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<ol class="chapter"><li class="affix"><a href="../../introduction.html">Introduction</a></li><li><a href="../../editions/index.html"><strong aria-hidden="true">1.</strong> What are editions?</a></li><li><ol class="section"><li><a href="../../editions/creating-a-new-project.html"><strong aria-hidden="true">1.1.</strong> Creating a new project</a></li><li><a href="../../editions/transitioning-an-existing-project-to-a-new-edition.html"><strong aria-hidden="true">1.2.</strong> Transitioning an existing project to a new edition</a></li></ol></li><li><a href="../../rust-2015/index.html"><strong aria-hidden="true">2.</strong> Rust 2015</a></li><li><a href="../../rust-2018/index.html"><strong aria-hidden="true">3.</strong> Rust 2018</a></li><li><ol class="section"><li><a href="../../rust-2018/edition-changes.html"><strong aria-hidden="true">3.1.</strong> 2018-Specific Changes</a></li><li><a href="../../rust-2018/module-system/index.html"><strong aria-hidden="true">3.2.</strong> Module system</a></li><li><ol class="section"><li><a href="../../rust-2018/module-system/raw-identifiers.html"><strong aria-hidden="true">3.2.1.</strong> Raw identifiers</a></li><li><a href="../../rust-2018/module-system/path-clarity.html"><strong aria-hidden="true">3.2.2.</strong> Path clarity</a></li><li><a href="../../rust-2018/module-system/more-visibility-modifiers.html"><strong aria-hidden="true">3.2.3.</strong> More visibility modifiers</a></li><li><a href="../../rust-2018/module-system/nested-imports-with-use.html"><strong aria-hidden="true">3.2.4.</strong> Nested imports with use</a></li></ol></li><li><a href="../../rust-2018/error-handling-and-panics/index.html"><strong aria-hidden="true">3.3.</strong> Error handling and panics</a></li><li><ol class="section"><li><a href="../../rust-2018/error-handling-and-panics/the-question-mark-operator-for-easier-error-handling.html"><strong aria-hidden="true">3.3.1.</strong> The ? operator for easier error handling</a></li><li><a href="../../rust-2018/error-handling-and-panics/question-mark-in-main-and-tests.html"><strong aria-hidden="true">3.3.2.</strong> ? in main and tests</a></li><li><a href="../../rust-2018/error-handling-and-panics/controlling-panics-with-std-panic.html"><strong aria-hidden="true">3.3.3.</strong> Controlling panics with std::panic</a></li><li><a href="../../rust-2018/error-handling-and-panics/aborting-on-panic.html"><strong aria-hidden="true">3.3.4.</strong> Aborting on panic</a></li></ol></li><li><a href="../../rust-2018/control-flow/index.html"><strong aria-hidden="true">3.4.</strong> Control flow</a></li><li><ol class="section"><li><a href="../../rust-2018/control-flow/loops-can-break-with-a-value.html"><strong aria-hidden="true">3.4.1.</strong> Loops can break with a value</a></li><li><a href="../../rust-2018/control-flow/async-await-for-easier-concurrency.html"><strong aria-hidden="true">3.4.2.</strong> async/await for easier concurrency</a></li></ol></li><li><a 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href="../../rust-2018/slice-patterns.html"><strong aria-hidden="true">3.6.</strong> Slice patterns</a></li><li><a href="../../rust-2018/ownership-and-lifetimes/index.html"><strong aria-hidden="true">3.7.</strong> Ownership and lifetimes</a></li><li><ol class="section"><li><a href="../../rust-2018/ownership-and-lifetimes/non-lexical-lifetimes.html"><strong aria-hidden="true">3.7.1.</strong> Non-lexical lifetimes</a></li><li><a href="../../rust-2018/ownership-and-lifetimes/default-match-bindings.html"><strong aria-hidden="true">3.7.2.</strong> Default match bindings</a></li><li><a href="../../rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html"><strong aria-hidden="true">3.7.3.</strong> '_, the anonymous lifetime</a></li><li><a href="../../rust-2018/ownership-and-lifetimes/lifetime-elision-in-impl.html"><strong aria-hidden="true">3.7.4.</strong> Lifetime elision in impl</a></li><li><a href="../../rust-2018/ownership-and-lifetimes/inference-in-structs.html"><strong aria-hidden="true">3.7.5.</strong> T: 'a inference in structs</a></li><li><a href="../../rust-2018/ownership-and-lifetimes/simpler-lifetimes-in-static-and-const.html"><strong aria-hidden="true">3.7.6.</strong> Simpler lifetimes in static and const</a></li></ol></li><li><a href="../../rust-2018/data-types/index.html"><strong aria-hidden="true">3.8.</strong> Data types</a></li><li><ol class="section"><li><a href="../../rust-2018/data-types/field-init-shorthand.html"><strong aria-hidden="true">3.8.1.</strong> Field init shorthand</a></li><li><a href="../../rust-2018/data-types/inclusive-ranges.html"><strong aria-hidden="true">3.8.2.</strong> ..= for inclusive ranges</a></li><li><a href="../../rust-2018/data-types/128-bit-integers.html"><strong aria-hidden="true">3.8.3.</strong> 128 bit integers</a></li><li><a href="../../rust-2018/data-types/operator-equals-are-now-implementable.html"><strong aria-hidden="true">3.8.4.</strong> "Operator-equals" are now implementable</a></li><li><a href="../../rust-2018/data-types/union-for-an-unsafe-form-of-enum.html"><strong aria-hidden="true">3.8.5.</strong> union for an unsafe form of enum</a></li><li><a href="../../rust-2018/data-types/choosing-alignment-with-the-repr-attribute.html"><strong aria-hidden="true">3.8.6.</strong> Choosing alignment with the repr attribute</a></li></ol></li><li><a href="../../rust-2018/simd-for-faster-computing.html"><strong aria-hidden="true">3.9.</strong> SIMD for faster computing</a></li><li><a href="../../rust-2018/macros/index.html"><strong aria-hidden="true">3.10.</strong> Macros</a></li><li><ol class="section"><li><a href="../../rust-2018/macros/custom-derive.html"><strong aria-hidden="true">3.10.1.</strong> Custom Derive</a></li><li><a href="../../rust-2018/macros/macro-changes.html"><strong aria-hidden="true">3.10.2.</strong> Macro changes</a></li><li><a href="../../rust-2018/macros/at-most-once.html"><strong aria-hidden="true">3.10.3.</strong> At most one repetition</a></li></ol></li><li><a href="../../rust-2018/the-compiler/index.html"><strong aria-hidden="true">3.11.</strong> The compiler</a></li><li><ol class="section"><li><a href="../../rust-2018/the-compiler/improved-error-messages.html"><strong aria-hidden="true">3.11.1.</strong> Improved error messages</a></li><li><a href="../../rust-2018/the-compiler/incremental-compilation-for-faster-compiles.html"><strong aria-hidden="true">3.11.2.</strong> Incremental Compilation for faster compiles</a></li><li><a href="../../rust-2018/the-compiler/an-attribute-for-deprecation.html"><strong aria-hidden="true">3.11.3.</strong> An attribute for deprecation</a></li></ol></li><li><a href="../../rust-2018/rustup-for-managing-rust-versions.html"><strong aria-hidden="true">3.12.</strong> Rustup for managing Rust versions</a></li><li><a href="../../rust-2018/cargo-and-crates-io/index.html"><strong aria-hidden="true">3.13.</strong> Cargo and crates.io</a></li><li><ol class="section"><li><a href="../../rust-2018/cargo-and-crates-io/cargo-check-for-faster-checking.html" class="active"><strong aria-hidden="true">3.13.1.</strong> cargo check for faster checking</a></li><li><a href="../../rust-2018/cargo-and-crates-io/cargo-install-for-easy-installation-of-tools.html"><strong aria-hidden="true">3.13.2.</strong> cargo install for easy installation of tools</a></li><li><a href="../../rust-2018/cargo-and-crates-io/cargo-new-defaults-to-a-binary-project.html"><strong aria-hidden="true">3.13.3.</strong> cargo new defaults to a binary project</a></li><li><a href="../../rust-2018/cargo-and-crates-io/cargo-rustc-for-passing-arbitrary-flags-to-rustc.html"><strong aria-hidden="true">3.13.4.</strong> cargo rustc for passing arbitrary flags to rustc</a></li><li><a href="../../rust-2018/cargo-and-crates-io/cargo-workspaces-for-multi-package-projects.html"><strong aria-hidden="true">3.13.5.</strong> Cargo workspaces for multi-package projects</a></li><li><a href="../../rust-2018/cargo-and-crates-io/multi-file-examples.html"><strong aria-hidden="true">3.13.6.</strong> Multi-file examples</a></li><li><a href="../../rust-2018/cargo-and-crates-io/replacing-dependencies-with-patch.html"><strong aria-hidden="true">3.13.7.</strong> Replacing dependencies with patch</a></li><li><a href="../../rust-2018/cargo-and-crates-io/cargo-can-use-a-local-registry-replacement.html"><strong aria-hidden="true">3.13.8.</strong> Cargo can use a local registry replacement</a></li><li><a href="../../rust-2018/cargo-and-crates-io/crates-io-disallows-wildcard-dependencies.html"><strong aria-hidden="true">3.13.9.</strong> Crates.io disallows wildcard dependencies</a></li></ol></li><li><a href="../../rust-2018/documentation/index.html"><strong aria-hidden="true">3.14.</strong> Documentation</a></li><li><ol class="section"><li><a href="../../rust-2018/documentation/new-editions-of-the-book.html"><strong aria-hidden="true">3.14.1.</strong> New editions of the "the 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<a class="header" href="#cargo-check-for-faster-checking" id="cargo-check-for-faster-checking"><h1><code>cargo check</code> for faster checking</h1></a>
<p><img src="https://img.shields.io/badge/Minimum%20Rust%20Version-1.16-brightgreen.svg" alt="Minimum Rust version: 1.16" /></p>
<p><code>cargo check</code> is a new subcommand should speed up the development
workflow in many cases.</p>
<p>What does it do? Let's take a step back and talk about how <code>rustc</code> compiles
your code. Compilation has many "passes", that is, there are many distinct
steps that the compiler takes on the road from your source code to producing
the final binary. However, you can think of this process in two big steps:
first, <code>rustc</code> does all of its safety checks, makes sure your syntax is
correct, all that stuff. Second, once it's satisfied that everything is in
order, it produces the actual binary code that you end up executing.</p>
<p>It turns out that that second step takes a lot of time. And most of the time,
it's not neccesary. That is, when you're working on some Rust code, many
developers will get into a workflow like this:</p>
<ol>
<li>Write some code.</li>
<li>Run <code>cargo build</code> to make sure it compiles.</li>
<li>Repeat 1-2 as needed.</li>
<li>Run <code>cargo test</code> to make sure your tests pass.</li>
<li>Try the binary yourself</li>
<li>GOTO 1.</li>
</ol>
<p>In step two, you never actually run your code. You're looking for feedback
from the compiler, not to actually run the binary. <code>cargo check</code> supports
exactly this use-case: it runs all of the compiler's checks, but doesn't
produce the final binary. To use it:</p>
<pre><code class="language-console">$ cargo check
</code></pre>
<p>where you may normally <code>cargo build</code>. The workflow now looks like:</p>
<ol>
<li>Write some code.</li>
<li>Run <code>cargo check</code> to make sure it compiles.</li>
<li>Repeat 1-2 as needed.</li>
<li>Run <code>cargo test</code> to make sure your tests pass.</li>
<li>Run <code>cargo build</code> to build a binary and try it yourself</li>
<li>GOTO 1.</li>
</ol>
<p>So how much speedup do you actually get? Like most performance related
questions, the answer is "it depends." Here are some very un-scientific
benchmarks at the time of writing.</p>
<table><thead><tr><th> build </th><th> performance </th><th> check performance </th><th> speedup </th></tr></thead><tbody>
<tr><td> initial compile </td><td> 11s </td><td> 5.6s </td><td> 1.96x </td></tr>
<tr><td> second compile (no changes) </td><td> 3s </td><td> 1.9s </td><td> 1.57x </td></tr>
<tr><td> third compile with small change </td><td> 5.8s </td><td> 3s </td><td> 1.93x </td></tr>
</tbody></table>
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