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aria-hidden="true">5.2.</strong> Type and lifetime parameters</a></li><li><a href="items/associated-items.html"><strong aria-hidden="true">5.3.</strong> Associated Items</a></li><li><a href="visibility-and-privacy.html"><strong aria-hidden="true">5.4.</strong> Visibility and Privacy</a></li><li><a href="attributes.html"><strong aria-hidden="true">5.5.</strong> Attributes</a></li></ol></li><li><a href="statements-and-expressions.html"><strong aria-hidden="true">6.</strong> Statements and expressions</a></li><li><ol class="section"><li><a href="statements.html"><strong aria-hidden="true">6.1.</strong> Statements</a></li><li><a href="expressions.html"><strong aria-hidden="true">6.2.</strong> Expressions</a></li><li><ol class="section"><li><a href="expressions/literal-expr.html"><strong aria-hidden="true">6.2.1.</strong> Literal expressions</a></li><li><a href="expressions/path-expr.html"><strong aria-hidden="true">6.2.2.</strong> Path expressions</a></li><li><a href="expressions/block-expr.html"><strong aria-hidden="true">6.2.3.</strong> Block expressions</a></li><li><a href="expressions/operator-expr.html"><strong aria-hidden="true">6.2.4.</strong> Operator expressions</a></li><li><a href="expressions/grouped-expr.html"><strong aria-hidden="true">6.2.5.</strong> Grouped expressions</a></li><li><a href="expressions/array-expr.html"><strong aria-hidden="true">6.2.6.</strong> Array and index expressions</a></li><li><a href="expressions/tuple-expr.html"><strong aria-hidden="true">6.2.7.</strong> Tuple and index expressions</a></li><li><a href="expressions/struct-expr.html"><strong aria-hidden="true">6.2.8.</strong> Struct expressions</a></li><li><a href="expressions/enum-variant-expr.html"><strong aria-hidden="true">6.2.9.</strong> Enum variant expressions</a></li><li><a href="expressions/call-expr.html"><strong aria-hidden="true">6.2.10.</strong> Call expressions</a></li><li><a href="expressions/method-call-expr.html"><strong 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aria-hidden="true">8.</strong> Special types and traits</a></li><li><a href="memory-model.html"><strong aria-hidden="true">9.</strong> Memory model</a></li><li><ol class="section"><li><a href="memory-allocation-and-lifetime.html"><strong aria-hidden="true">9.1.</strong> Memory allocation and lifetime</a></li><li><a href="memory-ownership.html"><strong aria-hidden="true">9.2.</strong> Memory ownership</a></li><li><a href="variables.html"><strong aria-hidden="true">9.3.</strong> Variables</a></li></ol></li><li><a href="linkage.html"><strong aria-hidden="true">10.</strong> Linkage</a></li><li><a href="unsafety.html"><strong aria-hidden="true">11.</strong> Unsafety</a></li><li><ol class="section"><li><a href="unsafe-functions.html"><strong aria-hidden="true">11.1.</strong> Unsafe functions</a></li><li><a href="unsafe-blocks.html"><strong aria-hidden="true">11.2.</strong> Unsafe blocks</a></li><li><a href="behavior-considered-undefined.html"><strong aria-hidden="true">11.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">11.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="influences.html">Appendix: Influences</a></li><li class="affix"><a href="undocumented.html">Appendix: As-yet-undocumented Features</a></li><li class="affix"><a href="glossary.html">Appendix: Glossary</a></li></ol>
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<a class="header" href="trait-bounds.html#trait-and-lifetime-bounds" id="trait-and-lifetime-bounds"><h1>Trait and lifetime bounds</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>TypeParamBounds</em> :<br />
<em>TypeParamBound</em> ( <code>+</code> <em>TypeParamBound</em> )<sup>*</sup> <code>+</code><sup>?</sup></p>
<p><em>TypeParamBound</em> :<br />
<em>Lifetime</em> | <em>TraitBound</em></p>
<p><em>TraitBound</em> :<br />
<code>?</code><sup>?</sup>
<a href="trait-bounds.html#higher-ranked-trait-bounds"><em>ForLifetimes</em></a><sup>?</sup> <a href="paths.html"><em>TraitPath</em></a><br />
| <code>(</code> <code>?</code><sup>?</sup>
<a href="trait-bounds.html#higher-ranked-trait-bounds"><em>ForLifetimes</em></a><sup>?</sup> <a href="paths.html"><em>TraitPath</em></a> <code>)</code></p>
<p><em>LifetimeBounds</em> :<br />
( <em>Lifetime</em> <code>+</code> )<sup>*</sup> <em>Lifetime</em><sup>?</sup></p>
<p><em>Lifetime</em> :<br />
<a href="tokens.html#lifetimes-and-loop-labels">LIFETIME_OR_LABEL</a><br />
| <code>'static</code><br />
| <code>'_</code></p>
</blockquote>
<p><a href="items/traits.html#trait-bounds">Trait</a> and lifetime bounds provide a way for <a href="items/generics.html">generic items</a> to
restrict which types and lifetimes are used as their parameters. Bounds can be
provided on any type in a <a href="items/generics.html#where-clauses">where clause</a>. There are also shorter forms for
certain common cases:</p>
<ul>
<li>Bounds written after declaring a <a href="items/generics.html">generic parameter</a>:
<code>fn f<A: Copy>() {}</code> is the same as <code>fn f<A> where A: Copy () {}</code>.</li>
<li>In trait declarations as <a href="items/traits.html#supertraits">supertraits</a>: <code>trait Circle : Shape {}</code> is
equivalent to <code>trait Circle where Self : Shape {}</code>.</li>
<li>In trait declarations as bounds on <a href="items/associated-items.html#associated-types">associated types</a>:
<code>trait A { type B: Copy; }</code> is equivalent to
<code>trait A where Self::B: Copy { type B; }</code>.</li>
</ul>
<p>Bounds on an item must be satisfied when using the item. When type checking and
borrow checking a generic item, the bounds can be used to determine that a
trait is implemented for a type. For example, given <code>Ty: Trait</code></p>
<ul>
<li>In the body of a generic function, methods from <code>Trait</code> can be called on <code>Ty</code>
values. Likewise associated constants on the <code>Trait</code> can be used.</li>
<li>Associated types from <code>Trait</code> can be used.</li>
<li>Generic functions and types with a <code>T: Trait</code> bounds can be used with <code>Ty</code>
being used for <code>T</code>.</li>
</ul>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# type Surface = i32;
trait Shape {
fn draw(&self, Surface);
fn name() -> &'static str;
}
fn draw_twice<T: Shape>(surface: Surface, sh: T) {
sh.draw(surface); // Can call method because T: Shape
sh.draw(surface);
}
fn copy_and_draw_twice<T: Copy>(surface: Surface, sh: T) where T: Shape {
let shape_copy = sh; // doesn't move sh because T: Copy
draw_twice(surface, sh); // Can use generic function because T: Shape
}
struct Figure<S: Shape>(S, S);
fn name_figure<U: Shape>(
figure: Figure<U>, // Type Figure<U> is well-formed because U: Shape
) {
println!(
"Figure of two {}",
U::name(), // Can use associated function
);
}
#}</code></pre></pre>
<p>Trait and lifetime bounds are also used to name <a href="types.html#trait-objects">trait objects</a>.</p>
<a class="header" href="trait-bounds.html#sized" id="sized"><h2><code>?Sized</code></h2></a>
<p><code>?</code> is only used to declare that the <a href="special-types-and-traits.html#sized"><code>Sized</code></a> trait may not be
implemented for a type parameter or associated type. <code>?Sized</code> may
not be used as a bound for other types.</p>
<a class="header" href="trait-bounds.html#lifetime-bounds" id="lifetime-bounds"><h2>Lifetime bounds</h2></a>
<p>Lifetime bounds can be applied to types or other lifetimes. The bound <code>'a: 'b</code>
is usually read as <code>'a</code> <em>outlives</em> <code>'b</code>. <code>'a: 'b</code> means that <code>'a</code> lasts longer
than <code>'b</code>, so a reference <code>&'a ()</code> is valid whenever <code>&'b ()</code> is valid.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn f<'a, 'b>(x: &'a i32, mut y: &'b i32) where 'a: 'b {
y = x; // &'a i32 is a subtype of &'b i32 because 'a: 'b
let r: &'b &'a i32 = &&0; // &'b &'a i32 is well formed because 'a: 'b
}
#}</code></pre></pre>
<p><code>T: 'a</code> means that all lifetime parameters of <code>T</code> outlive <code>'a</code>. For example if
<code>'a</code> is an unconstrained lifetime parameter then <code>i32: 'static</code> and
<code>&'static str: 'a</code> are satisfied but <code>Vec<&'a ()>: 'static</code> is not.</p>
<a class="header" href="trait-bounds.html#higher-ranked-trait-bounds" id="higher-ranked-trait-bounds"><h2>Higher-ranked trait bounds</h2></a>
<p>Type bounds may be <em>higher ranked</em> over lifetimes. These bounds specify a bound
is true <em>for all</em> lifetimes. For example, a bound such as <code>for<'a> &'a T: PartialEq<i32></code> would require an implementation like</p>
<pre><code class="language-rust ignore">impl<'a> PartialEq<i32> for &'a T {
// ...
}
</code></pre>
<p>and could then be used to compare a <code>&'a T</code> with any lifetime to an <code>i32</code>.</p>
<p>Only a higher-ranked bound can be used here as the lifetime of the reference is
shorter than a lifetime parameter on the function:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn call_on_ref_zero<F>(f: F) where for<'a> F: Fn(&'a i32) {
let zero = 0;
f(&zero);
}
#}</code></pre></pre>
<p>Higher-ranked lifetimes may also be specified just before the trait, the only
end of the following trait instead of the whole bound. This function is
difference is the scope of the lifetime parameter, which extends only to the
equivalent to the last one.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn call_on_ref_zero<F>(f: F) where F: for<'a> Fn(&'a i32) {
let zero = 0;
f(&zero);
}
#}</code></pre></pre>
<blockquote>
<p>Warning: lifetime bounds are allowed on lifetimes in a <code>for</code> binder, but have
no effect: <code>for<'a, 'b: 'a></code> is no different to <code>for<'a, 'b></code>.</p>
</blockquote>
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