Attributes

https://doc.rust-lang.org/book/attributes.html

#[hoge]という記法をattributeというものらしい。

#[cfg(target_os = "macos")]  // macosだけで動作する
fn only() {
    println!("mac");
}

#[cfg(target_os = "linux")] // linuxだけで動作する
fn only() {
    println!("linux");
}

#[test] // cargo testで動作する
fn hoge() {
    only();
}

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

$ cargo test
    Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/4_27_attributes-d9c4b09d37979797

running 1 test
test hoge ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

$ cargo run
   Compiling 4_27_attributes v0.1.0 (file:///Users/miura/work/git-work/exercises/The_Rust_Programming_Language/4_27_attributes)
    Finished debug [unoptimized + debuginfo] target(s) in 0.31 secs
     Running `target/debug/4_27_attributes`
Hello, world!
mac