有条件地编译内联汇编

使用条件编译来确保代码仅针对预期的指令集（例如 x86）进行编译。否则，如果为其他体系结构（如 ARM 处理器）编译程序，则代码可能会无效。

#![feature(asm)]

// Any valid x86 code is valid for x86_64 as well. Be careful
// not to write x86_64 only code while including x86 in the 
// compilation targets!
#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
fn do_nothing() {
    unsafe {
        asm!("NOP");
    }
}

#[cfg(not(any(target_arch = "x86", target_arch = "x86_64"))]
fn do_nothing() {
    // This is an alternative implementation that doesn't use any asm!
    // calls. Therefore, it should be safe to use as a fallback.
}