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<ul class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong>1.</strong> Meet Safe and Unsafe</a></li><li><ul class="section"><li><a href="safe-unsafe-meaning.html"><strong>1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="working-with-unsafe.html"><strong>1.2.</strong> Working with Unsafe</a></li></ul></li><li><a href="data.html"><strong>2.</strong> Data Layout</a></li><li><ul class="section"><li><a href="repr-rust.html"><strong>2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong>2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong>2.3.</strong> Other reprs</a></li></ul></li><li><a href="ownership.html"><strong>3.</strong> Ownership</a></li><li><ul class="section"><li><a href="references.html"><strong>3.1.</strong> References</a></li><li><a href="lifetimes.html"><strong>3.2.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong>3.3.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html" class="active"><strong>3.4.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong>3.5.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong>3.6.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong>3.7.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong>3.8.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong>3.9.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong>3.10.</strong> Splitting Borrows</a></li></ul></li><li><a href="conversions.html"><strong>4.</strong> Type Conversions</a></li><li><ul class="section"><li><a href="coercions.html"><strong>4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong>4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong>4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong>4.4.</strong> Transmutes</a></li></ul></li><li><a href="uninitialized.html"><strong>5.</strong> Uninitialized Memory</a></li><li><ul class="section"><li><a href="checked-uninit.html"><strong>5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong>5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong>5.3.</strong> Unchecked</a></li></ul></li><li><a href="obrm.html"><strong>6.</strong> Ownership Based Resource Management</a></li><li><ul class="section"><li><a href="constructors.html"><strong>6.1.</strong> Constructors</a></li><li><a href="destructors.html"><strong>6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong>6.3.</strong> Leaking</a></li></ul></li><li><a href="unwinding.html"><strong>7.</strong> Unwinding</a></li><li><ul class="section"><li><a href="exception-safety.html"><strong>7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong>7.2.</strong> Poisoning</a></li></ul></li><li><a href="concurrency.html"><strong>8.</strong> Concurrency</a></li><li><ul class="section"><li><a href="races.html"><strong>8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong>8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong>8.3.</strong> Atomics</a></li></ul></li><li><a href="vec.html"><strong>9.</strong> Implementing Vec</a></li><li><ul class="section"><li><a href="vec-layout.html"><strong>9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong>9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong>9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong>9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong>9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong>9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong>9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong>9.8.</strong> RawVec</a></li><li><a href="vec-drain.html"><strong>9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong>9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong>9.11.</strong> Final Code</a></li></ul></li><li><a href="arc-and-mutex.html"><strong>10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong>11.</strong> FFI</a></li></ul>
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<a class="header" href="lifetime-elision.html#lifetime-elision" id="lifetime-elision"><h1>Lifetime Elision</h1></a>
<p>In order to make common patterns more ergonomic, Rust allows lifetimes to be
<em>elided</em> in function signatures.</p>
<p>A <em>lifetime position</em> is anywhere you can write a lifetime in a type:</p>
<pre><code class="language-rust ignore">&'a T
&'a mut T
T<'a>
</code></pre>
<p>Lifetime positions can appear as either "input" or "output":</p>
<ul>
<li>
<p>For <code>fn</code> definitions, input refers to the types of the formal arguments
in the <code>fn</code> definition, while output refers to
result types. So <code>fn foo(s: &str) -> (&str, &str)</code> has elided one lifetime in
input position and two lifetimes in output position.
Note that the input positions of a <code>fn</code> method definition do not
include the lifetimes that occur in the method's <code>impl</code> header
(nor lifetimes that occur in the trait header, for a default method).</p>
</li>
<li>
<p>In the future, it should be possible to elide <code>impl</code> headers in the same manner.</p>
</li>
</ul>
<p>Elision rules are as follows:</p>
<ul>
<li>
<p>Each elided lifetime in input position becomes a distinct lifetime
parameter.</p>
</li>
<li>
<p>If there is exactly one input lifetime position (elided or not), that lifetime
is assigned to <em>all</em> elided output lifetimes.</p>
</li>
<li>
<p>If there are multiple input lifetime positions, but one of them is <code>&self</code> or
<code>&mut self</code>, the lifetime of <code>self</code> is assigned to <em>all</em> elided output lifetimes.</p>
</li>
<li>
<p>Otherwise, it is an error to elide an output lifetime.</p>
</li>
</ul>
<p>Examples:</p>
<pre><code class="language-rust ignore">fn print(s: &str); // elided
fn print<'a>(s: &'a str); // expanded
fn debug(lvl: usize, s: &str); // elided
fn debug<'a>(lvl: usize, s: &'a str); // expanded
fn substr(s: &str, until: usize) -> &str; // elided
fn substr<'a>(s: &'a str, until: usize) -> &'a str; // expanded
fn get_str() -> &str; // ILLEGAL
fn frob(s: &str, t: &str) -> &str; // ILLEGAL
fn get_mut(&mut self) -> &mut T; // elided
fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded
fn args<T: ToCStr>(&mut self, args: &[T]) -> &mut Command // elided
fn args<'a, 'b, T: ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded
fn new(buf: &mut [u8]) -> BufWriter; // elided
fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a> // expanded
</code></pre>
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