Rust (programming language)

Mozilla Foundation headquarters in Mountain View, California

Rust grew out of a personal project begun in 2006 by Mozilla Research employee Graydon Hoare. Mozilla began sponsoring the project in 2009 as a part of the ongoing development of an experimental browser engine called Servo.[21] The project was officially announced by Mozilla in 2010.[22][23] During the same year, work had shifted from the initial compiler written in OCaml to a self-hosting compiler based on LLVM written in Rust. The new Rust compiler successfully compiled itself in 2011.[21]

Rust's type system changed considerably between versions 0.2, 0.3, and 0.4. Version 0.2 introduced classes for the first time,[24] and version 0.3 added destructors and polymorphism through the use of interfaces.[25] In Rust 0.4, traits were added as a means to provide inheritance; interfaces were unified with traits and removed as a separate feature. Classes were also removed and replaced by a combination of implementations and structured types.[26] Along with conventional static typing, before version 0.4, Rust also supported typestate analysis through contracts. It was removed in release 0.4, though the same functionality can be achieved by leveraging Rust's type system.[27]

In January 2014, the editor-in-chief of Dr. Dobb's Journal, Andrew Binstock, commented on Rust's chances of becoming a competitor to C++ in addition to the languages D, Go, and Nim (then Nimrod). According to Binstock, while Rust was "widely viewed as a remarkably elegant language", adoption slowed because it repeatedly changed between versions.[28] The first stable release, Rust 1.0, was announced on May 15, 2015.[29][30]

In August 2020, Mozilla laid off 250 of its 1,000 employees worldwide as part of a corporate restructuring caused by the long-term impact of the COVID-19 pandemic.[31][32] The team behind Servo, a browser engine written in Rust, was completely disbanded. The event raised concerns about the future of Rust, as some members of the team were active contributors to Rust.[33] In the following week, the Rust Core Team acknowledged the severe impact of the layoffs and announced that plans for a Rust foundation were underway. The first goal of the foundation would be to take ownership of all trademarks and domain names, and take financial responsibility for their costs.[34]

On February 8, 2021, the formation of the Rust Foundation was announced by its five founding companies (AWS, Huawei, Google, Microsoft, and Mozilla).[35][36] In a blog post published on April 6, 2021, Google announced support for Rust within Android Open Source Project as an alternative to C/C++.[37][38]

On November 22, 2021, the Moderation team, responsible for enforcing community standards and the Code of Conduct, announced their resignation "in protest of the Core Team placing themselves unaccountable to anyone but themselves."[39] In May 2022, the Rust core team, other leads, and certain members of the Rust Foundation board sent out a statement with governance reforms in response to the incident.[40]

Below is a "Hello, World!" program in Rust. The println! macro prints the message to standard output.[41]

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

In Rust, blocks of code are delimited by curly brackets, and control flow is annotated with keywords such as if, else, while, and for.[42][43] Pattern matching is provided using the match keyword.[44] In the examples below, explanations are given in comments, which start with //.[45][43]

fn main() {
    let mut values = vec![1, 2, 3, 4];

    for value in &values {
        println!("value = {}", value);
    }

    if values.len() > 5 {
        println!("List is longer than five items");
    }

    // Pattern matching
    match values.len() {
        0 => println!("Empty"),
        1 => println!("One value"),
        2..=10 => println!("Between two and ten values"),
        11 => println!("Eleven values"),
        _ => println!("Many values"),
    };

    // while loop with predicate and pattern matching using let
    while let Some(value) = values.pop() {
        println!("value = {value}"); // using curly braces to format a local variable
    }
}

Despite the syntactic resemblance to C and C++,[46][47] Rust's design was more significantly influenced by functional programming languages.[48] For example, nearly every part of a function body is an expression, even control flow operators.[43] The ordinary if expression also takes the place of C's ternary conditional. A function does not need to end with a return expression: if the semicolon is omitted, the value of the last expression in the function will be used as the return value,[49] as seen in the following recursive implementation of the factorial function:

fn factorial(i: u64) -> u64 {
    if i == 0 {
        1
    } else {
        i * factorial(i - 1)
    }
}

The following iterative implementation uses the ..= operator to create an inclusive range:

fn factorial(i: u64) -> u64 {
    (2..=i).product()
}

Rust is strongly typed and statically typed, where all types of variables must be known during compilation, and assigning a value of a different type to a variable will result in a compilation error. The default integer type is i32, and the default floating point type is f64. If the type of a literal number is not explicitly provided, either it is inferred from the context or the default type is used.[50][43]

Unlike other languages, Rust does not use null pointers to indicate a lack of data, as doing so can lead to accidental dereferencing. Therefore, in order to uphold its safety guarantees, it is impossible to dereference null pointers unless the code block is manually checked and explicitly declared unsafe through the use of an unsafe block.[52] Rust instead uses an Option type, which has two variants, Some(T) (which indicates that a value is present) and None (analogous to the null pointer).[53] Option implements a "null pointer optimization" avoiding any overhead for types which cannot have a null value (references or the NonZero types for example). Option values must be handled using syntactic sugar such as the if let construction in order to access the inner value (in this case, a string):[54]

fn main() {
    let name: Option<String> = None;
    // If name was not None, it would print here.
    if let Some(name) = name {
        println!("{}", name);
    }
}

More advanced features in Rust include the use of generic functions to achieve polymorphism.[55] The following is a Rust program to calculate the sum of two things, for which addition is implemented using a generic function:

use std::ops::Add;

// sum is a generic function with one type parameter, T
fn sum<T>(num1: T, num2: T) -> T
where   
    T: Add<Output = T>,  // T must implement the Add trait where addition returns another T
{
    num1 + num2  // num1 + num2 is syntactic sugar for num1.add(num2) provided by the Add trait
}

fn main() {
    let result1 = sum(10, 20);
    println!("Sum is: {}", result1); // Sum is: 30

    let result2 = sum(10.23, 20.45);
    println!("Sum is: {}", result2); // Sum is: 30.68
}

Generics in Rust use trait bounds for their generic parameters, which precisely define what is required of a type in order to be used with a given generic function. This allows generics in Rust to be type-checked at compile time, without having to know the exact types.[55]

Rust uses an ownership system, where each value has a unique owner. Values are moved between different owners through assignment or passing a value as a function parameter. Functions taking ownership of a parameter are said to consume the value in question. Ownership in Rust is similar to move semantics in C++; unlike C++, however, Rust keeps track of moved values and all types are movable by default. This means that types in Rust do not have to implement an "empty" or "invalid" state (and corresponding runtime checks) to signify that a value has been moved out of, because using a moved value is a compile-time error in Rust:[56]

fn print_string(s: String) {
    println!("{}", s);
}

fn main() {
    let s = String::from("Hello, World");
    print_string(s);  // s consumed by print_string
    // s has been moved, so cannot be used any more
    // another print_string(s);  would result in a compile error
}

Lifetimes are a usually implicit part of all reference types in Rust. Each particular lifetime encompasses a set of locations in the code for which a variable is valid. The borrow checker in the Rust compiler uses lifetimes to ensure that the values pointed to by a reference remains valid. It also ensures that a mutable reference only exists if no immutable references exist at the same time.[55]

Rust defines the relationship between the lifetimes of the objects used and created by functions as part of their signature using lifetime parameters. In languages like C or C++, the information about which objects have to outlive others is informally specified in documentation or comments (if at all), and cannot be checked by the compiler.[55]

When a stack variable or temporary goes out of scope, it is dropped by running its destructor, which may be implicitly defined or a programmer-defined drop function. The latter can be used for design patterns akin to resource acquisition is initialization (RAII), in which resources, like file descriptors or network sockets, are tied to the lifetime of an object: When the object is dropped, the resource is closed.[57][58]

The example below parses some configuration options from a string and creates a struct containing the options. The struct only contains references to the data, so for the struct to remain valid, the data referred to by the struct needs to be valid as well. The function signature for parse_config specifies this relationship explicitly. In this example, the explicit lifetimes are unnecessary in newer Rust versions due to lifetime elision, which is an algorithm that automatically assigns lifetimes to functions if they are trivial.[55]

use std::collections::HashMap;

#[derive(Debug)]
// This struct has one lifetime parameter, 'src. The name is only used within the struct's definition.
struct Config<'src> {
    hostname: &'src str,
    username: &'src str,
}

// This function also has a lifetime parameter, 'cfg. 'cfg is attached to the "config" parameter, which
// establishes that the data in "config" lives at least as long as the 'cfg lifetime.
// The returned struct also uses 'cfg for its lifetime, so it can live at most as long as 'cfg.
fn parse_config<'cfg>(config: &'cfg str) -> Config<'cfg> {
    let key_values: HashMap<_, _> = config
        .lines()
        .filter(|line| !line.starts_with('#'))
        .filter_map(|line| line.split_once('='))
        .map(|(key, value)| (key.trim(), value.trim()))
        .collect();
    Config {
        hostname: key_values["hostname"],
        username: key_values["username"],
    }
}

fn main() {
    let config = parse_config(
        r#"hostname = foobar
username=barfoo"#,
    );
    println!("Parsed config: {:#?}", config);
}

Rust is designed to be memory safe. It does not permit null pointers, dangling pointers, or data races.[60][61][62] Data values can be initialized only through a fixed set of forms, all of which require their inputs to be already initialized.[63] To replicate pointers being either valid or NULL, such as in linked list or binary tree data structures, the Rust core library provides an option type, which can be used to test whether a pointer has Some value or None.[61] Rust has added syntax to manage lifetimes, which are checked at compile time by the borrow checker. Unsafe code can subvert some of these restrictions using the unsafe keyword.[52] Unsafe code may also be used for low-level functionality like volatile memory access, architecture-specific intrinsics, type punning, and inline assembly.[54]^{: 139, 376–379, 395}

Rust does not use automated garbage collection. Memory and other resources are managed through the resource acquisition in initialization convention,[64] with optional reference counting. Rust provides deterministic management of resources, with very low overhead.[65] Values are allocated on the stack by default and all dynamic allocations must be explicit.[66]

The built-in reference types using the & symbol do not involve run-time reference counting. The safety and validity of the underlying pointers is verified at compile time, preventing dangling pointers and other forms of undefined behavior. Rust's type system separates shared, immutable references of the form &T from unique, mutable references of the form &mut T. A mutable reference can be coerced to an immutable reference, but not vice versa.[56][67]

Rust's type system supports a mechanism called traits, inspired by type classes in the Haskell language. Traits annotate types and are used to define shared behavior between different types. For example, floats and integers both implement the Add trait because they can both be added; and any type that can be printed out as a string implements the Display or Debug traits. This facility is known as ad hoc polymorphism.[55]

Rust uses type inference for variables declared with the keyword let. Such variables do not require a value to be initially assigned to determine their type. A compile time error results if any branch of code leaves the variable without an assignment.[68] Variables assigned multiple times must be marked with the keyword mut (short for mutable).[43]

A function can be given generic parameters, which allows the same function to be applied to different types. Generic functions can constrain the generic type to implement a particular trait or traits; for example, an "add one" function might require the type to implement "Add". This means that a generic function can be type-checked as soon as it is defined. The implementation of Rust generics is similar to the typical implementation of C++ templates: a separate copy of the code is generated for each instantiation. This is called monomorphization and contrasts with the type erasure scheme typically used in Java and Haskell. Rust's type erasure is also available by using the keyword dyn (short for dynamic) .The benefit of monomorphization is optimized code for each specific use case; the drawback is increased compile time and size of the resulting binaries.[55]

In Rust, user-defined types are created with the struct or enum keywords. The struct keyword denotes a record type.[69] enums are kinds of algebraic data types commonly found in functional programming languages. These types can contain fields of other types.[53] The impl keyword can define methods for the types (data and functions are defined separately) or implement a trait for the types.[69] Traits are used to restrict generic parameters and because traits can provide a type with more methods than the user defined. For example, the trait Iterator requires that the next method be defined for the type. Once the next method is defined the trait provides common functional helper methods over the iterator like map or filter.[55]

Type aliases, including generic arguments, can also be defined with the type keyword.[52]

The type system within Rust is based around implementations, traits and structured types. Implementations fulfill a role similar to that of classes within other languages and are defined with the keyword impl. Traits provide inheritance and polymorphism; they allow methods to be defined and mixed in to implementations. Structured types are used to define fields. Implementations and traits cannot define fields themselves, and only traits can provide inheritance. Among other benefits, this prevents the diamond problem of multiple inheritance, as in C++. In other words, Rust supports interface inheritance but replaces implementation inheritance with composition; see composition over inheritance.[69][55]

Rust traits are implemented using static dispatch, meaning that the type of all values is known at compile time; however, Rust also uses a feature known as trait objects to accomplish dynamic dispatch (also known as duck typing).[70] Dynamically dispatched trait objects are declared using the syntax Box<dyn Tr> where Tr is a trait. For example, it is possible to create a list of objects which each can be printed out as follows: let v: Vec<Box<dyn Display>> = vec![Box::new(3), Box::new(5.0), Box::new("hi")].[70] Trait objects are dynamically sized; however, prior to the 2018 edition, the dyn keyword was optional.[71] A trait object is essentially a fat pointer that include a pointer as well as additional information about what type the pointer is.[72]

It is possible to extend the Rust language using macros.

A declarative macro (also called a "macro by example") is a macro that uses pattern matching to determine its expansion.[52]

Procedural macros use Rust functions that are compiled before other components to run and modify the compiler's input token stream. They are generally more flexible than declarative macros, but are more difficult to maintain due to their complexity.[73][52]

Procedural macros come in three flavors:

• Function-like macros custom!(...)
• Derive macros #[derive(CustomDerive)]
• Attribute macros #[custom_attribute]

The println! macro is an example of a function-like macro and serde_derive[74] is a commonly used library for generating code for reading and writing data in many formats such as JSON. Attribute macros are commonly used for language bindings such as the extendr library for Rust bindings to R.[75]

The following code shows the use of the Serialize, Deserialize and Debug derive procedural macros to implement JSON reading and writing as well as the ability to format a structure for debugging.

use serde_json::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let point = Point { x: 1, y: 2 };

    let serialized = serde_json::to_string(&point).unwrap();
    println!("serialized = {}", serialized);

    let deserialized: Point = serde_json::from_str(&serialized).unwrap();
    println!("deserialized = {:?}", deserialized);
}

Rust has a foreign function interface (FFI) that can be used both to call code written in languages such as C from Rust and to call Rust code from those languages. Rust also has a library, CXX, for calling to or from C++.[76] Rust and C differ in how they lay out structs in memory, so Rust structs may be given a #[repr(C)] attribute, forcing the same layout as the equivalent C struct.[77]

The Rust standard library is split into three crates: core, alloc, and std. When a project is annotated with the crate-level attribute #![no_std], the std crate is excluded.[79]

Screenshot of crates.io in June 2022

Cargo is Rust's build system and package manager. Cargo downloads, compiles, distributes, and uploads packages, called crates, maintained in the official registry. Cargo also acts as a front-end for Clippy and other Rust components.[12]

By default, Cargo sources its dependencies from the user-contributed registry crates.io, but Git repositories and crates in the local filesystem and other external sources can be specified as dependencies, too.[80]

Rustfmt is a code formatter for Rust. It takes Rust source code as input and changes the whitespace and indentation to produce code formatted in accordance to a common style unless specified otherwise. Rustfmt can be invoked as a standalone program or on a Rust project through Cargo.[81]

Example output of Clippy on a hello world Rust program

Clippy is Rust's built-in linting tool to improve the correctness, performance, and readability of Rust code. It was created in 2014[82] and named after the eponymous Microsoft Office feature.[83] As of 2021, Clippy has more than 450 rules,[84] which can be browsed online and filtered by category.[85][81]

Following Rust 1.0, new features are developed in nightly versions which release on a daily basis. During each release cycle of six weeks, changes on nightly versions are released to beta, while changes from the previous beta version are released to a new stable version.[86]

Every three years, a new "edition" is produced. Editions are released to provide an easy reference point for changes due to the frequent nature of Rust's train release schedule, and to provide a window to make limited breaking changes. Editions are largely compatible and migration to a new edition is trivialized with automated tooling.[87]

The most popular language server for Rust is rust-analyzer.[81] The original language server, RLS was officially deprecated in favor of rust-analyzer in July 2022.[88] These projects provide IDEs and text editors with more information about a Rust project, with basic features including autocompletion, and display of compilation errors while editing.[81]

Early homepage of Mozilla's Servo browser engine

• Firefox has two projects written in Rust: the Servo parallel browser engine[110] developed by Mozilla in collaboration with Samsung;[111] and Quantum, which is composed of several sub-projects for improving Mozilla's Gecko browser engine.[112]
• OpenDNS uses Rust in some of its internal projects.[113]
• Deno, a secure runtime for JavaScript and TypeScript, is built with V8, Rust, and Tokio.[114]
• Amazon Web Services has multiple projects written in Rust, including Firecracker, a virtualization solution,[115] and Bottlerocket, a Linux distribution and containerization solution.[116]
• Cloudflare's implementations of the QUIC protocol and firewall rules are written in Rust.[117][118]
• Arti is a Rust implementation of Tor server in Rust by The Tor Project.[119]

exa, a command-line alternative to ls, written in Rust

• Redox is a "full-blown Unix-like operating system" including a microkernel written in Rust.[120]
• Theseus, an experimental operating system described as having "intralingual design": leveraging Rust's programming language mechanisms for implementing the OS.[121]
• The Google Fuchsia capability-based operating system has components written in Rust,[122] including a TCP/IP library. [123]
• Stratis is a file system manager written in Rust for Fedora[124] and RHEL.[125]
• exa is a Unix/Linux command line alternative to ls written in Rust.
• Rust for Linux is a patch series started in 2021 for adding Rust support to the Linux kernel.[126]

Ruffle, a web emulator for Adobe Flash SWF files

• Discord uses Rust for portions of its backend, as well as client-side video encoding,[104] to augment the core infrastructure written in Elixir.[127]
• Microsoft Azure IoT Edge, a platform used to run Azure services and artificial intelligence on IoT devices, has components implemented in Rust.[128]
• Polkadot is an open source blockchain platform and cryptocurrency written in Rust.[129]
• Ruffle is an open-source SWF emulator written in Rust.[130]
• TerminusDB, an open source graph database designed for collaboratively building and curating knowledge graphs, is written in Prolog and Rust.[131]

Rust's official website lists online forums, messaging platforms, and in-person meetups for the Rust community.[133] Conferences dedicated to Rust development include:

• RustConf: an annual conference in Portland, Oregon. Held annually since 2016 (except in 2020 and 2021 because of the COVID-19 pandemic).[134]
• Rust Belt Rust: a #rustlang conference in the Rust Belt[135]
• RustFest: Europe's @rustlang conference[136]
• RustCon Asia[137]
• Rust LATAM[138]
• Oxidize Global[139]

Rust Foundation

Formation	February 8, 2021 (2021-02-08)
Founders	Amazon Web Services Google Huawei Microsoft Mozilla Foundation
Type	Nonprofit organization
Location	United States
Chairperson	Shane Miller
Executive Director	Rebecca Rumbul
Website	foundation.rust-lang.org

The Rust Foundation is a non-profit membership organization incorporated in United States, with the primary purposes of backing the technical project as a legal entity and helping to manage the trademark and infrastructure assets.[140][47]

It was established on February 8, 2021, with five founding corporate members (Amazon Web Services, Huawei, Google, Microsoft, and Mozilla).[141] The foundation's board is chaired by Shane Miller.[142] Starting in late 2021, its Executive Director and CEO is Rebecca Rumbul.[143] Prior to this, Ashley Williams was interim executive director.[144]

The Rust project is composed of teams that are responsible for different subareas of the development. For example, the Core team is responsible for "Managing the overall direction of Rust, subteam leadership, and any cross-cutting issues," the Compiler team is responsible for "Developing and managing compiler internals and optimizations," and the Language team is responsible for "Designing and helping to implement new language features," according to the official website.[145][146]