% Conditional Compilation 条件编译

Rust has a special attribute, #[cfg], which allows you to compile codebased on a flag passed to the compiler. It has two forms:

Rust语言有一个特殊的属性——#[cfg]——它允许你基于一个标记参数来编译代码。它有两种形式：

#[cfg(foo)]
# fn foo() {}

#[cfg(bar = "baz")]
# fn bar() {}

They also have some helpers:

他们有一些辅助项

#[cfg(any(unix, windows))]
# fn foo() {}

#[cfg(all(unix, target_pointer_width = "32"))]
# fn bar() {}

#[cfg(not(foo))]
# fn not_foo() {}

These can nest arbitrarily:

他们可以任意嵌套：

#[cfg(any(not(unix), all(target_os="macos", target_arch = "powerpc")))]
# fn foo() {}

As for how to enable or disable these switches, if you’re using Cargo,they get set in the [features] section of your Cargo.toml:

那么当你使用Cargo的时候，如何开启和禁用这些开关呢？他们被设定在你的Cargo.toml文件的[features] section 配置块中。

[features]
# no features by default
default = []

# The “secure-password” feature depends on the bcrypt package.
secure-password = ["bcrypt"]

When you do this, Cargo passes along a flag to rustc:

当你做这么做时，Cargo传递给rustc一个标记：

--cfg feature="${feature_name}"

The sum of these cfg flags will determine which ones get activated, and therefore, which code gets compiled. Let’s take this code:

这些cfg标记汇总起来将决定哪些代码能够被激活，因此那部分代码得以被编译。让我们看下面的代码：

#[cfg(feature = "foo")]
mod foo {
}

If we compile it with cargo build --features "foo", it will send the --cfg feature="foo" flag to rustc, and the output will have the mod foo in it.If we compile it with a regular cargo build, no extra flags get passed on,and so, no foo module will exist.

如果我们使用cargo build --features "foo"来编译它，它将会给tustc添加一个--cfg feature="foo"的标记，并且在输出信息中有mod foo。如果我们使用普通的cargo build来编译它，那么就没有给rustc添加任何标记，因此也就没有foo存在。

cfg_attr

You can also set another attribute based on a cfg variable with cfg_attr:

你同样可以使用cfg_attr给cfg变量设置额外的属性值。

#[cfg_attr(a, b)]
# fn foo() {}

Will be the same as #[b] if a is set by cfg attribute, and nothing otherwise.

如果a被cfg属性设置，#[b]也将同样被设计，否则反之。

cfg!

The cfg! syntax extension lets you use these kinds of flags elsewhere in your code, too:

cfg!语法扩展也能让你在代码中其他地方使用这种标记：

if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
    println!("Think Different!");
}

These will be replaced by a true or false at compile-time, depending on the configuration settings.

根据不同的配置设置，在编译时，这些标记被true或者false替代。