rust 条件编译

条件编译可以通过两种不同的操作符实现，如下：

-   cfg属性：在属性位置中使用#[cfg(...)]

-   cfg！宏：在布尔表达式中使用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") {
        println!("Yes. It's definitely linux!");
    } else {
        println!("Yes. It's definitely *not* linux!");
    }
}

[root@bogon cfg]# ls
Cargo.lock  Cargo.toml  README.md  src
[root@bogon cfg]# cargo build
   Compiling hello_world v0.1.0 (/data2/rust/cfg)
    Finished dev [unoptimized + debuginfo] target(s) in 0.39s
[root@bogon cfg]# cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/hello_world`
You are running linux!
Are you sure?
Yes. It's definitely linux!
[root@bogon cfg]#

//main.rs
#[cfg(some_condition)]
fn conditional_function() {
    println!("condition met!");
}

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

[root@bogon src]# rustc --cfg some_condition main.rs
[root@bogon src]# ./main
condition met!
Hello, world!
[root@bogon src]# rustc  main.rs
error[E0425]: cannot find function `conditional_function` in this scope
 --> main.rs:8:5
  |
8 |     conditional_function();
  |     ^^^^^^^^^^^^^^^^^^^^ not found in this scope

error: aborting due to previous error

For more information about this error, try `rustc --explain E0425`.
[root@bogon src]#