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< h1 id = "条件编译-features" > < a class = "header" href = "#条件编译-features" > 条件编译 Features< / a > < / h1 >
< p > < code > Cargo Feature< / code > 是非常强大的机制，可以为大家提供< a href = "https://doc.rust-lang.org/stable/reference/conditional-compilation.html" > 条件编译< / a > 和可选依赖的高级特性。< / p >
< h2 id = "features" > < a class = "header" href = "#features" > [features]< / a > < / h2 >
< p > < code > Feature< / code > 可以通过 < code > Cargo.toml< / code > 中的 < code > [features]< / code > 部分来定义：其中每个 < code > feature< / code > 通过列表的方式指定了它所能启用的其他 < code > feature< / code > 或可选依赖。< / p >
< p > 假设我们有一个 2D 图像处理库，然后该库所支持的图片格式可以通过以下方式启用：< / p >
< pre > < code class = "language-toml" > [features]
# 定义一个 feature : webp, 但它并没有启用其它 feature
webp = []
< / code > < / pre >
< p > 当定义了 < code > webp< / code > 后，我们就可以在代码中通过 < a href = "https://doc.rust-lang.org/stable/reference/conditional-compilation.html" > < code > cfg< / code > 表达式< / a > 来进行条件编译。例如项目中的 < code > lib.rs< / code > 可以使用以下代码对 < code > webp< / code > 模块进行条件引入:< / p >
< pre > < code class = "language-toml" > #[cfg(feature = "webp")]
pub mod webp;
< / code > < / pre >
< p > < code > #[cfg(feature = "webp")]< / code > 的含义是：只有在 < code > webp< / code > feature 被定义后，以下的 < code > webp< / code > 模块才能被引入进来。由于我们之前在 < code > [features]< / code > 里定义了 < code > webp< / code > ，因此以上代码的 < code > webp< / code > 模块会被成功引入。< / p >
< p > 在 < code > Cargo.toml< / code > 中定义的 < code > feature< / code > 会被 < code > Cargo< / code > 通过命令行参数 < code > --cfg< / code > 传给 < code > rustc< / code > ，最终由后者完成编译：例如，定义 “hello” 和 “hi” 两个 < code > feature< / code > ，等价于 < code > rustc --cfg 'feature="hello"' --cfg 'feature="hi" ...'< / code > 。若项目中的代码想要测试 < code > feature< / code > 是否存在，可以使用 < a href = "https://doc.rust-lang.org/stable/reference/conditional-compilation.html#the-cfg-attribute" > < code > cfg< / code > 属性< / a > 或 < a href = "https://doc.rust-lang.org/stable/std/macro.cfg.html" > < code > cfg< / code > 宏< / a > 。< / p >
< p > 之前我们提到了一个 < code > feature< / code > 还可以开启其他 < code > feature< / code > ，举个例子，例如 ICO 图片格式包含 BMP 和 PNG 格式，因此当 < code > ico< / code > 被启用后，它还得确保启用 < code > bmp< / code > 和 < code > png< / code > ： < / p >
< pre > < code class = "language-toml" > [features]
bmp = []
png = []
ico = ["bmp", "png"]
webp = []
< / code > < / pre >
< p > 对此，我们可以理解为： < strong > < code > bmp< / code > 和 < code > png< / code > 是开启 < code > ico< / code > 的先决条件< / strong > （注：开启 < code > ico< / code > ，会自动开启 < code > bmp< / code > , < code > png< / code > ）。< / p >
< p > Feature 名称可以包含来自 < a href = "" > Unicode XID standard< / a > 定义的字母，允许使用 < code > _< / code > 或 < code > 0-9< / code > 的数字作为起始字符，在起始字符后，还可以使用 < code > -< / code > 、< code > +< / code > 或 < code > .< / code > 。< / p >
< p > 但是我们< strong > 还是推荐按照 crates.io 的方式来设置 Feature 名称< / strong > : < code > crate.io< / code > 要求名称只能由 ASCII 字母数字、< code > _< / code > 、< code > -< / code > 或 < code > +< / code > 组成。< / p >
< h2 id = "default-feature" > < a class = "header" href = "#default-feature" > default feature< / a > < / h2 >
< p > 默认情况下，所有的 < code > feature< / code > 都会被自动禁用，可以通过 < code > default< / code > 来启用它们：< / p >
< pre > < code class = "language-toml" > [features]
default = ["ico", "webp"]
bmp = []
png = []
ico = ["bmp", "png"]
webp = []
< / code > < / pre >
< p > 使用如上配置的项目被构建时，< code > default< / code > feature 首先会被启用，然后它接着启用了 < code > ico< / code > 和 < code > webp< / code > feature， < code > default< / code > ： < / p >
< ul >
< li > < code > --no-default-features< / code > 命令行参数可以禁用 < code > default< / code > feature< / li >
< li > < code > default-features = false< / code > 选项可以在依赖声明中指定< / li >
< / ul >
< blockquote >
< p > 当你要去改变某个依赖库的 < code > default< / code > 启用的 feature 列表时(例如觉得该库引入的 feature 过多，导致最终编译出的文件过大)，需要格外的小心，因为这可能会导致某些功能的缺失< / p >
< / blockquote >
< h2 id = "可选依赖" > < a class = "header" href = "#可选依赖" > 可选依赖< / a > < / h2 >
< p > 当依赖被标记为 “可选 optional” 时，意味着它默认不会被编译。假设我们的 2D 图片处理库需要用到一个外部的包来处理 GIF 图片：< / p >
< pre > < code class = "language-toml" > [dependencies]
gif = { version = "0.11.1", optional = true }
< / code > < / pre >
< p > < strong > 这种可选依赖的写法会自动定义一个与依赖同名的 feature， < code > gif< / code > feature< / strong > ，这样一来，当我们启用 < code > gif< / code > feature 时，该依赖库也会被自动引入并启用：例如通过 < code > --features gif< / code > 的方式启用 feature 。< / p >
< blockquote >
< p > 注意：目前来说，< code > [feature]< / code > 中定义的 feature 还不能与已引入的依赖库同名。但是在 < code > nightly< / code > 中已经提供了实验性的功能用于改变这一点: < a href = "https://doc.rust-lang.org/stable/cargo/reference/unstable.html#namespaced-features" > namespaced features< / a > < / p >
< / blockquote >
< p > 当然，< strong > 我们还可以通过显式定义 feature 的方式来启用这些可选依赖库< / strong > ，例如为了支持 AVIF 图片格式，我们需要引入两个依赖包，由于 < code > avif< / code > 是通过 feature 引入的可选格式，因此它依赖的两个包也必须声明为可选的:< / p >
< pre > < code class = "language-toml" > [dependencies]
ravif = { version = "0.6.3", optional = true }
rgb = { version = "0.8.25", optional = true }
[features]
avif = ["ravif", "rgb"]
< / code > < / pre >
< p > 之后，< code > avif< / code > feature 一旦被启用，那这两个依赖库也将自动被引入。< / p >
< blockquote >
< p > 注意：我们之前也讲过条件引入依赖的方法，那就是使用< a href = "https://course.rs/cargo/reference/specify-deps.html#根据平台引入依赖" > 平台相关的依赖< / a > ，与基于 feature 的可选依赖不同，它们是基于特定平台的可选依赖< / p >
< / blockquote >
< h2 id = "依赖库自身的-feature" > < a class = "header" href = "#依赖库自身的-feature" > 依赖库自身的 feature< / a > < / h2 >
< p > 就像我们的项目可以定义 < code > feature< / code > 一样，依赖库也可以定义它自己的 < code > feature< / code > ，也有需要启用的 < code > feature< / code > 列表，当引入该依赖库时，我们可以通过以下方式为其启用相关的 < code > features< / code > :< / p >
< pre > < code class = "language-toml" > [dependencies]
serde = { version = "1.0.118", features = ["derive"] }
< / code > < / pre >
< p > 以上配置为 < code > serde< / code > 依赖开启了 < code > derive< / code > feature， < code > default-features = false< / code > 来禁用依赖库的 < code > default< / code > feature :< / p >
< pre > < code class = "language-toml" > [dependencies]
flate2 = { version = "1.0.3", default-features = false, features = ["zlib"] }
< / code > < / pre >
< p > 这里我们禁用了 < code > flate2< / code > 的 < code > default< / code > feature， < code > zlib< / code > feature。< / p >
< blockquote >
< p > 注意：这种方式未必能成功禁用 < code > default< / code > ，原因是可能会有其它依赖也引入了 < code > flate2< / code > ，并且没有对 < code > default< / code > 进行禁用，那此时 < code > default< / code > 依然会被启用。< / p >
< p > 查看下文的 < a href = "#feature-同一化" > feature 同一化< / a > 获取更多信息< / p >
< / blockquote >
< p > 除此之外，还能通过下面的方式来间接开启依赖库的 feature :< / p >
< pre > < code class = "language-toml" > [dependencies]
jpeg-decoder = { version = "0.1.20", default-features = false }
[features]
# Enables parallel processing support by enabling the "rayon" feature of jpeg-decoder.
parallel = ["jpeg-decoder/rayon"]
< / code > < / pre >
< p > 如上所示，我们定义了一个 < code > parallel< / code > feature， < code > jpeg-decoder< / code > 依赖的 < code > rayon< / code > feature。< / p >
< blockquote >
< p > 注意: 上面的 “package-name/feature-name” 语法形式不仅会开启指定依赖的指定 feature， ， < / p >
< p > 在 < code > nightly< / code > 版本中，可以对这种行为进行禁用：< a href = ""package-name/feature-name"" > weak dependency features< / a > < / p >
< / blockquote >
< h2 id = "通过命令行参数启用-feature" > < a class = "header" href = "#通过命令行参数启用-feature" > 通过命令行参数启用 feature< / a > < / h2 >
< p > 以下的命令行参数可以启用指定的 < code > feature< / code > :< / p >
< ul >
< li > < code > --features FEATURES< / code > : 启用给出的 feature 列表，可以使用逗号或空格进行分隔，若你是在终端中使用，还需要加上双引号，例如 < code > --features "foo bar"< / code > 。 若在工作空间中构建多个 < code > package< / code > ，可以使用 < code > package-name/feature-name< / code > 为特定的成员启用 features< / li >
< li > < code > --all-features< / code > : 启用命令行上所选择的所有包的所有 features< / li >
< li > < code > --no-default-features< / code > : 对选择的包禁用 < code > default< / code > feature< / li >
< / ul >
< h2 id = "feature-同一化" > < a class = "header" href = "#feature-同一化" > feature 同一化< / a > < / h2 >
< p > < code > feature< / code > 只有在定义的包中才是唯一的，不同包之间的 < code > feature< / code > 允许同名。因此，在一个包上启用 < code > feature< / code > 不会导致另一个包的同名 < code > feature< / code > 被误启用。< / p >
< p > < strong > 当一个依赖被多个包所使用时，这些包对该依赖所设置的 < code > feature< / code > 将被进行合并，这样才能确保该依赖只有一个拷贝存在，这个过程就被称之为同一化< / strong > 。大家可以查看< a href = "https://doc.rust-lang.org/stable/cargo/reference/resolver.html#features" > 这里< / a > 了解下解析器如何对 feature 进行解析处理。< / p >
< p > 这里，我们使用 < code > winapi< / code > 为例来说明这个过程。首先，< code > winapi< / code > 使用了大量的 < code > features< / code > ；然后我们有两个包 < code > foo< / code > 和 < code > bar< / code > 分别使用了它的两个 features， ， < code > winapi< / code > 将同时启四个 features :< / p >
< img src = "https://pic2.zhimg.com/80/v2-251973b0cc83f35cd6858bf21dd00ed6_1440w.png" / >
< p > 由于这种不可控性，我们需要让 < code > 启用feature = 添加特性< / code > 这个等式成立，换而言之，< strong > 启用一个 feature 不应该导致某个功能被禁止< / strong > 。这样才能让多个包启用同一个依赖的不同 features。< / p >
< p > 例如，如果我们想可选的支持 < code > no_std< / code > 环境(不使用标准库)，那么有两种做法：< / p >
< ul >
< li > 默认代码使用标准库的，当 < code > no_std< / code > feature 启用时，禁用相关的标准库代码< / li >
< li > 默认代码使用非标准库的，当 < code > std< / code > feature 启用时，才使用标准库的代码< / li >
< / ul >
< p > 前者就是功能削减，与之相对，后者是功能添加，根据之前的内容，我们应该选择后者的做法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > #![no_std]
< span class = "boring" > fn main() {
< / span > #[cfg(feature = "std")]
extern crate std;
#[cfg(feature = "std")]
pub fn function_that_requires_std() {
// ...
}
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "彼此互斥的-feature" > < a class = "header" href = "#彼此互斥的-feature" > 彼此互斥的 feature< / a > < / h4 >
< p > 某极少数情况下， < / p >
< p > 如果实在没有办法，可以考虑增加一个编译错误来让报错更清晰:< / p >
< pre > < code class = "language-toml" > #[cfg(all(feature = "foo", feature = "bar"))]
compile_error!("feature \"foo\" and feature \"bar\" cannot be enabled at the same time");
< / code > < / pre >
< p > 当同时启用 < code > foo< / code > 和 < code > bar< / code > 时， ： < code > foo< / code > 和 < code > bar< / code > 无法同时启用。< / p >
< p > 总之，我们还是应该在设计上避免这种情况的发生，例如：< / p >
< ul >
< li > 将某个功能分割到多个包中< / li >
< li > 当冲突时，设置 feature 优先级，< a href = "https://crates.io/crates/cfg-if" > cfg-if< / a > 包可以帮助我们写出更复杂的 < code > cfg< / code > 表达式< / li >
< / ul >
< h4 id = "检视已解析的-features" > < a class = "header" href = "#检视已解析的-features" > 检视已解析的 features< / a > < / h4 >
< p > 在复杂的依赖图中，如果想要了解不同的 features 是如何被多个包多启用的，这是相当困难的。好在 < code > cargo tree< / code > 命令提供了几个选项可以帮组我们更好的检视哪些 features 被启用了:< / p >
< p > < code > cargo tree -e features< / code > ，该命令以依赖图的方式来展示已启用的 features， ： < / p >
< pre > < code class = "language-shell" > $ cargo tree -e features
test_cargo v0.1.0 (/Users/sunfei/development/rust/demos/test_cargo)
└── uuid feature "default"
├── uuid v0.8.2
└── uuid feature "std"
└── uuid v0.8.2
< / code > < / pre >
< p > < code > cargo tree -f "{p} {f}"< / code > 命令会提供一个更加紧凑的视图：< / p >
< pre > < code class = "language-shell" > $ cargo tree -f "{p} {f}"
test_cargo v0.1.0 (/Users/sunfei/development/rust/demos/test_cargo)
└── uuid v0.8.2 default,std
< / code > < / pre >
< p > < code > cargo tree -e features -i foo< / code > ，该命令会显示 < code > features< / code > 会如何“流入“指定的包 < code > foo< / code > 中:< / p >
< pre > < code class = "language-shell" > $ cargo tree -e features -i uuid
uuid v0.8.2
├── uuid feature "default"
│ └── test_cargo v0.1.0 (/Users/sunfei/development/rust/demos/test_cargo)
│ └── test_cargo feature "default" (command-line)
└── uuid feature "std"
└── uuid feature "default" (*)
< / code > < / pre >
< p > 该命令在依赖图较为复杂时非常有用，使用它可以让你了解某个依赖包上开启了哪些 < code > features< / code > 以及其中的原因。< / p >
< p > 大家可以查看官方的 < code > cargo tree< / code > < a href = "https://doc.rust-lang.org/stable/cargo/commands/cargo-tree.html" > 文档< / a > 获取更加详细的使用信息。< / p >
< h2 id = "feature-解析器-v2-版本" > < a class = "header" href = "#feature-解析器-v2-版本" > Feature 解析器 V2 版本< / a > < / h2 >
< p > 我们还能通过以下配置指定使用 V2 版本的解析器( < a href = "https://doc.rust-lang.org/stable/cargo/reference/resolver.html#resolver-versions" > resolver< / a > ):< / p >
< pre > < code class = "language-toml" > [package]
name = "my-package"
version = "1.0.0"
resolver = "2"
< / code > < / pre >
< p > V2 版本的解析器可以在某些情况下避免 feature 同一化的发生，具体的情况在< a href = "https://doc.rust-lang.org/stable/cargo/reference/resolver.html#feature-resolver-version-2" > 这里< / a > 有描述，下面做下简单的总结:< / p >
< ul >
< li > 为特定平台开启的 < code > features< / code > 且此时并没有被构建，会被忽略< / li >
< li > < code > build-dependencies< / code > 和 < code > proc-macros< / code > 不再跟普通的依赖共享 < code > features< / code > < / li >
< li > < code > dev-dependencies< / code > 的 < code > features< / code > 不会被启用，除非正在构建的对象需要它们(例如测试对象、示例对象等)< / li >
< / ul >
< p > 对于部分场景而言， < code > std< / code > feature， < code > no_std< / code > 环境，很明显，开启 < code > std< / code > 将导致错误的发生。< / p >
< p > 说完优点，我们再来看看 V2 的缺点，其中增加编译构建时间就是其中之一，原因是同一个依赖会被构建多次(每个都拥有不同的 feature 列表)。< / p >
< blockquote >
< p > 由于此部分内容可能只有极少数的用户需要，因此我们并没有对其进行扩展，如果大家希望了解更多关于 V2 的内容，可以查看< a href = "https://doc.rust-lang.org/stable/cargo/reference/features.html#feature-resolver-version-2" > 官方文档< / a > < / p >
< / blockquote >
< h2 id = "构建脚本" > < a class = "header" href = "#构建脚本" > 构建脚本< / a > < / h2 >
< p > < a href = "https://course.rs/cargo/reference/build-script/intro.html" > 构建脚本< / a > 可以通过 < code > CARGO_FEATURE_< name> < / code > 环境变量获取启用的 < code > feature< / code > 列表，其中 < code > < name> < / code > 是 feature 的名称，该名称被转换成大全写字母，且 < code > -< / code > 被转换为 < code > _< / code > 。< / p >
< h2 id = "required-features" > < a class = "header" href = "#required-features" > required-features< / a > < / h2 >
< p > 该字段可以用于禁用特定的 Cargo Target： < a href = "https://course.rs/cargo/reference/cargo-target.html#required-features" > 这里< / a > 获取更多信息。< / p >
< h2 id = "semver-兼容性" > < a class = "header" href = "#semver-兼容性" > SemVer 兼容性< / a > < / h2 >
< p > 启用一个 feature 不应该引入一个不兼容 SemVer 的改变。例如，启用的 feature 不应该改变现有的 API， < a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html" > 官方文档< / a > 。< / p >
< p > 总之，在新增/移除 feature 或可选依赖时，你需要小心，因此这些可能会造成向后不兼容性。更多信息参见< a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo" > 这里< / a > ，简单总结如下：< / p >
< ul >
< li > 在发布 < code > minor< / code > 版本时，以下通常是安全的:
< ul >
< li > < a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-feature-add" > 新增 feature< / a > 或< a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-dep-add" > 可选依赖< / a > < / li >
< li > < a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-change-dep-feature" > 修改某个依赖的 features< / a > < / li >
< / ul >
< / li >
< li > 在发布 < code > minor< / code > 版本时，以下操作应该避免：
< ul >
< li > < a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-feature-remove" > 移除 feature< / a > 或< a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-remove-opt-dep" > 可选依赖< / a > < / li >
< li > < a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-remove-opt-dep" > 将现有的公有代码放在某个 feature 之后< / a > < / li >
< li > < a href = "https://doc.rust-lang.org/stable/cargo/reference/semver.html#cargo-feature-remove-another" > 从 feature 列表中移除一个 feature< / a > < / li >
< / ul >
< / li >
< / ul >
< h2 id = "feature-文档和发现" > < a class = "header" href = "#feature-文档和发现" > feature 文档和发现< / a > < / h2 >
< p > 将你的项目支持的 feature 信息写入到文档中是非常好的选择:< / p >
< ul >
< li > 我们可以通过在 < code > lib.rs< / code > 的顶部添加< a href = "https://course.rs/basic/comment.html#文档注释" > 文档注释< / a > 的方式来实现。例如 < code > regex< / code > 就是< a href = "https://github.com/rust-lang/regex/blob/1.4.2/src/lib.rs#L488-L583" > 这么做的< / a > 。< / li >
< li > 若项目拥有一个用户手册，那也可以在那里添加说明，例如 < a href = "https://github.com/rust-lang/regex/blob/1.4.2/src/lib.rs#L488-L583" > serde.rs< / a > 。< / li >
< li > 若项目是二进制类型(可运行的应用服务，包含 < code > fn main< / code > 入口)，可以将说明放在 < code > README< / code > 文件或其他文档中，例如 < a href = "https://github.com/mozilla/sccache/blob/0.2.13/README.md#build-requirements" > sccache< / a > 。< / li >
< / ul >
< p > 特别是对于不稳定的或者不该再被使用的 feature 而言，它们更应该被放在文档中进行清晰的说明。< / p >
< p > 当构建发布到 < code > docs.rs< / code > 上的文档时，会使用 < code > Cargo.toml< / code > 中的元数据来控制哪些 features 会被启用。查看 < a href = "https://docs.rs/about/metadata" > docs.rs 文档< / a > 获取更多信息。< / p >
< h4 id = "如何发现-features" > < a class = "header" href = "#如何发现-features" > 如何发现 features< / a > < / h4 >
< p > 若依赖库的文档中对其使用的 < code > features< / code > 做了详细描述，那你会更容易知道他们使用了哪些 < code > features< / code > 以及该如何使用。< / p >
< p > 当依赖库的文档没有相关信息时，你也可以通过源码仓库的 < code > Cargo.toml< / code > 文件来获取，但是有些时候，使用这种方式来跟踪并获取全部相关的信息是相当困难的。< / p >
< / main >
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