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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 href="../../rust-2018/trait-system/index.html"><strong aria-hidden="true">3.5.</strong> Trait system</a></li><li><ol class="section"><li><a href="../../rust-2018/trait-system/impl-trait-for-returning-complex-types-with-ease.html"><strong aria-hidden="true">3.5.1.</strong> impl Trait for returning complex types with ease</a></li><li><a href="../../rust-2018/trait-system/dyn-trait-for-trait-objects.html"><strong aria-hidden="true">3.5.2.</strong> dyn Trait for trait objects</a></li><li><a href="../../rust-2018/trait-system/more-container-types-support-trait-objects.html"><strong aria-hidden="true">3.5.3.</strong> More container types support trait objects</a></li><li><a href="../../rust-2018/trait-system/associated-constants.html"><strong aria-hidden="true">3.5.4.</strong> Associated constants</a></li><li><a href="../../rust-2018/trait-system/no-anon-params.html"><strong aria-hidden="true">3.5.5.</strong> No more anonymous parameters</a></li></ol></li><li><a 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" class="active"><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"><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 book"</a></li><li><a href="../../rust-2018/documentation/the-rust-bookshelf.html"><strong aria-hidden="true">3.14.2.</strong> The Rust Bookshelf</a></li><li><a href="../../rust-2018/documentation/the-rustonomicon.html"><strong aria-hidden="true">3.14.3.</strong> The Rustonomicon</a></li><li><a href="../../rust-2018/documentation/std-os-has-documentation-for-all-platforms.html"><strong aria-hidden="true">3.14.4.</strong> Full documentation for std::os</a></li></ol></li><li><a href="../../rust-2018/rustdoc/index.html"><strong aria-hidden="true">3.15.</strong> rustdoc</a></li><li><ol class="section"><li><a href="../../rust-2018/rustdoc/documentation-tests-can-now-compile-fail.html"><strong aria-hidden="true">3.15.1.</strong> Documentation tests can now compile-fail</a></li><li><a href="../../rust-2018/rustdoc/rustdoc-uses-commonmark.html"><strong aria-hidden="true">3.15.2.</strong> Rustdoc uses 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fully static binaries</a></li><li><a href="../../rust-2018/platform-and-target-support/cdylib-crates-for-c-interoperability.html"><strong aria-hidden="true">3.16.6.</strong> cdylib crates for C interoperability</a></li></ol></li></ol></li></ol>
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<a class="header" href="#lifetime-elision-in-impl" id="lifetime-elision-in-impl"><h1>Lifetime elision in impl</h1></a>
<p><img src="https://img.shields.io/badge/Minimum%20Rust%20Version-1.31-brightgreen.svg" alt="Minimum Rust version: 1.31" /></p>
<p>When writing <code>impl</code> blocks, you can now elide lifetime annotations in some
situations.</p>
<p>Consider a trait like <code>MyIterator</code>:</p>
<pre><code class="language-rust ignore">trait MyIterator {
type Item;
fn next(&mut self) -> Option<Self::Item>;
}
</code></pre>
<p>In Rust 2015, if we wanted to implement this iterator for mutable references
to <code>Iterators</code>, we'd need to write this:</p>
<pre><code class="language-rust ignore">impl<'a, I: MyIterator> MyIterator for &'a mut I {
type Item = I::Item;
fn next(&mut self) -> Option<Self::Item> {
(*self).next()
}
}
</code></pre>
<p>Note all of the <code>'a</code> annotations. In Rust 2018, we can write this:</p>
<pre><code class="language-rust ignore">impl<I: MyIterator> MyIterator for &mut I {
type Item = I::Item;
fn next(&mut self) -> Option<Self::Item> {
(*self).next()
}
}
</code></pre>
<p>Similarly, lifetime annotations can appear due to a struct that contains
references:</p>
<pre><code class="language-rust ignore">struct SetOnDrop<'a, T> {
borrow: &'a mut T,
value: Option<T>,
}
</code></pre>
<p>In Rust 2015, to implement <code>Drop</code> on this struct, we'd write:</p>
<pre><code class="language-rust ignore">impl<'a, T> Drop for SetOnDrop<'a, T> {
fn drop(&mut self) {
if let Some(x) = self.value.take() {
*self.borrow = x;
}
}
}
</code></pre>
<p>But in Rust 2018, we can combine elision with <a href="the-anonymous-lifetime.html">the anonymous lifetime</a> and
write this instead.</p>
<pre><code class="language-rust ignore">impl<T> Drop for SetOnDrop<'_, T> {
fn drop(&mut self) {
if let Some(x) = self.value.take() {
*self.borrow = x;
}
}
}
</code></pre>
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