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<h1 id="构建-build-缓存"><a class="header" href="#构建-build-缓存">构建( Build )缓存</a></h1>
<p><code>cargo build</code> 的结果会被放入项目根目录下的 <code>target</code> 文件夹中,当然,这个位置可以三种方式更改:设置 <code>CARGO_TARGET_DIR</code> <a href="https://doc.rust-lang.org/stable/cargo/reference/environment-variables.html">环境变量</a><a href="https://course.rs/cargo/reference/configuration.html#%E9%85%8D%E7%BD%AE%E6%96%87%E4%BB%B6%E6%A6%82%E8%A7%88"><code>build.target-dir</code></a> 配置项以及 <code>--target-dir</code> 命令行参数。</p>
<h2 id="target-目录结构"><a class="header" href="#target-目录结构">target 目录结构</a></h2>
<p><code>target</code> 目录的结构取决于是否使用 <code>--target</code> 标志为特定的平台构建。</p>
<h4 id="不使用---target"><a class="header" href="#不使用---target">不使用 --target</a></h4>
<p><code>--target</code> 标志没有指定,<code>Cargo</code> 会根据宿主机架构进行构建,构建结果会放入项目根目录下的 <code>target</code> 目录中,<code>target</code> 下每个子目录中包含了相应的 <a href="https://course.rs/cargo/reference/profiles.html"><code>发布配置profile</code></a> 的构建结果,例如 <code>release</code><code>debug</code> 是自带的<code>profile</code>,前者往往用于生产环境,因为会做大量的性能优化,而后者则用于开发环境,此时的编译效率和报错信息是最好的。</p>
<p>除此之外我们还可以定义自己想要的 <code>profile</code> ,例如用于测试环境的 <code>profile</code> <code>test</code>,用于预发环境的 <code>profile</code> <code>pre-prod</code> 等。</p>
<div class="table-wrapper"><table><thead><tr><th>目录</th><th>描述</th></tr></thead><tbody>
<tr><td><code>target/debug/</code></td><td>包含了 <code>dev</code> profile 的构建输出(<code>cargo build</code><code>cargo build --debug</code>)</td></tr>
<tr><td><code>target/release/</code></td><td><code>release</code> profile 的构建输出,<code>cargo build --release</code></td></tr>
<tr><td><code>target/foo/</code></td><td>自定义 <code>foo</code> profile 的构建输出,<code>cargo build --profile=foo</code></td></tr>
</tbody></table>
</div>
<p>出于历史原因:</p>
<ul>
<li><code>dev</code><code>test</code> profile 的构建结果都存放在 <code>debug</code> 目录下</li>
<li><code>release</code><code>bench</code> profile 则存放在 <code>release</code> 目录下</li>
<li>用户定义的 profile 存在同名的目录下</li>
</ul>
<h4 id="使用---target"><a class="header" href="#使用---target">使用 --target</a></h4>
<p>当使用 <code>--target XXX</code> 为特定的平台编译后,输出会放在 <code>target/XXX/</code> 目录下:</p>
<div class="table-wrapper"><table><thead><tr><th>目录</th><th>示例</th></tr></thead><tbody>
<tr><td><code>target/&lt;triple&gt;/debug/</code></td><td><code>target/thumbv7em-none-eabihf/debug/</code></td></tr>
<tr><td><code>target/&lt;triple&gt;/release/</code></td><td><code>target/thumbv7em-none-eabihf/release/</code></td></tr>
</tbody></table>
</div>
<blockquote>
<p><strong>注意:</strong>,当没有使用 <code>--target</code> 时,<code>Cargo</code> 会与构建脚本和过程宏一起共享你的依赖包,对于每个 <code>rustc</code> 命令调用而言,<a href="https://course.rs/cargo/reference/configuration.html#%E9%85%8D%E7%BD%AE%E6%96%87%E4%BB%B6%E6%A6%82%E8%A7%88"><code>RUSTFLAGS</code></a> 也将被共享。</p>
<p>而使用 <code>--target</code> 后,构建脚本、过程宏会针对宿主机的 CPU 架构进行各自构建,且不会共享 <code>RUSTFLAGS</code></p>
</blockquote>
<h4 id="target-子目录说明"><a class="header" href="#target-子目录说明">target 子目录说明</a></h4>
<p>在 profile 文件夹中(例如 <code>debug</code><code>release</code>),包含编译后的最终成果:</p>
<div class="table-wrapper"><table><thead><tr><th>目录</th><th>描述</th></tr></thead><tbody>
<tr><td><code>target/debug/</code></td><td>包含编译后的输出,例如二进制可执行文件、<a href="https://course.rs/cargo/reference/cargo-target.html#%E5%BA%93%E5%AF%B9%E8%B1%A1library">库对象( library target )</a></td></tr>
<tr><td><code>target/debug/examples/</code></td><td>包含<a href="https://course.rs/cargo/reference/cargo-target.html#%E7%A4%BA%E4%BE%8B%E5%AF%B9%E8%B1%A1examples">示例对象( example target )</a></td></tr>
</tbody></table>
</div>
<p>还有一些命令会在 <code>target</code> 下生成自己的独立目录:</p>
<div class="table-wrapper"><table><thead><tr><th>目录</th><th>描述</th></tr></thead><tbody>
<tr><td><code>target/doc/</code></td><td>包含通过 <code>cargo doc</code> 生成的文档</td></tr>
<tr><td><code>target/package/</code></td><td>包含 <code>cargo package</code><code>cargo publish</code> 生成的输出</td></tr>
</tbody></table>
</div>
<p>Cargo 还会创建几个用于构建过程的其它类型目录,它们的目录结构只应该被 Cargo 自身使用,因此可能会在未来发生变化:</p>
<div class="table-wrapper"><table><thead><tr><th>目录</th><th>描述</th></tr></thead><tbody>
<tr><td><code>target/debug/deps</code></td><td>依赖和其它输出成果</td></tr>
<tr><td><code>target/debug/incremental</code></td><td><code>rustc</code> <a href="https://course.rs/cargo/reference/profiles.html#incremental">增量编译</a>的输出,该缓存可以用于提升后续的编译速度</td></tr>
<tr><td><code>target/debug/build/</code></td><td><a href="https://course.rs/cargo/reference/build-script/intro.html">构建脚本</a>的输出</td></tr>
</tbody></table>
</div>
<h2 id="依赖信息文件"><a class="header" href="#依赖信息文件">依赖信息文件</a></h2>
<p>在每一个编译成果的旁边,都有一个依赖信息文件,文件后缀是 <code>.d</code>。该文件的语法类似于 <code>Makefile</code>,用于说明构建编译成果所需的所有依赖包。</p>
<p>该文件往往用于提供给外部的构建系统,这样它们就可以判断 <code>Cargo</code> 命令是否需要再次被执行。</p>
<p>文件中的路径默认是绝对路径,你可以通过 <a href="https://course.rs/cargo/reference/configuration.html#%E9%85%8D%E7%BD%AE%E6%96%87%E4%BB%B6%E6%A6%82%E8%A7%88"><code>build.dep-info-basedir</code></a> 配置项来修改为相对路径。</p>
<pre><code class="language-shell"># 关于 `.d` 文件的一个示例 : target/debug/foo.d
/path/to/myproj/target/debug/foo: /path/to/myproj/src/lib.rs /path/to/myproj/src/main.rs
</code></pre>
<h2 id="共享缓存"><a class="header" href="#共享缓存">共享缓存</a></h2>
<p><a href="https://github.com/mozilla/sccache">sccache</a> 是一个三方工具,可以用于在不同的工作空间中共享已经构建好的依赖包。</p>
<p>为了设置 <code>sccache</code>,首先需要使用 <code>cargo install sccache</code> 进行安装,然后在调用 <code>Cargo</code> 之前将 <code>RUSTC_WRAPPER</code> 环境变量设置为 <code>sccache</code></p>
<ul>
<li>如果用的 <code>bash</code>,可以将 <code>export RUSTC_WRAPPER=sccache</code> 添加到 <code>.bashrc</code></li>
<li>也可以使用 <a href="https://course.rs/cargo/reference/configuration.html#%E9%85%8D%E7%BD%AE%E6%96%87%E4%BB%B6%E6%A6%82%E8%A7%88"><code>build.rustc-wrapper</code></a> 配置项</li>
</ul>
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