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<h1 id="构建脚本-build-scripts"><a class="header" href="#构建脚本-build-scripts">构建脚本( Build Scripts)</a></h1>
<p>一些项目希望编译第三方的非 Rust 代码,例如 C 依赖库;一些希望链接本地或者基于源码构建的 C 依赖库;还有一些项目需要功能性的工具,例如在构建之间执行一些代码生成的工作等。</p>
<p>对于这些目标社区已经提供了一些工具来很好的解决Cargo 并不想替代它们但是为了给用户带来一些便利Cargo 提供了自定义构建脚本的方式,来帮助用户更好的解决类似的问题。</p>
<h2 id="buildrs"><a class="header" href="#buildrs">build.rs</a></h2>
<p>若要创建构建脚本,我们只需在项目的根目录下添加一个 <code>build.rs</code> 文件即可。这样一来, Cargo 就会先编译和执行该构建脚本,然后再去构建整个项目。</p>
<p>以下是一个非常简单的脚本示例:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
// 以下代码告诉 Cargo ,一旦指定的文件 `src/hello.c` 发生了改变,就重新运行当前的构建脚本
println!(&quot;cargo:rerun-if-changed=src/hello.c&quot;);
// 使用 `cc` 来构建一个 C 文件,然后进行静态链接
cc::Build::new()
.file(&quot;src/hello.c&quot;)
.compile(&quot;hello&quot;);
}</code></pre></pre>
<p>关于构建脚本的一些使用场景如下:</p>
<ul>
<li>构建 C 依赖库</li>
<li>在操作系统中寻找指定的 C 依赖库</li>
<li>根据某个说明描述文件生成一个 Rust 模块</li>
<li>执行一些平台相关的配置</li>
</ul>
<p>下面的部分我们一起来看看构建脚本具体是如何工作的,然后在<a href="https://course.rs/cargo/reference/build-script/examples.html">下个章节</a>中还提供了一些关于如何编写构建脚本的示例。</p>
<blockquote>
<p>Note: <a href="https://course.rs/cargo/reference/manifest.html#build"><code>package.build</code></a> 可以用于改变构建脚本的名称,或者直接禁用该功能</p>
</blockquote>
<h2 id="构建脚本的生命周期"><a class="header" href="#构建脚本的生命周期">构建脚本的生命周期</a></h2>
<p>在项目被构建之前Cargo 会将构建脚本编译成一个可执行文件,然后运行该文件并执行相应的任务。</p>
<p>在运行的过程中,<strong>脚本可以使用之前 <code>println</code> 的方式跟 Cargo 进行通信</strong>:通信内容是以 <code>cargo:</code> 开头的格式化字符串。</p>
<p>需要注意的是Cargo 也不是每次都会重新编译构建脚本,只有当脚本的内容或依赖发生变化时才会。默认情况下,任何文件变化都会触发重新编译,如果你希望对其进行定制,可以使用 <code>rerun-if</code>命令,后文会讲。</p>
<p>在构建脚本成功执行后,我们的项目就会开始进行编译。如果构建脚本的运行过程中发生错误,脚本应该通过返回一个非 0 码来立刻退出,在这种情况下,构建脚本的输出会被打印到终端中。</p>
<h2 id="构建脚本的输入"><a class="header" href="#构建脚本的输入">构建脚本的输入</a></h2>
<p>我们可以通过<a href="https://doc.rust-lang.org/stable/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts">环境变量</a>的方式给构建脚本提供一些输入值,除此之外,构建脚本所在的当前目录也可以。</p>
<h2 id="构建脚本的输出"><a class="header" href="#构建脚本的输出">构建脚本的输出</a></h2>
<p>构建脚本如果会产出文件,那么这些文件需要放在统一的目录中,该目录可以通过 <a href="https://doc.rust-lang.org/stable/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts"><code>OUT_DIR</code> 环境变量</a>来指定,<strong>构建脚本不应该修改该目录之外的任何文件!</strong></p>
<p>在之前提到过,构建脚本可以通过 <code>println!</code> 输出内容跟 Cargo 进行通信Cargo 会将每一行带有 <code>cargo:</code> 前缀的输出解析为一条指令,其它的输出内容会自动被忽略。</p>
<p>通过 <code>println!</code> 输出的内容在构建过程中默认是隐藏的,如果大家想要在终端中看到这些内容,你可以使用 <code>-vv</code> 来调用,以下 <code>build.rs</code> </p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
println!(&quot;hello, build.rs&quot;);
}</code></pre></pre>
<p>将输出:</p>
<pre><code class="language-shell">$ cargo run -vv
[study_cargo 0.1.0] hello, build.rs
</code></pre>
<p>构建脚本打印到标准输出 <code>stdout</code> 的所有内容将保存在文件 <code>target/debug/build/&lt;pkg&gt;/output</code> 中 (具体的位置可能取决于你的配置)<code>stderr</code> 的输出内容也将保存在同一个目录中。</p>
<p>以下是 Cargo 能识别的通信指令以及简介,如果大家希望深入了解每个命令,可以点击具体的链接查看官方文档的说明。</p>
<ul>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rerun-if-changed"><code>cargo:rerun-if-changed=PATH</code></a> — 当指定路径的文件发生变化时Cargo 会重新运行脚本</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rerun-if-env-changed"><code>cargo:rerun-if-env-changed=VAR</code></a> — 当指定的环境变量发生变化时Cargo 会重新运行脚本</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-arg"><code>cargo:rustc-link-arg=FLAG</code></a> 将自定义的 flags 传给 linker用于后续的基准性能测试 benchmark、 可执行文件 binary,、<code>cdylib</code> 包、示例和测试</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-arg-bin"><code>cargo:rustc-link-arg-bin=BIN=FLAG</code></a> 自定义的 flags 传给 linker用于可执行文件 <code>BIN</code></li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-arg-bins"><code>cargo:rustc-link-arg-bins=FLAG</code></a> 自定义的 flags 传给 linker用于可执行文件</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-arg-tests"><code>cargo:rustc-link-arg-tests=FLAG</code></a> 自定义的 flags 传给 linker用于测试</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-arg-examples"><code>cargo:rustc-link-arg-examples=FLAG</code></a> 自定义的 flags 传给 linker用于示例</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-arg-benches"><code>cargo:rustc-link-arg-benches=FLAG</code></a> 自定义的 flags 传给 linker用于基准性能测试 benchmark</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-cdylib-link-arg"><code>cargo:rustc-cdylib-link-arg=FLAG</code></a> — 自定义的 flags 传给 linker用于 <code>cdylib</code></li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-lib"><code>cargo:rustc-link-lib=[KIND=]NAME</code></a> — 告知 Cargo 通过 <code>-l</code> 去链接一个指定的库,往往用于链接一个本地库,通过 FFI</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-search"><code>cargo:rustc-link-search=[KIND=]PATH</code></a> — 告知 Cargo 通过 <code>-L</code> 将一个目录添加到依赖库的搜索路径中</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-flags"><code>cargo:rustc-flags=FLAGS</code></a> — 将特定的 flags 传给编译器</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-cfg"><code>cargo:rustc-cfg=KEY[=&quot;VALUE&quot;]</code></a> — 开启编译时 <code>cfg</code> 设置</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-env"><code>cargo:rustc-env=VAR=VALUE</code></a> — 设置一个环境变量</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#cargo-warning"><code>cargo:warning=MESSAGE</code></a> — 在终端打印一条 warning 信息</li>
<li><a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#the-links-manifest-key"><code>cargo:KEY=VALUE</code></a><code>links</code> 脚本使用的元数据</li>
</ul>
<h2 id="构建脚本的依赖"><a class="header" href="#构建脚本的依赖">构建脚本的依赖</a></h2>
<p>构建脚本也可以引入其它基于 Cargo 的依赖包,只需要在 <code>Cargo.toml</code> 中添加或修改以下内容:</p>
<pre><code class="language-toml">[build-dependencies]
cc = &quot;1.0.46&quot;
</code></pre>
<p>需要这么配置的原因在于构建脚本无法使用通过 <code>[dependencies]</code><code>[dev-dependencies]</code> 引入的依赖包,因为构建脚本的编译运行过程跟项目本身的编译过程是分离的的,且前者先于后者发生。同样的,我们项目也无法使用 <code>[build-dependencies]</code> 中的依赖包。</p>
<p><strong>大家在引入依赖的时候,需要仔细考虑它会给编译时间、开源协议和维护性等方面带来什么样的影响</strong>。如果你在 <code>[build-dependencies]</code><code>[dependencies]</code> 引入了同样的包,这种情况下 Cargo 也许会对依赖进行复用,也许不会,例如在交叉编译时,如果不会,那编译速度自然会受到不小的影响。</p>
<h2 id="links"><a class="header" href="#links">links</a></h2>
<p><code>Cargo.toml</code> 中可以配置 <code>package.links</code> 选项,它的目的是告诉 Cargo 当前项目所链接的本地库,同时提供了一种方式可以在项目构建脚本之间传递元信息。</p>
<pre><code class="language-toml">[package]
# ...
links = &quot;foo&quot;
</code></pre>
<p>以上配置表明项目链接到一个 <code>libfoo</code> 本地库,当使用 <code>links</code> 时,项目必须拥有一个构建脚本,并且该脚本需要使用 <a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#rustc-link-lib"><code>rustc-link-lib</code></a> 指令来链接目标库。</p>
<p>Cargo 要求一个本地库最多只能被一个项目所链接,换而言之,你无法让两个项目链接到同一个本地库,但是有一种方法可以降低这种限制,感兴趣的同学可以看看<a href="https://doc.rust-lang.org/stable/cargo/reference/build-scripts.html#-sys-packages">官方文档</a></p>
<p>假设 A 项目的构建脚本生成任意数量的 kv 形式的元数据,那这些元数据将传递给 A 用作依赖包的项目的构建脚本。例如,如果包 <code>bar</code> 依赖于 <code>foo</code>,当 <code>foo</code> 生成 <code>key=value</code> 形式的构建脚本元数据时,那么 <code>bar</code> 的构建脚本就可以通过环境变量的形式使用该元数据:<code>DEP_FOO_KEY=value</code></p>
<p>需要注意的是,该元数据只能传给直接相关者,对于间接的,例如依赖的依赖,就无能为力了。</p>
<h2 id="覆盖构建脚本"><a class="header" href="#覆盖构建脚本">覆盖构建脚本</a></h2>
<p><code>Cargo.toml</code> 设置了 <code>links</code> 时, Cargo 就允许我们使用自定义库对现有的构建脚本进行覆盖。在 <a href="https://course.rs/cargo/reference/configuration.html">Cargo 使用的配置文件</a>中添加以下内容:</p>
<pre><code class="language-toml">[target.x86_64-unknown-linux-gnu.foo]
rustc-link-lib = [&quot;foo&quot;]
rustc-link-search = [&quot;/path/to/foo&quot;]
rustc-flags = &quot;-L /some/path&quot;
rustc-cfg = ['key=&quot;value&quot;']
rustc-env = {key = &quot;value&quot;}
rustc-cdylib-link-arg = [&quot;&quot;]
metadata_key1 = &quot;value&quot;
metadata_key2 = &quot;value&quot;
</code></pre>
<p>增加这个配置后,在未来,一旦我们的某个项目声明了它链接到 <code>foo</code> ,那项目的构建脚本将不会被编译和运行,替代的是这里的配置将被使用。</p>
<p><code>warning</code>, <code>rerun-if-changed</code><code>rerun-if-env-changed</code> 这三个 key 在这里不应该被使用,就算用了也会被忽略。</p>
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