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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" class="active"><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"><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 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<a class="header" href="#union-for-an-unsafe-form-of-enum" id="union-for-an-unsafe-form-of-enum"><h1><code>union</code> for an unsafe form of <code>enum</code></h1></a>
<p><img src="https://img.shields.io/badge/Minimum%20Rust%20Version-1.19-brightgreen.svg" alt="Minimum Rust version: 1.19" /></p>
<p>Rust now supports <code>unions</code>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
union MyUnion {
f1: u32,
f2: f32,
}
#}</code></pre></pre>
<p>Unions are kind of like enums, but they are “untagged”. Enums have a “tag”
that stores which variant is the correct one at runtime; unions don't have
this tag.</p>
<p>Since we can interpret the data held in the union using the wrong variant and
Rust can’t check this for us, that means reading a union’s field is unsafe:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# union MyUnion {
# f1: u32,
# f2: f32,
# }
let mut u = MyUnion { f1: 1 };
u.f1 = 5;
let value = unsafe { u.f1 };
#}</code></pre></pre>
<p>Pattern matching works too:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# union MyUnion {
# f1: u32,
# f2: f32,
# }
fn f(u: MyUnion) {
unsafe {
match u {
MyUnion { f1: 10 } => { println!("ten"); }
MyUnion { f2 } => { println!("{}", f2); }
}
}
}
#}</code></pre></pre>
<p>When are unions useful? One major use-case is interoperability with C. C APIs
can (and depending on the area, often do) expose unions, and so this makes
writing API wrappers for those libraries significantly easier. Additionally,
unions also simplify Rust implementations of space-efficient or
cache-efficient structures relying on value representation, such as
machine-word-sized unions using the least-significant bits of aligned
pointers to distinguish cases.</p>
<p>There’s still more improvements to come. For now, unions can only include
<code>Copy</code> types and may not implement <code>Drop</code>. We expect to lift these
restrictions in the future.</p>
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