:

fn main() {
    let n = Number { odd: true, value: 51 };
    let m = n.clone();
    let o = n.clone();
}

But Number values will no longer be moved:

fn main() {
    let n = Number { odd: true, value: 51 };
    let m = n; // `m` is a copy of `n`
    let o = n; // same. `n` is neither moved nor borrowed.
}

Deriving traits

Some traits are so common, they can be implemented automatically by using the derive attribute:

#[derive(Clone, Copy)]
struct Number {
    odd: bool,
    value: i32,
}

// this expands to `impl Clone for Number` and `impl Copy for Number` blocks.

Generics

Generic functions

Functions can be generic:

fn foobar<T>(arg: T) {
    // do something with `arg`
}

They can have multiple type parameters, which can then be used in the function's declaration and its body, instead of concrete types:

fn foobar<L, R>(left: L, right: R) {
    // do something with `left` and `right`
}

Type parameter constraints (trait bounds)

Type parameters usually have constraints, so you can actually do something with them.

The simplest constraints are just trait names:

fn print<T: Display>(value: T) {
    println!("value = {}", value);
}

fn print<T: Debug>(value: T) {
    println!("value = {:?}", value);
}

There's a longer syntax for type parameter constraints:

fn print<T>(value: T)
where
    T: Display,
{
    println!("value = {}", value);
}

Constraints can be more complicated: they can require a type parameter to implement multiple traits:

use std::fmt::Debug;

fn compare<T>(left: T, right: T)
where
    T: Debug + PartialEq,
{
    println!("{:?} {} {:?}", left, if left == right { "==" } else { "!=" }, right);
}

fn main() {
    compare("tea", "coffee");
    // prints: "tea" != "coffee"
}

Monomorphization

Generic functions can be thought of as namespaces, containing an infinity of functions with different concrete types.

Same as with crates, and modules, and types, generic functions can be "explored" (navigated?) using ::

fn main() {
    use std::any::type_name;
    println!("{}", type_name::<i32>()); // prints "i32"
    println!("{}", type_name::<(f64, char)>()); // prints "(f64, char)"
}

This is lovingly called turbofish syntax, because ::<> looks like a fish.

Generic structs

Structs can be generic too:

struct Pair<T> {
    a: T,
    b: T,
}

fn print_type_name<T>(_val: &T) {
    println!("{}", std::any::type_name::<T>());
}

fn main() {
    let p1 = Pair { a: 3, b: 9 };
    let p2 = Pair { a: true, b: false };
    print_type_name(&p1); // prints "Pair<i32>"
    print_type_name(&p2); // prints "Pair<bool>"
}

Example: Vec<T>

The standard library type Vec (~ a heap-allocated array), is generic:

fn main() {
    let mut v1 = Vec::new();
    v1.push(1);
    let mut v2 = Vec::new();
    v2.push(false);
    print_type_name(&v1); // prints "Vec<i32>"
    print_type_name(&v2); // prints "Vec<bool>"
}

Speaking of Vec, it comes with a macro that gives more or less "vec literals":

fn main() {
    let v1 = vec![1, 2, 3];
    let v2 = vec![true, false, true];
    print_type_name(&v1); // prints "Vec<i32>"
    print_type_name(&v2); // prints "Vec<bool>"
}

Macros

All of name!(), name![] or name!{} invoke a macro. Macros just expand to regular code.

In fact, println is a macro:

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

This expands to something that has the same effect as:

fn main() {
    use std::io::{self, Write};
    io::stdout().lock().write_all(b"Hello there!\n").unwrap();
}

The panic! macro

panic is also a macro. It violently stops execution with an error message, and the file name / line number of the error, if enabled:

fn main() {
    panic!("This panics");
}
// output: thread 'main' panicked at 'This panics', src/main.rs:3:5

Functions that panic

Some methods also panic. For example, the Option type can contain something, or it can contain nothing. If .unwrap() is called on it, and it contains nothing, it panics:

fn main() {
    let o1: Option<i32> = Some(128);
    o1.unwrap(); // this is fine

    let o2: Option<i32> = None;
    o2.unwrap(); // this panics!
}

// output: thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21

Enums (sum types)

Option is not a struct - it's an enum, with two variants.

enum Option<T> {
    None,
    Some(T),
}

impl<T> Option<T> {
    fn unwrap(self) -> T {
        // enums variants can be used in patterns:
        match self {
            Self::Some(t) => t,
            Self::None => panic!(".unwrap() called on a None option"),
        }
    }
}

use self::Option::{None, Some};

fn main() {
    let o1: Option<i32> = Some(128);
    o1.unwrap(); // this is fine

    let o2: Option<i32> = None;
    o2.unwrap(); // this panics!
}

// output: thread 'main' panicked at '.unwrap() called on a None option', src/main.rs:11:27

Result is also an enum, it can either contain something, or an error:

enum Result<T, E> {
    Ok(T),
    Err(E),
}

It also panics when unwrapped and containing an error.

Lifetimes

Variables bindings have a "lifetime":

fn main() {
    // `x` doesn't exist yet
    {
        let x = 42; // `x` starts existing
        println!("x = {}", x);
        // `x` stops existing
    }
    // `x` no longer exists
}

Similarly, references have a lifetime:

fn main() {
    // `x` doesn't exist yet
    {
        let x = 42; // `x` starts existing
        let x_ref = &x; // `x_ref` starts existing - it borrows `x`
        println!("x_ref = {}", x_ref);
        // `x_ref` stops existing
        // `x` stops existing
    }
    // `x` no longer exists
}

The lifetime of a reference cannot exceed the lifetime of the variable binding it borrows:

fn main() {
    let x_ref = {
        let x = 42;
        &x
    };
    println!("x_ref = {}", x_ref);
    // error: `x` does not live long enough
}

Borrowing rules (one or more immutable borrows XOR one mutable borrow)

A variable binding can be immutably borrowed multiple times:

fn main() {
    let x = 42;
    let x_ref1 = &x;
    let x_ref2 = &x;
    let x_ref3 = &x;
    println!("{} {} {}", x_ref1, x_ref2, x_ref3);
}

While borrowed, a variable binding cannot be mutated:

fn main() {
    let mut x = 42;
    let x_ref = &x;
    x = 13;
    println!("x_ref = {}", x_ref);
    // error: cannot assign to `x` because it is borrowed
}

While immutably borrowed, a variable cannot be mutably borrowed:

fn main() {
    let mut x = 42;
    let x_ref1 = &x;
    let x_ref2 = &mut x;
    // error: cannot borrow `x` as mutable because it is also borrowed as immutable
    println!("x_ref1 = {}", x_ref1);
}

Functions generic over lifetimes

References in function arguments also have lifetimes:

fn print(x: &i32) {
    // `x` is borrowed (from the outside) for the
    // entire time this function is called.
}

Functions with reference arguments can be called with borrows that have different lifetimes, so:

  • All functions that take references are generic
  • Lifetimes are generic parameters

Lifetimes' names start with a single quote, ':

// elided (non-named) lifetimes:
fn print(x: &i32) {}

// named lifetimes:
fn print<'a>(x: &'a i32) {}

This allows returning references whose lifetime depend on the lifetime of the arguments:

struct Number {
    value: i32,
}

fn number_value<'a>(num: &'a Number) -> &'a i32 {
    &num.value
}

fn main() {
    let n = Number { value: 47 };
    let v = number_value(&n);
    // `v` borrows `n` (immutably), thus: `v` cannot outlive `n`.
    // While `v` exists, `n` cannot be mutably borrowed, mutated, moved, etc.
}

Lifetime elision

When there is a single input lifetime, it doesn't need to be named, and everything has the same lifetime, so the two functions below are equivalent:

fn number_value<'a>(num: &'a Number) -> &'a i32 {
    &num.value
}

fn number_value(num: &Number) -> &i32 {
    &num.value
}

Structs generic over lifetimes

Structs can also be generic over lifetimes, which allows them to hold references:

struct NumRef<'a> {
    x: &'a i32,
}

fn main() {
    let x: i32 = 99;
    let x_ref = NumRef { x: &x };
    // `x_ref` cannot outlive `x`, etc.
}

The same code, but with an additional function:

struct NumRef<'a> {
    x: &'a i32,
}

fn as_num_ref<'a>(x: &'a i32) -> NumRef<'a> {
    NumRef { x: &x }
}

fn main() {
    let x: i32 = 99;
    let x_ref = as_num_ref(&x);
    // `x_ref` cannot outlive `x`, etc.
}

The same code, but with "elided" lifetimes:

struct NumRef<'a> {
    x: &'a i32,
}

fn as_num_ref(x: &i32) -> NumRef<'_> {
    NumRef { x: &x }
}

fn main() {
    let x: i32 = 99;
    let x_ref = as_num_ref(&x);
    // `x_ref` cannot outlive `x`, etc.
}

Implementations generic over lifetimes

impl blocks can be generic over lifetimes too:

impl<'a> NumRef<'a> {
    fn as_i32_ref(&'a self) -> &'a i32 {
        self.x
    }
}

fn main() {
    let x: i32 = 99;
    let x_num_ref = NumRef { x: &x };
    let x_i32_ref = x_num_ref.as_i32_ref();
    // neither ref can outlive `x`
}

But you can do elision ("to elide") there too:

impl<'a> NumRef<'a> {
    fn as_i32_ref(&self) -> &i32 {
        self.x
    }
}

You can elide even harder, if you never need the name:

impl NumRef<'_> {
    fn as_i32_ref(&self) -> &i32 {
        self.x
    }
}

The 'static lifetime

There is a special lifetime, named 'static, which is valid for the entire program's lifetime.

String literals are 'static:

struct Person {
    name: &'static str,
}

fn main() {
    let p = Person {
        name: "fasterthanlime",
    };
}

But references to a String are not static:

struct Person {
    name: &'static str,
}

fn main() {
    let name = format!("fasterthan{}", "lime");
    let p = Person { name: &name };
    // error: `name` does not live long enough
}

In that last example, the local name is not a &'static str, it's a String. It's been allocated dynamically, and it will be freed. Its lifetime is less than the whole program (even though it happens to be in main).

To store a non-'static string in Person, it needs to either:

A) Be generic over a lifetime:

struct Person<'a> {
    name: &'a str,
}

fn main() {
    let name = format!("fasterthan{}", "lime");
    let p = Person { name: &name };
    // `p` cannot outlive `name`
}

or

B) Take ownership of the string

struct Person {
    name: String,
}

fn main() {
    let name = format!("fasterthan{}", "lime");
    let p = Person { name: name };
    // `name` was moved into `p`, their lifetimes are no longer tied.
}

Struct literal assignment shorthand

Speaking of: in a struct literal, when a field is set to a variable binding of the same name:

    let p = Person { name: name };

It can be shortened like this:

    let p = Person { name };

Tools like clippy will suggest making those changes, and even apply the fix programmatically if you let it.

Owned types vs reference types

For many types in Rust, there are owned and non-owned variants:

  • Strings: String is owned, &str is a reference.
  • Paths: PathBuf is owned, &Path is a reference.
  • Collections: Vec<T> is owned, &[T] is a reference.

Slices

Rust has slices - they're a reference to multiple contiguous elements.

You can borrow a slice of a vector, for example:

fn main() {
    let v = vec![1, 2, 3, 4, 5];
    let v2 = &v[2..4];
    println!("v2 = {:?}", v2);
}

// output:
// v2 = [3, 4]

Operator overloading

The above is not magical. The indexing operator (foo[index]) is overloaded with the Index and IndexMut traits.

The .. syntax is just range literals. Ranges are just a few structs defined in the standard library.

They can be open-ended, and their rightmost bound can be inclusive, if it's preceded by =.

fn main() {
    // 0 or greater
    println!("{:?}", (0..).contains(&100)); // true
    // strictly less than 20
    println!("{:?}", (..20).contains(&20)); // false
    // 20 or less than 20
    println!("{:?}", (..=20).contains(&20)); // true
    // only 3, 4, 5
    println!("{:?}", (3..6).contains(&4)); // true
}

Borrowing rules and slices

Borrowing rules apply to slices.

fn tail(s: &[u8]) -> &[u8] {
  &s[1..] 
}

fn main() {
    let x = &[1, 2, 3, 4, 5];
    let y = tail(x);
    println!("y = {:?}", y);
}

This is the same as:

fn tail<'a>(s: &'a [u8]) -> &'a [u8] {
  &s[1..] 
}

This is legal:

fn main() {
    let y = {
        let x = &[1, 2, 3, 4, 5];
        tail(x)
    };
    println!("y = {:?}", y);
}

...but only because [1, 2, 3, 4, 5] is a 'static array.

So, this is illegal:

fn main() {
    let y = {
        let v = vec![1, 2, 3, 4, 5];
        tail(&v)
        // error: `v` does not live long enough
    };
    println!("y = {:?}", y);
}

...because a vector is heap-allocated, and it has a non-'static lifetime.

String slices (&str)

&str values are really slices.

fn file_ext(name: &str) -> Option<&str> {
    // this does not create a new string - it returns
    // a slice of the argument.
    name.split(".").last()
}

fn main() {
    let name = "Read me. Or don't.txt";
    if let Some(ext) = file_ext(name) {
        println!("file extension: {}", ext);
    } else {
        println!("no file extension");
    }
}

...so the borrow rules apply here too:

fn main() {
    let ext = {
        let name = String::from("Read me. Or don't.txt");
        file_ext(&name).unwrap_or("")
        // error: `name` does not live long enough
    };
    println!("extension: {:?}", ext);
}

Fallible functions (Result<T, E>)

Functions that can fail typically return a Result:

fn main() {
    let s = std::str::from_utf8(&[240, 159, 141, 137]);
    println!("{:?}", s);
    // prints: Ok("🍉")

    let s = std::str::from_utf8(&[195, 40]);
    println!("{:?}", s);
    // prints: Err(Utf8Error { valid_up_to: 0, error_len: Some(1) })
}

If you want to panic in case of failure, you can .unwrap():

fn main() {
    let s = std::str::from_utf8(&[240, 159, 141, 137]).unwrap();
    println!("{:?}", s);
    // prints: "🍉"

    let s = std::str::from_utf8(&[195, 40]).unwrap();
    // prints: thread 'main' panicked at 'called `Result::unwrap()`
    // on an `Err` value: Utf8Error { valid_up_to: 0, error_len: Some(1) }',
    // src/libcore/result.rs:1165:5
}

Or .expect(), for a custom message:

fn main() {
    let s = std::str::from_utf8(&[195, 40]).expect("valid utf-8");
    // prints: thread 'main' panicked at 'valid utf-8: Utf8Error
    // { valid_up_to: 0, error_len: Some(1) }', src/libcore/result.rs:1165:5
}

Or, you can match:

fn main() {
    match std::str::from_utf8(&[240, 159, 141, 137]) {
        Ok(s) => println!("{}", s),
        Err(e) => panic!(e),
    }
    // prints 🍉
}

Or you can if let:

fn main() {
    if let Ok(s) = std::str::from_utf8(&[240, 159, 141, 137]) {
        println!("{}", s);
    }
    // prints 🍉
}

Or you can bubble up the error:

fn main() -> Result<(), std::str::Utf8Error> {
    match std::str::from_utf8(&[240, 159, 141, 137]) {
        Ok(s) => println!("{}", s),
        Err(e) => return Err(e),
    }
    Ok(())
}

Or you can use ? to do it the concise way:

fn main() -> Result<(), std::str::Utf8Error> {
    let s = std::str::from_utf8(&[240, 159, 141, 137])?;
    println!("{}", s);
    Ok(())
}

Dereferencing

The * operator can be used to dereference, but you don't need to do that to access fields or call methods:

struct Point {
    x: f64,
    y: f64,
}

fn main() {
    let p = Point { x: 1.0, y: 3.0 };
    let p_ref = &p;
    println!("({}, {})", p_ref.x, p_ref.y);
}

// prints `(1, 3)`

And you can only do it if the type is Copy:

struct Point {
    x: f64,
    y: f64,
}

fn negate(p: Point) -> Point {
    Point {
        x: -p.x,
        y: -p.y,
    }
}

fn main() {
    let p = Point { x: 1.0, y: 3.0 };
    let p_ref = &p;
    negate(*p_ref);
    // error: cannot move out of `*p_ref` which is behind a shared reference
}

// now `Point` is `Copy`
#[derive(Clone, Copy)]
struct Point {
    x: f64,
    y: f64,
}

fn negate(p: Point) -> Point {
    Point {
        x: -p.x,
        y: -p.y,
    }
}

fn main() {
    let p = Point { x: 1.0, y: 3.0 };
    let p_ref = &p;
    negate(*p_ref); // ...and now this works
}

Function types, closures

Closures are just functions of type Fn, FnMut or FnOnce with some captured context.

Their parameters are a comma-separated list of names within a pair of pipes (|). They don't need curly braces, unless you want to have multiple statements.

fn for_each_planet<F>(f: F)
    where F: Fn(&'static str)
{
    f("Earth");
    f("Mars");
    f("Jupiter");
}
 
fn main() {
    for_each_planet(|planet| println!("Hello, {}", planet));
}

// prints:
// Hello, Earth
// Hello, Mars
// Hello, Jupiter

The borrow rules apply to them too:

fn for_each_planet<F>(f: F)
    where F: Fn(&'static str)
{
    f("Earth");
    f("Mars");
    f("Jupiter");
}
 
fn main() {
    let greeting = String::from("Good to see you");
    for_each_planet(|planet| println!("{}, {}", greeting, planet));
    // our closure borrows `greeting`, so it cannot outlive it
}

For example, this would not work:

fn for_each_planet<F>(f: F)
    where F: Fn(&'static str) + 'static // `F` must now have "'static" lifetime
{
    f("Earth");
    f("Mars");
    f("Jupiter");
}

fn main() {
    let greeting = String::from("Good to see you");
    for_each_planet(|planet| println!("{}, {}", greeting, planet));
    // error: closure may outlive the current function, but it borrows
    // `greeting`, which is owned by the current function
}

But this would:

fn main() {
    let greeting = String::from("You're doing great");
    for_each_planet(move |planet| println!("{}, {}", greeting, planet));
    // `greeting` is no longer borrowed, it is *moved* into
    // the closure.
}

FnMut and borrowing rules

An FnMut needs to be mutably borrowed to be called, so it can only be called once at a time.

This is legal:

fn foobar<F>(f: F)
    where F: Fn(i32) -> i32
{
    println!("{}", f(f(2))); 
}
 
fn main() {
    foobar(|x| x * 2);
}

// output: 8

This isn't:

fn foobar<F>(mut f: F)
    where F: FnMut(i32) -> i32
{
    println!("{}", f(f(2))); 
    // error: cannot borrow `f` as mutable more than once at a time
}
 
fn main() {
    foobar(|x| x * 2);
}

This is legal again:

fn foobar<F>(mut f: F)
    where F: FnMut(i32) -> i32
{
    let tmp = f(2);
    println!("{}", f(tmp)); 
}
 
fn main() {
    foobar(|x| x * 2);
}

// output: 8

FnMut exists because some closures mutably borrow local variables:

fn foobar<F>(mut f: F)
    where F: FnMut(i32) -> i32
{
    let tmp = f(2);
    println!("{}", f(tmp)); 
}
 
fn main() {
    let mut acc = 2;
    foobar(|x| {
        acc += 1;
        x * acc
    });
}

// output: 24

Those closures cannot be passed to functions expecting Fn:

fn foobar<F>(f: F)
    where F: Fn(i32) -> i32
{
    println!("{}", f(f(2))); 
}
 
fn main() {
    let mut acc = 2;
    foobar(|x| {
        acc += 1;
        // error: cannot assign to `acc`, as it is a
        // captured variable in a `Fn` closure.
        // the compiler suggests "changing foobar
        // to accept closures that implement `FnMut`"
        x * acc
    });
}

FnOnce closures can only be called once. They exist because some closure move out variables that have been moved when captured:

fn foobar<F>(f: F)
    where F: FnOnce() -> String
{
    println!("{}", f()); 
}
 
fn main() {
    let s = String::from("alright");
    foobar(move || s);
    // `s` was moved into our closure, and our
    // closures moves it to the caller by returning
    // it. Remember that `String` is not `Copy`.
}

This is enforced naturally, as FnOnce closures need to be moved in order to be called.

So, for example, this is illegal:

fn foobar<F>(f: F)
    where F: FnOnce() -> String
{
    println!("{}", f()); 
    println!("{}", f()); 
    // error: use of moved value: `f`
}

And, if you need convincing that our closure does move s, this is illegal too:

fn main() {
    let s = String::from("alright");
    foobar(move || s);
    foobar(move || s);
    // use of moved value: `s`
}

But this is fine:

fn main() {
    let s = String::from("alright");
    foobar(|| s.clone());
    foobar(|| s.clone());
}

Here's a closure with two arguments:

fn foobar<F>(x: i32, y: i32, is_greater: F)
    where F: Fn(i32, i32) -> bool
{
    let (greater, smaller) = if is_greater(x, y) {
        (x, y)
    } else {
        (y, x)
    };
    println!("{} is greater than {}", greater, smaller);
}
 
fn main() {
    foobar(32, 64, |x, y| x > y);
}

Here's a closure ignoring both its arguments:

fn main() {
    foobar(32, 64, |_, _| panic!("Comparing is futile!"));
}

Here's a slightly worrying closure:

fn countdown<F>(count: usize, tick: F)
    where F: Fn(usize)
{
    for i in (1..=count).rev() {
        tick(i);
    }
}
 
fn main() {
    countdown(3, |i| println!("tick {}...", i));
}

// output:
// tick 3...
// tick 2...
// tick 1...

The toilet closure

And here's a toilet closure:

fn main() {
    countdown(3, |_| ());
}

It's called that because |_| () looks like a toilet.

Loops, iterators

Anything that is iterable can be used in a for in loop.

We've just seen a range being used, but it also works with a Vec:

fn main() {
    for i in vec![52, 49, 21] {
        println!("I like the number {}", i);
    }
}

Or a slice:

fn main() {
    for i in &[52, 49, 21] {
        println!("I like the number {}", i);
    }
}

// output:
// I like the number 52
// I like the number 49
// I like the number 21

Or an actual iterator:

fn main() {
    // note: `&str` also has a `.bytes()` iterator.
    // Rust's `char` type is a "Unicode scalar value"
    for c in "rust".chars() {
        println!("Give me a {}", c);
    }
}

// output:
// Give me a r
// Give me a u
// Give me a s
// Give me a t

Even if the iterator items are filtered and mapped and flattened:

fn main() {
    for c in "SuRPRISE INbOUND"
        .chars()
        .filter(|c| c.is_lowercase())
        .flat_map(|c| c.to_uppercase())
    {
        print!("{}", c);
    }
    println!();
}

// output: UB

Returning closures

You can return a closure from a function:

fn make_tester(answer: String) -> impl Fn(&str) -> bool {
    move |challenge| {
        challenge == answer
    }
}

fn main() {
    // you can use `.into()` to perform conversions
    // between various types, here `&'static str` and `String`
    let test = make_tester("hunter2".into());
    println!("{}", test("******"));
    println!("{}", test("hunter2"));
}

Capturing into a closure

You can even move a reference to some of a function's arguments, into a closure it returns:

fn make_tester<'a>(answer: &'a str) -> impl Fn(&str) -> bool + 'a {
    move |challenge| {
        challenge == answer
    }
}

fn main() {
    let test = make_tester("hunter2");
    println!("{}", test("*******"));
    println!("{}", test("hunter2"));
}

// output:
// false
// true

Or, with elided lifetimes:

fn make_tester(answer: &str) -> impl Fn(&str) -> bool + '_ {
    move |challenge| {
        challenge == answer
    }
}

Conclusion

And with that, we hit the 30-minute estimated reading time mark, and you should be able to read most of the Rust code you find online.

Writing Rust is a very different experience from reading Rust. On one hand, you're not reading the solution to a problem, you're actually solving it. On the other hand, the Rust compiler helps out a lot.

The Rust compiler has high-quality diagnostics (which include suggestions) for all the mistakes featured in this article.

And when there's a hint missing, the compiler team is not afraid to add it.

For more Rust material, you may want to check out:

I also blog about Rust and post a lot about Rust on Mastodon and Twitter a lot, so if you liked this article, you know what to do!

Have fun!

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Aug 12, 2020
clock icon 1 hour 13 min #rust · #traits · #lifetimes

As I've said before, I'm working on a book about lifetimes. Or maybe it's just a long series - I haven't decided the specifics yet. Like every one of my series/book things, it's long, and it starts you off way in the periphery of the subject, and takes a lot of detours to get there.

In other words - it's great if you want an adventure (which truly understanding Rust definitely is), but it's not the best if you are currently on the puzzled end of a conversation with your neighborhood lifetime enforcer, the Rust compiler.

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