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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"><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" class="active"><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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teams will work together to figure out how best to do this. Until then, this
is a best-effort attempt. If you find something wrong or missing, file an
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<a class="header" href="attributes/diagnostics.html#diagnostic-attributes" id="diagnostic-attributes"><h1>Diagnostic attributes</h1></a>
<p>The following <a href="attributes.html">attributes</a> are used for controlling or generating diagnostic
messages during compilation.</p>
<a class="header" href="attributes/diagnostics.html#lint-check-attributes" id="lint-check-attributes"><h2>Lint check attributes</h2></a>
<p>A lint check names a potentially undesirable coding pattern, such as
unreachable code or omitted documentation. The lint attributes <code>allow</code>,
<code>warn</code>, <code>deny</code>, and <code>forbid</code> use the <a href="attributes.html#meta-item-attribute-syntax"><em>MetaListPaths</em></a> syntax to specify a
list of lint names to change the lint level for the entity to which the
attribute applies.</p>
<p>For any lint check <code>C</code>:</p>
<ul>
<li><code>allow(C)</code> overrides the check for <code>C</code> so that violations will go
unreported,</li>
<li><code>warn(C)</code> warns about violations of <code>C</code> but continues compilation.</li>
<li><code>deny(C)</code> signals an error after encountering a violation of <code>C</code>,</li>
<li><code>forbid(C)</code> is the same as <code>deny(C)</code>, but also forbids changing the lint
level afterwards,</li>
</ul>
<blockquote>
<p>Note: The lint checks supported by <code>rustc</code> can be found via <code>rustc -W help</code>,
along with their default settings and are documented in the <a href="../rustc/lints/index.html">rustc book</a>.</p>
</blockquote>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub mod m1 {
// Missing documentation is ignored here
#[allow(missing_docs)]
pub fn undocumented_one() -> i32 { 1 }
// Missing documentation signals a warning here
#[warn(missing_docs)]
pub fn undocumented_too() -> i32 { 2 }
// Missing documentation signals an error here
#[deny(missing_docs)]
pub fn undocumented_end() -> i32 { 3 }
}
#}</code></pre></pre>
<p>This example shows how one can use <code>allow</code> and <code>warn</code> to toggle a particular
check on and off:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[warn(missing_docs)]
pub mod m2{
#[allow(missing_docs)]
pub mod nested {
// Missing documentation is ignored here
pub fn undocumented_one() -> i32 { 1 }
// Missing documentation signals a warning here,
// despite the allow above.
#[warn(missing_docs)]
pub fn undocumented_two() -> i32 { 2 }
}
// Missing documentation signals a warning here
pub fn undocumented_too() -> i32 { 3 }
}
#}</code></pre></pre>
<p>This example shows how one can use <code>forbid</code> to disallow uses of <code>allow</code> for
that lint check:</p>
<pre><pre class="playpen"><code class="language-rust compile_fail">
# #![allow(unused_variables)]
#fn main() {
#[forbid(missing_docs)]
pub mod m3 {
// Attempting to toggle warning signals an error here
#[allow(missing_docs)]
/// Returns 2.
pub fn undocumented_too() -> i32 { 2 }
}
#}</code></pre></pre>
<a class="header" href="attributes/diagnostics.html#tool-lint-attributes" id="tool-lint-attributes"><h3>Tool lint attributes</h3></a>
<p>Tool lints allows using scoped lints, to <code>allow</code>, <code>warn</code>, <code>deny</code> or <code>forbid</code>
lints of certain tools.</p>
<p>Currently <code>clippy</code> is the only available lint tool.</p>
<p>Tool lints only get checked when the associated tool is active. If a lint
attribute, such as <code>allow</code>, references a nonexistent tool lint, the compiler
will not warn about the nonexistent lint until you use the tool.</p>
<p>Otherwise, they work just like regular lint attributes:</p>
<pre><pre class="playpen"><code class="language-rust">// set the entire `pedantic` clippy lint group to warn
#![warn(clippy::pedantic)]
// silence warnings from the `filter_map` clippy lint
#![allow(clippy::filter_map)]
fn main() {
// ...
}
// silence the `cmp_nan` clippy lint just for this function
#[allow(clippy::cmp_nan)]
fn foo() {
// ...
}
</code></pre></pre>
<a class="header" href="attributes/diagnostics.html#the-deprecated-attribute" id="the-deprecated-attribute"><h2>The <code>deprecated</code> attribute</h2></a>
<p>The <em><code>deprecated</code> attribute</em> marks an item as deprecated. <code>rustc</code> will issue
warnings on usage of <code>#[deprecated]</code> items. <code>rustdoc</code> will show item
deprecation, including the <code>since</code> version and <code>note</code>, if available.</p>
<p>The <code>deprecated</code> attribute has several forms:</p>
<ul>
<li><code>deprecated</code> — Issues a generic message.</li>
<li><code>deprecated = "message"</code> — Includes the given string in the deprecation
message.</li>
<li><a href="attributes.html#meta-item-attribute-syntax"><em>MetaListNameValueStr</em></a> syntax with two optional fields:
<ul>
<li><code>since</code> — Specifies a version number when the item was deprecated. <code>rustc</code>
does not currently interpret the string, but external tools like <a href="https://github.com/rust-lang/rust-clippy">Clippy</a>
may check the validity of the value.</li>
<li><code>note</code> — Specifies a string that should be included in the deprecation
message. This is typically used to provide an explanation about the
deprecation and preferred alternatives.</li>
</ul>
</li>
</ul>
<p>The <code>deprecated</code> attribute may be applied to any <a href="items.html">item</a>, <a href="items/traits.html">trait item</a>, <a href="items/enumerations.html">enum
variant</a>, <a href="items/structs.html">struct field</a>, or <a href="items/external-blocks.html">external block item</a>. It cannot be applied to <a href="items/implementations.html#trait-implementations">trait
implementation items</a>. When applied to an item containing other items, such as
a <a href="items/modules.html">module</a> or <a href="items/implementations.html">implementation</a>, all child items inherit the deprecation attribute.</p>
<!-- NOTE: Currently broken for macros, see https://github.com/rust-lang/rust/issues/49912
Also, it is only rejected for trait impl items (AnnotationKind::Prohibited). In all
other locations, it is silently ignored. Tuple struct fields are ignored.
-->
<p>Here is an example:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[deprecated(since = "5.2", note = "foo was rarely used. Users should instead use bar")]
pub fn foo() {}
pub fn bar() {}
#}</code></pre></pre>
<p>The <a href="https://github.com/rust-lang/rfcs/blob/master/text/1270-deprecation.md">RFC</a> contains motivations and more details.</p>
<a class="header" href="attributes/diagnostics.html#the-must_use-attribute" id="the-must_use-attribute"><h2>The <code>must_use</code> attribute</h2></a>
<p>The <em><code>must_use</code> attribute</em> is used to issue a diagnostic warning when a value
is not "used". It can be applied to user-defined composite types
(<a href="items/structs.html"><code>struct</code>s</a>, <a href="items/enumerations.html"><code>enum</code>s</a>, and <a href="items/unions.html"><code>union</code>s</a>), <a href="items/functions.html">functions</a>,
and <a href="items/traits.html">traits</a>.</p>
<p>The <code>must_use</code> attribute may include a message by using the
<a href="attributes.html#meta-item-attribute-syntax"><em>MetaNameValueStr</em></a> syntax such as <code>#[must_use = "example message"]</code>. The
message will be given alongside the warning.</p>
<p>When used on user-defined composite types, if the <a href="expressions.html">expression</a> of an
<a href="statements.html#expression-statements">expression statement</a> has that type, then the <code>unused_must_use</code> lint is
violated.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[must_use]
struct MustUse {
// some fields
}
# impl MustUse {
# fn new() -> MustUse { MustUse {} }
# }
#
// Violates the `unused_must_use` lint.
MustUse::new();
#}</code></pre></pre>
<p>When used on a function, if the <a href="expressions.html">expression</a> of an <a href="statements.html#expression-statements">expression statement</a> is a
<a href="expressions/call-expr.html">call expression</a> to that function, then the <code>unused_must_use</code> lint is
violated.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[must_use]
fn five() -> i32 { 5i32 }
// Violates the unused_must_use lint.
five();
#}</code></pre></pre>
<p>When used on a <a href="items/traits.html">trait declaration</a>, a <a href="expressions/call-expr.html">call expression</a> of an <a href="statements.html#expression-statements">expression
statement</a> to a function that returns an <a href="types/impl-trait.html">impl trait</a> of that trait violates
the <code>unsued_must_use</code> lint.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[must_use]
trait Critical {}
impl Critical for i32 {}
fn get_critical() -> impl Critical {
4i32
}
// Violates the `unused_must_use` lint.
get_critical();
#}</code></pre></pre>
<p>When used on a function in a trait declaration, then the behavior also applies
when the call expression is a function from an implementation of the trait.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Trait {
#[must_use]
fn use_me(&self) -> i32;
}
impl Trait for i32 {
fn use_me(&self) -> i32 { 0i32 }
}
// Violates the `unused_must_use` lint.
5i32.use_me();
#}</code></pre></pre>
<p>When used on a function in a trait implementation, the attribute does nothing.</p>
<blockquote>
<p>Note: Trivial no-op expressions containing the value will not violate the
lint. Examples include wrapping the value in a type that does not implement
<a href="special-types-and-traits.html#drop"><code>Drop</code></a> and then not using that type and being the final expression of a
<a href="expressions/block-expr.html">block expression</a> that is not used.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[must_use]
fn five() -> i32 { 5i32 }
// None of these violate the unused_must_use lint.
(five(),);
Some(five());
{ five() };
if true { five() } else { 0i32 };
match true {
_ => five()
};
#}</code></pre></pre>
</blockquote>
<blockquote>
<p>Note: It is idiomatic to use a <a href="statements.html#let-statements">let statement</a> with a pattern of <code>_</code>
when a must-used value is purposely discarded.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[must_use]
fn five() -> i32 { 5i32 }
// Does not violate the unused_must_use lint.
let _ = five();
#}</code></pre></pre>
</blockquote>
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