Remarks#

Rust is a systems programming language designed for safety, speed, and concurrency. Rust has numerous compile-time features and safety checks to avoid data races and common bugs, all with minimal to zero runtime overhead.

Versions#

Stable

Beta

Advanced usage of println!

println! (and its sibling, print!) provides a convenient mechanism for producing and printing text that contains dynamic data, similar to the printf family of functions found in many other languages. Its first argument is a format string, which dictates how the other arguments should be printed as text. The format string may contain placeholders (enclosed in {}) to specify that a substitution should occur:

// No substitution -- the simplest kind of format string
println!("Hello World");
// Output: Hello World

// The first {} is substituted with a textual representation of
// the first argument following the format string. The second {}
// is substituted with the second argument, and so on.
println!("{} {} {}", "Hello", true, 42);
// Output: Hello true 42

At this point, you may be asking: How did println! know to print the boolean value true as the string “true”? {} is really an instruction to the formatter that the value should be converted to text using the Display trait. This trait is implemented for most primitive Rust types (strings, numbers, booleans, etc.), and is meant for “user-facing output”. Hence, the number 42 will be printed in decimal as 42, and not, say, in binary, which is how it is stored internally.

How do we print types, then, that do not implement Display, examples being Slices ([i32]), vectors (Vec<i32>), or options (Option<&str>)? There is no clear user-facing textual representation of these (i.e. one you could trivially insert into a sentence). To facilitate the printing of such values, Rust also has the Debug trait, and the corresponding {:?} placeholder. From the documentation: ”Debug should format the output in a programmer-facing, debugging context.” Let’s see some examples:

println!("{:?}", vec!["a", "b", "c"]);
// Output: ["a", "b", "c"]

println!("{:?}", Some("fantastic"));
// Output: Some("fantastic")

println!("{:?}", "Hello");
// Output: "Hello"
// Notice the quotation marks around "Hello" that indicate
// that a string was printed.

Debug also has a built-in pretty-print mechanism, which you can enable by using the # modifier after the colon:

println!("{:#?}", vec![Some("Hello"), None, Some("World")]);
// Output: [
//    Some(
//        "Hello"
//    ),
//    None,
//    Some(
//        "World"
//    )
// ]

The format strings allow you to express fairly complex substitutions:

// You can specify the position of arguments using numerical indexes.
println!("{1} {0}", "World", "Hello");
// Output: Hello World

// You can use named arguments with format
println!("{greeting} {who}!", greeting="Hello", who="World");
// Output: Hello World

// You can mix Debug and Display prints:
println!("{greeting} {1:?}, {0}", "and welcome", Some(42), greeting="Hello");
// Output: Hello Some(42), and welcome

println! and friends will also warn you if you are trying to do something that won’t work, rather than crashing at runtime:

// This does not compile, since we don't use the second argument.
println!("{}", "Hello World", "ignored");

// This does not compile, since we don't give the second argument.
println!("{} {}", "Hello");

// This does not compile, since Option type does not implement Display
println!("{}", Some(42));

At their core, the Rust printing macros are simply wrappers around the format! macro, which allows constructing a string by stitching together textual representations of different data values. Thus, for all the examples above, you can substitute println! for format! to store the formatted string instead of printing it:

let x: String = format!("{} {}", "Hello", 42);
assert_eq!(x, "Hello 42");

Console output without macros

// use Write trait that contains write() function
use std::io::Write;

fn main() {
    std::io::stdout().write(b"Hello, world!\n").unwrap();
}

• The std::io::Write trait is designed for objects which accept byte streams. In this case, a handle to standard output is acquired with std::io::stdout().

• Write::write() accepts a byte slice (&[u8]), which is created with a byte-string literal (b"<string>"). Write::write() returns a Result<usize, IoError>, which contains either the number of bytes written (on success) or an error value (on failure).

• The call to Result::unwrap() indicates that the call is expected to succeed (Result<usize, IoError> -> usize), and the value is discarded.

Minimal example

To write the traditional Hello World program in Rust, create a text file called hello.rs containing the following source code:

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

This defines a new function called main, which takes no parameters and returns no data. This is where your program starts execution when run. Inside it, you have a println!, which is a macro that prints text into the console.

To generate a binary application, invoke the Rust compiler by passing it the name of the source file:

$ rustc hello.rs

The resulting executable will have the same name as the main source module, so to run the program on a Linux or MacOS system, run:

$ ./hello
Hello World!

On a Windows system, run:

C:\Rust> hello.exe
Hello World!

Getting started

Installing

Before you can do anything using the Rust programming language, you’re going to need to acquire it—either for Windows or by using your terminal on Unix-like systems, where $ symbolizes input into the terminal:

$ curl https://sh.rustup.rs -sSf | sh

This will retrieve the required files and set up the latest version of Rust for you, no matter what system you’re on. For more information, see project page.

Note: Some Linux distributions (e.g. Arch Linux) provide rustup as a package, which can be installed instead. And although many Unix-like systems provide rustc and cargo as separate packages, it is still recommended to use rustup instead since it makes it much easier to manage multiple release channels and do cross-compilation.

Rust Compiler

We can now check to see if Rust was in fact successfully installed onto our computers by running the following command either in our terminal—if on UNIX—or the command prompt—if on Windows:

$ rustc --version

Should this command be successful, the version of Rust’s compiler installed onto our computers will be displayed before our eyes.

Cargo

With Rust comes Cargo, which is a build tool used for managing your Rust packages and projects. To make sure this, too, is present on your computer, run the following inside the console—console referring to either terminal or command prompt depending on what system you’re on:

$ cargo --version

Just like the equivalent command for the Rust compiler, this will return and display the current version of Cargo.

To create your first Cargo project, you may head to Cargo.

Alternatively, you could compile programs directly using rustc as shown in Minimal example.