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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> Macros</a></li><li><ol class="section"><li><a href="macros-by-example.html"><strong aria-hidden="true">3.1.</strong> Macros By Example</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">3.2.</strong> Procedural Macros</a></li></ol></li><li><a href="crates-and-source-files.html"><strong aria-hidden="true">4.</strong> Crates and source files</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">5.</strong> Conditional compilation</a></li><li><a href="items.html"><strong aria-hidden="true">6.</strong> Items</a></li><li><ol class="section"><li><a href="items/modules.html"><strong aria-hidden="true">6.1.</strong> Modules</a></li><li><a href="items/extern-crates.html"><strong aria-hidden="true">6.2.</strong> Extern crates</a></li><li><a href="items/use-declarations.html"><strong aria-hidden="true">6.3.</strong> Use declarations</a></li><li><a href="items/functions.html"><strong aria-hidden="true">6.4.</strong> Functions</a></li><li><a href="items/type-aliases.html"><strong aria-hidden="true">6.5.</strong> Type aliases</a></li><li><a href="items/structs.html"><strong aria-hidden="true">6.6.</strong> Structs</a></li><li><a href="items/enumerations.html"><strong aria-hidden="true">6.7.</strong> Enumerations</a></li><li><a href="items/unions.html"><strong aria-hidden="true">6.8.</strong> Unions</a></li><li><a href="items/constant-items.html"><strong aria-hidden="true">6.9.</strong> Constant items</a></li><li><a href="items/static-items.html"><strong aria-hidden="true">6.10.</strong> Static items</a></li><li><a href="items/traits.html"><strong aria-hidden="true">6.11.</strong> Traits</a></li><li><a href="items/implementations.html" class="active"><strong aria-hidden="true">6.12.</strong> Implementations</a></li><li><a href="items/external-blocks.html"><strong aria-hidden="true">6.13.</strong> External blocks</a></li><li><a href="items/generics.html"><strong aria-hidden="true">6.14.</strong> Type and lifetime parameters</a></li><li><a href="items/associated-items.html"><strong aria-hidden="true">6.15.</strong> Associated Items</a></li><li><a href="visibility-and-privacy.html"><strong aria-hidden="true">6.16.</strong> Visibility and Privacy</a></li></ol></li><li><a href="attributes.html"><strong aria-hidden="true">7.</strong> Attributes</a></li><li><ol class="section"><li><a href="attributes/testing.html"><strong aria-hidden="true">7.1.</strong> Testing</a></li><li><a href="attributes/derive.html"><strong aria-hidden="true">7.2.</strong> Derive</a></li><li><a href="attributes/diagnostics.html"><strong aria-hidden="true">7.3.</strong> Diagnostics</a></li><li><a href="attributes/codegen.html"><strong aria-hidden="true">7.4.</strong> Code generation</a></li><li><a href="attributes/limits.html"><strong aria-hidden="true">7.5.</strong> Limits</a></li></ol></li><li><a href="statements-and-expressions.html"><strong aria-hidden="true">8.</strong> Statements and expressions</a></li><li><ol class="section"><li><a href="statements.html"><strong aria-hidden="true">8.1.</strong> Statements</a></li><li><a href="expressions.html"><strong aria-hidden="true">8.2.</strong> Expressions</a></li><li><ol class="section"><li><a href="expressions/literal-expr.html"><strong aria-hidden="true">8.2.1.</strong> Literal expressions</a></li><li><a href="expressions/path-expr.html"><strong aria-hidden="true">8.2.2.</strong> Path expressions</a></li><li><a href="expressions/block-expr.html"><strong aria-hidden="true">8.2.3.</strong> Block expressions</a></li><li><a href="expressions/operator-expr.html"><strong aria-hidden="true">8.2.4.</strong> Operator expressions</a></li><li><a href="expressions/grouped-expr.html"><strong aria-hidden="true">8.2.5.</strong> Grouped expressions</a></li><li><a href="expressions/array-expr.html"><strong aria-hidden="true">8.2.6.</strong> Array and index expressions</a></li><li><a href="expressions/tuple-expr.html"><strong aria-hidden="true">8.2.7.</strong> Tuple and index expressions</a></li><li><a href="expressions/struct-expr.html"><strong aria-hidden="true">8.2.8.</strong> Struct expressions</a></li><li><a href="expressions/enum-variant-expr.html"><strong aria-hidden="true">8.2.9.</strong> Enum variant expressions</a></li><li><a href="expressions/call-expr.html"><strong aria-hidden="true">8.2.10.</strong> Call expressions</a></li><li><a href="expressions/method-call-expr.html"><strong aria-hidden="true">8.2.11.</strong> Method call expressions</a></li><li><a href="expressions/field-expr.html"><strong aria-hidden="true">8.2.12.</strong> Field access expressions</a></li><li><a href="expressions/closure-expr.html"><strong aria-hidden="true">8.2.13.</strong> Closure expressions</a></li><li><a href="expressions/loop-expr.html"><strong aria-hidden="true">8.2.14.</strong> Loop expressions</a></li><li><a href="expressions/range-expr.html"><strong aria-hidden="true">8.2.15.</strong> Range expressions</a></li><li><a href="expressions/if-expr.html"><strong aria-hidden="true">8.2.16.</strong> If and if let expressions</a></li><li><a href="expressions/match-expr.html"><strong aria-hidden="true">8.2.17.</strong> Match expressions</a></li><li><a href="expressions/return-expr.html"><strong aria-hidden="true">8.2.18.</strong> Return expressions</a></li></ol></li></ol></li><li><a href="patterns.html"><strong aria-hidden="true">9.</strong> Patterns</a></li><li><a href="type-system.html"><strong aria-hidden="true">10.</strong> Type system</a></li><li><ol class="section"><li><a href="types.html"><strong aria-hidden="true">10.1.</strong> Types</a></li><li><ol class="section"><li><a href="types/boolean.html"><strong aria-hidden="true">10.1.1.</strong> Boolean type</a></li><li><a href="types/numeric.html"><strong aria-hidden="true">10.1.2.</strong> Numeric types</a></li><li><a href="types/textual.html"><strong aria-hidden="true">10.1.3.</strong> Textual types</a></li><li><a href="types/never.html"><strong aria-hidden="true">10.1.4.</strong> Never type</a></li><li><a href="types/tuple.html"><strong aria-hidden="true">10.1.5.</strong> Tuple types</a></li><li><a href="types/array.html"><strong aria-hidden="true">10.1.6.</strong> Array types</a></li><li><a href="types/slice.html"><strong aria-hidden="true">10.1.7.</strong> Slice types</a></li><li><a href="types/struct.html"><strong aria-hidden="true">10.1.8.</strong> Struct types</a></li><li><a href="types/enum.html"><strong aria-hidden="true">10.1.9.</strong> Enumerated types</a></li><li><a href="types/union.html"><strong aria-hidden="true">10.1.10.</strong> Union types</a></li><li><a href="types/function-item.html"><strong aria-hidden="true">10.1.11.</strong> Function item types</a></li><li><a href="types/closure.html"><strong aria-hidden="true">10.1.12.</strong> Closure types</a></li><li><a href="types/pointer.html"><strong aria-hidden="true">10.1.13.</strong> Pointer types</a></li><li><a href="types/function-pointer.html"><strong aria-hidden="true">10.1.14.</strong> Function pointer types</a></li><li><a href="types/trait-object.html"><strong aria-hidden="true">10.1.15.</strong> Trait object types</a></li><li><a href="types/impl-trait.html"><strong aria-hidden="true">10.1.16.</strong> Impl trait type</a></li><li><a href="types/parameters.html"><strong aria-hidden="true">10.1.17.</strong> Type parameters</a></li><li><a href="types/inferred.html"><strong aria-hidden="true">10.1.18.</strong> Inferred type</a></li></ol></li><li><a href="dynamically-sized-types.html"><strong aria-hidden="true">10.2.</strong> Dynamically Sized Types</a></li><li><a href="type-layout.html"><strong aria-hidden="true">10.3.</strong> Type layout</a></li><li><a href="interior-mutability.html"><strong aria-hidden="true">10.4.</strong> Interior mutability</a></li><li><a href="subtyping.html"><strong aria-hidden="true">10.5.</strong> Subtyping and Variance</a></li><li><a href="trait-bounds.html"><strong aria-hidden="true">10.6.</strong> Trait and lifetime bounds</a></li><li><a href="type-coercions.html"><strong aria-hidden="true">10.7.</strong> Type coercions</a></li><li><a href="destructors.html"><strong aria-hidden="true">10.8.</strong> Destructors</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">10.9.</strong> Lifetime elision</a></li></ol></li><li><a href="special-types-and-traits.html"><strong aria-hidden="true">11.</strong> Special types and traits</a></li><li><a href="memory-model.html"><strong aria-hidden="true">12.</strong> Memory model</a></li><li><ol class="section"><li><a href="memory-allocation-and-lifetime.html"><strong aria-hidden="true">12.1.</strong> Memory allocation and lifetime</a></li><li><a href="memory-ownership.html"><strong aria-hidden="true">12.2.</strong> Memory ownership</a></li><li><a href="variables.html"><strong aria-hidden="true">12.3.</strong> Variables</a></li></ol></li><li><a href="linkage.html"><strong aria-hidden="true">13.</strong> Linkage</a></li><li><a href="unsafety.html"><strong aria-hidden="true">14.</strong> Unsafety</a></li><li><ol class="section"><li><a href="unsafe-functions.html"><strong aria-hidden="true">14.1.</strong> Unsafe functions</a></li><li><a href="unsafe-blocks.html"><strong aria-hidden="true">14.2.</strong> Unsafe blocks</a></li><li><a href="behavior-considered-undefined.html"><strong aria-hidden="true">14.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">14.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="const_eval.html"><strong aria-hidden="true">15.</strong> Constant Evaluation</a></li><li><a href="abi.html"><strong aria-hidden="true">16.</strong> Application Binary Interface</a></li><li><a href="runtime.html"><strong aria-hidden="true">17.</strong> The Rust runtime</a></li><li><a href="appendices.html"><strong aria-hidden="true">18.</strong> Appendices</a></li><li><ol class="section"><li><a href="macro-ambiguity.html"><strong aria-hidden="true">18.1.</strong> Macro Follow-Set Ambiguity Formal Specification</a></li><li><a href="influences.html"><strong aria-hidden="true">18.2.</strong> Influences</a></li><li><a href="glossary.html"><strong aria-hidden="true">18.3.</strong> Glossary</a></li></ol></li></ol>
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<a class="header" href="items/implementations.html#implementations" id="implementations"><h1>Implementations</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>Implementation</em> :<br />
<em>InherentImpl</em> | <em>TraitImpl</em></p>
<p><em>InherentImpl</em> :<br />
<code>impl</code> <a href="items/generics.html"><em>Generics</em></a> <a href="types.html#type-expressions"><em>Type</em></a> <a href="items/generics.html#where-clauses"><em>WhereClause</em></a><sup>?</sup> <code>{</code><br />
<a href="attributes.html"><em>InnerAttribute</em></a><sup>*</sup><br />
<em>InherentImplItem</em><sup>*</sup><br />
<code>}</code></p>
<p><em>InherentImplItem</em> :<br />
<a href="attributes.html"><em>OuterAttribute</em></a><sup>*</sup> (<br />
<a href="macros.html#macro-invocation"><em>MacroInvocationSemi</em></a><br />
| ( <a href="visibility-and-privacy.html"><em>Visibility</em></a><sup>?</sup> ( <a href="items/constant-items.html"><em>ConstantItem</em></a> | <a href="items/functions.html"><em>Function</em></a> | <a href="items/associated-items.html#methods"><em>Method</em></a> ) )<br />
)</p>
<p><em>TraitImpl</em> :<br />
<code>unsafe</code><sup>?</sup> <code>impl</code> <a href="items/generics.html"><em>Generics</em></a> <code>!</code><sup>?</sup>
<a href="paths.html#paths-in-types"><em>TypePath</em></a> <code>for</code> <a href="types.html#type-expressions"><em>Type</em></a><br />
<a href="items/generics.html#where-clauses"><em>WhereClause</em></a><sup>?</sup><br />
<code>{</code><br />
<a href="attributes.html"><em>InnerAttribute</em></a><sup>*</sup><br />
<em>TraitImplItem</em><sup>*</sup><br />
<code>}</code></p>
<p><em>TraitImplItem</em> :<br />
<a href="attributes.html"><em>OuterAttribute</em></a><sup>*</sup> (<br />
<a href="macros.html#macro-invocation"><em>MacroInvocationSemi</em></a><br />
| ( <a href="visibility-and-privacy.html"><em>Visibility</em></a><sup>?</sup> ( <a href="items/type-aliases.html"><em>TypeAlias</em></a> | <a href="items/constant-items.html"><em>ConstantItem</em></a> | <a href="items/functions.html"><em>Function</em></a> | <a href="items/associated-items.html#methods"><em>Method</em></a> ) )<br />
)</p>
</blockquote>
<p>An <em>implementation</em> is an item that associates items with an <em>implementing type</em>.
Implementations are defined with the keyword <code>impl</code> and contain functions
that belong to an instance of the type that is being implemented or to the
type statically.</p>
<p>There are two types of implementations:</p>
<ul>
<li>inherent implementations</li>
<li><a href="items/traits.html">trait</a> implementations</li>
</ul>
<a class="header" href="items/implementations.html#inherent-implementations" id="inherent-implementations"><h2>Inherent Implementations</h2></a>
<p>An inherent implementation is defined as the sequence of the <code>impl</code> keyword,
generic type declarations, a path to a nominal type, a where clause, and a
bracketed set of associable items.</p>
<p>The nominal type is called the <em>implementing type</em> and the associable items are
the <em>associated items</em> to the implementing type.</p>
<p>Inherent implementations associate the contained items to the
implementing type. Inherent implementations can contain <a href="items/associated-items.html#associated-functions-and-methods">associated
functions</a> (including methods) and <a href="items/associated-items.html#associated-constants">associated constants</a>. They cannot
contain associated type aliases.</p>
<p>The <a href="paths.html">path</a> to an associated item is any path to the implementing type,
followed by the associated item's identifier as the final path
component.</p>
<p>A type can also have multiple inherent implementations. An implementing type
must be defined within the same crate as the original type definition.</p>
<pre><pre class="playpen"><code class="language-rust">pub mod color {
pub struct Color(pub u8, pub u8, pub u8);
impl Color {
pub const WHITE: Color = Color(255, 255, 255);
}
}
mod values {
use super::color::Color;
impl Color {
pub fn red() -> Color {
Color(255, 0, 0)
}
}
}
pub use self::color::Color;
fn main() {
// Actual path to the implementing type and impl in the same module.
color::Color::WHITE;
// Impl blocks in different modules are still accessed through a path to the type.
color::Color::red();
// Re-exported paths to the implementing type also work.
Color::red();
// Does not work, because use in `values` is not pub.
// values::Color::red();
}
</code></pre></pre>
<a class="header" href="items/implementations.html#trait-implementations" id="trait-implementations"><h2>Trait Implementations</h2></a>
<p>A <em>trait implementation</em> is defined like an inherent implementation except that
the optional generic type declarations is followed by a <a href="items/traits.html">trait</a> followed
by the keyword <code>for</code>. Followed by a path to a nominal type.</p>
<!-- To understand this, you have to back-reference to the previous section. :( -->
<p>The trait is known as the <em>implemented trait</em>. The implementing type
implements the implemented trait.</p>
<p>A trait implementation must define all non-default associated items declared
by the implemented trait, may redefine default associated items defined by the
implemented trait, and cannot define any other items.</p>
<p>The path to the associated items is <code><</code> followed by a path to the implementing
type followed by <code>as</code> followed by a path to the trait followed by <code>></code> as a path
component followed by the associated item's path component.</p>
<p><a href="items/traits.html#unsafe-traits">Unsafe traits</a> require the trait implementation to begin with the <code>unsafe</code>
keyword.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# #[derive(Copy, Clone)]
# struct Point {x: f64, y: f64};
# type Surface = i32;
# struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
# trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
# fn do_draw_circle(s: Surface, c: Circle) { }
struct Circle {
radius: f64,
center: Point,
}
impl Copy for Circle {}
impl Clone for Circle {
fn clone(&self) -> Circle { *self }
}
impl Shape for Circle {
fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
fn bounding_box(&self) -> BoundingBox {
let r = self.radius;
BoundingBox {
x: self.center.x - r,
y: self.center.y - r,
width: 2.0 * r,
height: 2.0 * r,
}
}
}
#}</code></pre></pre>
<a class="header" href="items/implementations.html#trait-implementation-coherence" id="trait-implementation-coherence"><h3>Trait Implementation Coherence</h3></a>
<p>A trait implementation is considered incoherent if either the orphan check fails
or there are overlapping implementation instances.</p>
<p>Two trait implementations overlap when there is a non-empty intersection of the
traits the implementation is for, the implementations can be instantiated with
the same type. <!-- This is probably wrong? Source: No two implementations can
be instantiable with the same set of types for the input type parameters. --></p>
<p>The <code>Orphan Check</code> states that every trait implementation must meet either of
the following conditions:</p>
<ol>
<li>
<p>The trait being implemented is defined in the same crate.</p>
</li>
<li>
<p>At least one of either <code>Self</code> or a generic type parameter of the trait must
meet the following grammar, where <code>C</code> is a nominal type defined
within the containing crate:</p>
<pre><code class="language-ignore"> T = C
| &C
| &mut C
| Box<C>
</code></pre>
</li>
</ol>
<a class="header" href="items/implementations.html#generic-implementations" id="generic-implementations"><h2>Generic Implementations</h2></a>
<p>An implementation can take type and lifetime parameters, which can be used in
the rest of the implementation. Type parameters declared for an implementation
must be used at least once in either the trait or the implementing type of an
implementation. Implementation parameters are written directly after the <code>impl</code>
keyword.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# trait Seq<T> { fn dummy(&self, _: T) { } }
impl<T> Seq<T> for Vec<T> {
/* ... */
}
impl Seq<bool> for u32 {
/* Treat the integer as a sequence of bits */
}
#}</code></pre></pre>
<a class="header" href="items/implementations.html#attributes-on-implementations" id="attributes-on-implementations"><h2>Attributes on Implementations</h2></a>
<p>Implementations may contain outer <a href="attributes.html">attributes</a> before the <code>impl</code> keyword and
inner <a href="attributes.html">attributes</a> inside the brackets that contain the associated items. Inner
attributes must come before any associated items. That attributes that have
meaning here are <a href="conditional-compilation.html"><code>cfg</code></a>, <a href="attributes/diagnostics.html#the-deprecated-attribute"><code>deprecated</code></a>, <a href="../rustdoc/the-doc-attribute.html"><code>doc</code></a>, and <a href="attributes/diagnostics.html#lint-check-attributes">the lint check
attributes</a>.</p>
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