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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="notation.html"><strong aria-hidden="true">1.</strong> Notation</a></li><li><a href="lexical-structure.html"><strong aria-hidden="true">2.</strong> Lexical structure</a></li><li><ol class="section"><li><a href="input-format.html"><strong aria-hidden="true">2.1.</strong> Input format</a></li><li><a href="keywords.html"><strong aria-hidden="true">2.2.</strong> Keywords</a></li><li><a href="identifiers.html"><strong aria-hidden="true">2.3.</strong> Identifiers</a></li><li><a href="comments.html"><strong aria-hidden="true">2.4.</strong> Comments</a></li><li><a href="whitespace.html"><strong aria-hidden="true">2.5.</strong> Whitespace</a></li><li><a href="tokens.html"><strong aria-hidden="true">2.6.</strong> Tokens</a></li><li><a href="paths.html"><strong aria-hidden="true">2.7.</strong> Paths</a></li></ol></li><li><a href="macros.html"><strong aria-hidden="true">3.</strong> 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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="items/generics.html#type-and-lifetime-parameters" id="type-and-lifetime-parameters"><h1>Type and Lifetime Parameters</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>Generics</em> :<br />
<code><</code> <em>GenericParams</em> <code>></code></p>
<p><em>GenericParams</em> :<br />
<em>LifetimeParams</em><br />
| ( <em>LifetimeParam</em> <code>,</code> )<sup>*</sup> <em>TypeParams</em></p>
<p><em>LifetimeParams</em> :<br />
( <em>LifetimeParam</em> <code>,</code> )<sup>*</sup> <em>LifetimeParam</em><sup>?</sup></p>
<p><em>LifetimeParam</em> :<br />
<a href="attributes.html"><em>OuterAttribute</em></a><sup>?</sup> <a href="tokens.html#lifetimes-and-loop-labels">LIFETIME_OR_LABEL</a> <code>:</code> <a href="trait-bounds.html"><em>LifetimeBounds</em></a><sup>?</sup></p>
<p><em>TypeParams</em>:<br />
( <em>TypeParam</em> <code>,</code> )<sup>*</sup> <em>TypeParam</em> <sup>?</sup></p>
<p><em>TypeParam</em> :<br />
<a href="attributes.html"><em>OuterAttribute</em></a><sup>?</sup> <a href="identifiers.html">IDENTIFIER</a> ( <code>:</code> <a href="trait-bounds.html"><em>TypeParamBounds</em></a> )<sup>?</sup> ( <code>=</code> <a href="types.html"><em>Type</em></a> )<sup>?</sup></p>
</blockquote>
<p>Functions, type aliases, structs, enumerations, unions, traits and
implementations may be <em>parameterized</em> by types and lifetimes. These parameters
are listed in angle <span class="parenthetical">brackets (<code><...></code>)</span>,
usually immediately after and before its definition the name of the item. For
implementations, which don't have a name, they come directly after <code>impl</code>.
Lifetime parameters must be declared before type parameters. Some examples of
items with type and lifetime parameters:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn foo<'a, T>() {}
trait A<U> {}
struct Ref<'a, T> where T: 'a { r: &'a T }
#}</code></pre></pre>
<p><a href="types.html#shared-references-">References</a>, <a href="types.html#raw-pointers-const-and-mut">raw pointers</a>, <a href="types.html#array-and-slice-types">arrays</a>, <a href="types.html#array-and-slice-types">slices</a>, <a href="types.html#tuple-types">tuples</a> and
<a href="types.html#function-pointer-types">function pointers</a> have lifetime or type parameters as well, but are not
referred to with path syntax.</p>
<a class="header" href="items/generics.html#where-clauses" id="where-clauses"><h2>Where clauses</h2></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>WhereClause</em> :<br />
<code>where</code> ( <em>WhereClauseItem</em> <code>,</code> )<sup>*</sup> <em>WhereClauseItem</em> <sup>?</sup></p>
<p><em>WhereClauseItem</em> :<br />
<em>LifetimeWhereClauseItem</em><br />
| <em>TypeBoundWhereClauseItem</em></p>
<p><em>LifetimeWhereClauseItem</em> :<br />
<a href="trait-bounds.html"><em>Lifetime</em></a> <code>:</code> <a href="trait-bounds.html"><em>LifetimeBounds</em></a></p>
<p><em>TypeBoundWhereClauseItem</em> :<br />
<em>ForLifetimes</em><sup>?</sup> <a href="types.html"><em>Type</em></a> <code>:</code> <a href="trait-bounds.html"><em>TypeParamBounds</em></a><sup>?</sup></p>
<p><em>ForLifetimes</em> :<br />
<code>for</code> <code><</code> <a href="items/generics.html#type-and-lifetime-parameters"><em>LifetimeParams</em></a> <code>></code></p>
</blockquote>
<p><em>Where clauses</em> provide an another way to specify bounds on type and lifetime
parameters as well as a way to specify bounds on types that aren't type
parameters.</p>
<p>Bounds that don't use the item's parameters or higher-ranked lifetimes are
checked when the item is defined. It is an error for such a bound to be false.</p>
<p><a href="special-types-and-traits.html#copy"><code>Copy</code></a>, <a href="special-types-and-traits.html#clone"><code>Clone</code></a> and <a href="special-types-and-traits.html#sized"><code>Sized</code></a> bounds are also checked for certain generic
types when defining the item. It is an error to have <code>Copy</code> or <code>Clone</code>as a
bound on a mutable reference, <a href="types.html#trait-objects">trait object</a> or <a href="types.html#array-and-slice-types">slice</a> or <code>Sized</code> as a
bound on a trait object or slice.</p>
<pre><code class="language-rust ignore">struct A<T>
where
T: Iterator, // Could use A<T: Iterator> instead
T::Item: Copy,
String: PartialEq<T>,
i32: Default, // Allowed, but not useful
i32: Iterator, // Error: the trait bound is not satisfied
[T]: Copy, // Error: the trait bound is not satisfied
{
f: T,
}
</code></pre>
<a class="header" href="items/generics.html#attributes" id="attributes"><h2>Attributes</h2></a>
<p>Generic lifetime and type parameters allow <a href="attributes.html">attributes</a> on them. There are no
built-in attributes that do anything in this position, although custom derive
attributes may give meaning to it.</p>
<p>This example shows using a custom derive attribute to modify the meaning of a
generic parameter.</p>
<pre><code class="language-ignore">// Assume that the derive for MyFlexibleClone declared `my_flexible_clone` as
// an attribute it understands.
#[derive(MyFlexibleClone)] struct Foo<#[my_flexible_clone(unbounded)] H> {
a: *const H
}
</code></pre>
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