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.
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
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") {
