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!

let introduces a variable binding:

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

This can also be written as a single line:

Rust code
let x = 42;

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

Rust code
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:

Rust code
let x: i32 = 42;

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

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

However, doing this is completely fine:

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

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

Rust code
// 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:

Rust code
// 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;

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

Rust code
let x = 13; let x = x + 3; // using `x` after that line only refers to the second `x`, // the first `x` no longer exists.

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

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

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

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

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

Rust code
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:

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

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

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

The semi-colon marks the end of a statement:

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

Which means statements can span multiple lines:

Rust code
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).

fn declares a function.

Here's a void function:

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

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

Rust code
fn fair_dice_roll() -> i32 { 4 }

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

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

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

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

Inside a block, there can be multiple statements:

Rust code
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 };

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

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

if conditionals are also expressions:

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

A match is also an expression:

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

Dots are typically used to access fields of a value:

Rust code
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:

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

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:

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

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

Rust code
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:

Rust code
// 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:

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

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

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

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

Rust code
// `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:

Rust code
use std::prelude::v1::*;

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

Structs are declared with the struct keyword:

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

They can be initialized using struct literals:

Rust code
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

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

Rust code
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:

Rust code
let v4 = Vec2 { ..v3 };

Structs, like tuples, can be destructured.

Just like this is a valid let pattern:

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

So is this:

Rust code
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:

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

let patterns can be used as conditions in if:

Rust code
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 also patterns, just like if let:

Rust code
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

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

Rust code
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:

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

You can declare methods on your own types:

Rust code
struct Number { odd: bool, value: i32, } impl Number { fn is_strictly_positive(self) -> bool { self.value > 0 } }

And use them like usual:

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

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

Rust code
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:

Rust code
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:

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

Traits are something multiple types can have in common:

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

You can implement:

These are called the "orphan rules".

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

Rust code
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):

Rust code
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:

Rust code
// 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" }

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

Rust code
impl std::ops::Neg for Number { type Output = Self; fn neg(self) -> Self { Self { value: -self.value, odd: self.odd, } } }

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:

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

And this also works:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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.

Rust code
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 methods can also take self by reference or mutable reference:

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

When invoking trait methods, the receiver is borrowed implicitly:

Rust code
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:

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

Marker traits like Copy have no methods:

Rust code
// 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:

Rust code
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:

Rust code
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. }

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

Rust code
#[derive(Clone, Copy)] struct Number { odd: bool, value: i32, } // this expands to `impl Clone for Number` and `impl Copy for Number` blocks.

Functions can be generic:

Rust code
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:

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

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

The simplest constraints are just trait names:

Rust code
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:

Rust code
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:

Rust code
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" }

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 ::

Rust code
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.

Structs can be generic too:

Rust code
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>" }

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

Rust code
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":

Rust code
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>" }

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

In fact, println is a macro:

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

This expands to something that has the same effect as:

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

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

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

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:

Rust code
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

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

Rust code
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:

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

It also panics when unwrapped and containing an error.

Variables bindings have a "lifetime":

Rust code
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:

Rust code
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:

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

A variable binding can be immutably borrowed multiple times:

Rust code
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:

Rust code
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:

Rust code
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); }

References in function arguments also have lifetimes:

Rust code
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:

Lifetimes' names start with a single quote, ':

Rust code
// 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:

Rust code
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. }

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:

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

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

Rust code
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:

Rust code
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:

Rust code
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. }

impl blocks can be generic over lifetimes too:

Rust code
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:

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

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

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

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

String literals are 'static:

Rust code
struct Person { name: &'static str, } fn main() { let p = Person { name: "fasterthanlime", }; }

But owned strings are not static:

Rust code
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:

Rust code
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

Rust code
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. }

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

Rust code
let p = Person { name: name };

It can be shortened like this:

Rust code
let p = Person { name };

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

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

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

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

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 =.

Rust code
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 apply to slices.

Rust code
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:

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

This is legal:

Rust code
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:

Rust code
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.

&str values are really slices.

Rust code
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:

Rust code
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); }

Functions that can fail typically return a Result:

Rust code
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():

Rust code
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:

Rust code
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:

Rust code
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:

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

Or you can bubble up the error:

Rust code
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:

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

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

Rust code
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:

Rust code
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 }
Rust code
// 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 }

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.

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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. }

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

This is legal:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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:

Rust code
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:

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

But this is fine:

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

Here's a closure with two arguments:

Rust code
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:

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

Here's a slightly worrying closure:

Rust code
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...

And here's a toilet closure:

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

Called thusly because |_| () looks like a toilet.

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:

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

Or a slice:

Rust code
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:

Rust code
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:

Rust code
fn main() { for c in "sHE'S brOKen" .chars() .filter(|c| c.is_uppercase() || !c.is_ascii_alphabetic()) .flat_map(|c| c.to_lowercase()) { print!("{}", c); } println!(); } // output: he's ok

You can return a closure from a function:

Rust code
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")); }

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

Rust code
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:

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

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.

For all of the intentional mistakes made above ("this code is illegal", etc.), rustc always has very good error messages and insightful suggestions.

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 tweet about Rust a lot, so if you liked this article, you know what to do.

Have fun!