Rust by Example

fn noisy_unused_function() {}

// FIXME ^ Add an attribute to suppress the warning

fn main() {

used_function();

}

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Note that in real programs, you should eliminate dead code. In these examples we'll allow dead code in some places because of the interactive nature of the examples.

Crates

The crate_type attribute can be used to tell the compiler whether a crate is a binary or a library (and even which type of library), and the crate_name attribute can be used to set the name of the crate.

However, it is important to note that both the crate_type and crate_name attributes have no effect whatsoever when using Cargo, the Rust package manager. Since Cargo is used for the majority of Rust projects, this means real-world uses of crate_type and crate_name are relatively limited.

// This crate is a library

#![crate_type = "lib"]

// The library is named "rary"

#![crate_name = "rary"]

pub fn public_function() {

println!("called rary's `public_function()`");

}

fn private_function() {

println!("called rary's `private_function()`");

}

pub fn indirect_access() {

print!("called rary's `indirect_access()`, that\n> ");

private_function();

}

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When the crate_type attribute is used, we no longer need to pass the --crate-type flag to rustc.

$ rustc lib.rs

$ ls lib*

library.rlib

cfg

Configuration conditional checks are possible through two different operators:

   • the cfg attribute: #[cfg(...)] in attribute position

   • the cfg! macro: cfg!(...) in boolean expressions

While the former enables conditional compilation, the latter conditionally evaluates to true or false literals allowing for checks at run-time. Both utilize identical argument syntax.

// This function only gets compiled if the target OS is linux

#[cfg(target_os = "linux")]

fn are_you_on_linux() {

println!("You are running linux!");

}

// And this function only gets compiled if the target OS is *not* linux

#[cfg(not(target_os = "linux"))]

fn are_you_on_linux() {

println!("You are *not* running linux!");

}

fn main() {

are_you_on_linux();

println!("Are you sure?");

if cfg!(target_os = "linux") {