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<ol class="chapter"><li><a href="what-is-rustc.html"><strong aria-hidden="true">1.</strong> What is rustc?</a></li><li><a href="command-line-arguments.html"><strong aria-hidden="true">2.</strong> Command-line arguments</a></li><li><a href="lints/index.html"><strong aria-hidden="true">3.</strong> Lints</a></li><li><ol class="section"><li><a href="lints/levels.html" class="active"><strong aria-hidden="true">3.1.</strong> Lint levels</a></li><li><a href="lints/groups.html"><strong aria-hidden="true">3.2.</strong> Lint Groups</a></li><li><a href="lints/listing/index.html"><strong aria-hidden="true">3.3.</strong> Lint listing</a></li><li><ol class="section"><li><a href="lints/listing/allowed-by-default.html"><strong aria-hidden="true">3.3.1.</strong> Allowed-by-default lints</a></li><li><a href="lints/listing/warn-by-default.html"><strong aria-hidden="true">3.3.2.</strong> Warn-by-default lints</a></li><li><a href="lints/listing/deny-by-default.html"><strong aria-hidden="true">3.3.3.</strong> Deny-by-default lints</a></li></ol></li></ol></li><li><a href="codegen-options/index.html"><strong aria-hidden="true">4.</strong> Codegen options</a></li><li><a href="targets/index.html"><strong aria-hidden="true">5.</strong> Targets</a></li><li><ol class="section"><li><a href="targets/built-in.html"><strong aria-hidden="true">5.1.</strong> Built-in Targets</a></li><li><a href="targets/custom.html"><strong aria-hidden="true">5.2.</strong> Custom Targets</a></li></ol></li><li><a href="linker-plugin-lto.html"><strong aria-hidden="true">6.</strong> Linker-plugin based LTO</a></li><li><a href="contributing.html"><strong aria-hidden="true">7.</strong> Contributing to rustc</a></li></ol>
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<a class="header" href="lints/levels.html#lint-levels" id="lint-levels"><h1>Lint levels</h1></a>
<p>In <code>rustc</code>, lints are divided into four <em>levels</em>:</p>
<ol>
<li>allow</li>
<li>warn</li>
<li>deny</li>
<li>forbid</li>
</ol>
<p>Each lint has a default level (explained in the lint listing later in this
chapter), and the compiler has a default warning level. First, let's explain
what these levels mean, and then we'll talk about configuration.</p>
<a class="header" href="lints/levels.html#allow" id="allow"><h2>allow</h2></a>
<p>These lints exist, but by default, do nothing. For example, consider this
source:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn foo() {}
#}</code></pre></pre>
<p>Compiling this file produces no warnings:</p>
<pre><code class="language-bash">$ rustc lib.rs --crate-type=lib
$
</code></pre>
<p>But this code violates the <code>missing_docs</code> lint.</p>
<p>These lints exist mostly to be manually turned on via configuration, as we'll
talk about later in this section.</p>
<a class="header" href="lints/levels.html#warn" id="warn"><h2>warn</h2></a>
<p>The 'warn' lint level will produce a warning if you violate the lint. For example,
this code runs afoul of the <code>unused_variable</code> lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn foo() {
let x = 5;
}
#}</code></pre></pre>
<p>This will produce this warning:</p>
<pre><code class="language-bash">$ rustc lib.rs --crate-type=lib
warning: unused variable: `x`
--> lib.rs:2:9
|
2 | let x = 5;
| ^
|
= note: #[warn(unused_variables)] on by default
= note: to avoid this warning, consider using `_x` instead
</code></pre>
<a class="header" href="lints/levels.html#deny" id="deny"><h2>deny</h2></a>
<p>A 'deny' lint produces an error if you violate it. For example, this code
runs into the <code>exceeding_bitshifts</code> lint.</p>
<pre><code class="language-rust ignore">fn main() {
100u8 << 10;
}
</code></pre>
<pre><code class="language-bash">$ rustc main.rs
error: bitshift exceeds the type's number of bits
--> main.rs:2:13
|
2 | 100u8 << 10;
| ^^^^^^^^^^^
|
= note: #[deny(exceeding_bitshifts)] on by default
</code></pre>
<p>What's the difference between an error from a lint and a regular old error?
Lints are configurable via levels, so in a similar way to 'allow' lints,
warnings that are 'deny' by default let you allow them. Similarly, you may
wish to set up a lint that is <code>warn</code> by default to produce an error instead.
This lint level gives you that.</p>
<a class="header" href="lints/levels.html#forbid" id="forbid"><h2>forbid</h2></a>
<p>'forbid' is a special lint level that's stronger than 'deny'. It's the same
as 'deny' in that a lint at this level will produce an error, but unlike the
'deny' level, the 'forbid' level can not be overridden to be anything lower
than an error. However, lint levels may still be capped with <code>--cap-lints</code>
(see below) so <code>rustc --cap-lints warn</code> will make lints set to 'forbid' just
warn.</p>
<a class="header" href="lints/levels.html#configuring-warning-levels" id="configuring-warning-levels"><h2>Configuring warning levels</h2></a>
<p>Remember our <code>missing_docs</code> example from the 'allow' lint level?</p>
<pre><code class="language-bash">$ cat lib.rs
pub fn foo() {}
$ rustc lib.rs --crate-type=lib
$
</code></pre>
<p>We can configure this lint to operate at a higher level, both with
compiler flags, as well as with an attribute in the source code.</p>
<p>You can also "cap" lints so that the compiler can choose to ignore
certain lint levels. We'll talk about that last.</p>
<a class="header" href="lints/levels.html#via-compiler-flag" id="via-compiler-flag"><h3>Via compiler flag</h3></a>
<p>The <code>-A</code>, <code>-W</code>, <code>-D</code>, and <code>-F</code> flags let you turn one or more lints
into allowed, warning, deny, or forbid levels, like this:</p>
<pre><code class="language-bash">$ rustc lib.rs --crate-type=lib -W missing-docs
warning: missing documentation for crate
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
|
= note: requested on the command line with `-W missing-docs`
warning: missing documentation for a function
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
</code></pre>
<pre><code class="language-bash">$ rustc lib.rs --crate-type=lib -D missing-docs
error: missing documentation for crate
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
|
= note: requested on the command line with `-D missing-docs`
error: missing documentation for a function
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
error: aborting due to 2 previous errors
</code></pre>
<p>You can also pass each flag more than once for changing multiple lints:</p>
<pre><code class="language-bash">$ rustc lib.rs --crate-type=lib -D missing-docs -D unused-variables
</code></pre>
<p>And of course, you can mix these four flags together:</p>
<pre><code class="language-bash">$ rustc lib.rs --crate-type=lib -D missing-docs -A unused-variables
</code></pre>
<a class="header" href="lints/levels.html#via-an-attribute" id="via-an-attribute"><h3>Via an attribute</h3></a>
<p>You can also modify the lint level with a crate-wide attribute:</p>
<pre><code class="language-bash">$ cat lib.rs
#![warn(missing_docs)]
pub fn foo() {}
$ rustc lib.rs --crate-type=lib
warning: missing documentation for crate
--> lib.rs:1:1
|
1 | / #![warn(missing_docs)]
2 | |
3 | | pub fn foo() {}
| |_______________^
|
note: lint level defined here
--> lib.rs:1:9
|
1 | #![warn(missing_docs)]
| ^^^^^^^^^^^^
warning: missing documentation for a function
--> lib.rs:3:1
|
3 | pub fn foo() {}
| ^^^^^^^^^^^^
</code></pre>
<p>All four, <code>warn</code>, <code>allow</code>, <code>deny</code>, and <code>forbid</code> all work this way.</p>
<p>You can also pass in multiple lints per attribute:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#![warn(missing_docs, unused_variables)]
#fn main() {
pub fn foo() {}
#}</code></pre></pre>
<p>And use multiple attributes together:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#![warn(missing_docs)]
#![deny(unused_variables)]
#fn main() {
pub fn foo() {}
#}</code></pre></pre>
<a class="header" href="lints/levels.html#capping-lints" id="capping-lints"><h3>Capping lints</h3></a>
<p><code>rustc</code> supports a flag, <code>--cap-lints LEVEL</code> that sets the "lint cap level."
This is the maximum level for all lints. So for example, if we take our
code sample from the "deny" lint level above:</p>
<pre><code class="language-rust ignore">fn main() {
100u8 << 10;
}
</code></pre>
<p>And we compile it, capping lints to warn:</p>
<pre><code class="language-bash">$ rustc lib.rs --cap-lints warn
warning: bitshift exceeds the type's number of bits
--> lib.rs:2:5
|
2 | 100u8 << 10;
| ^^^^^^^^^^^
|
= note: #[warn(exceeding_bitshifts)] on by default
warning: this expression will panic at run-time
--> lib.rs:2:5
|
2 | 100u8 << 10;
| ^^^^^^^^^^^ attempt to shift left with overflow
</code></pre>
<p>It now only warns, rather than errors. We can go further and allow all lints:</p>
<pre><code class="language-bash">$ rustc lib.rs --cap-lints allow
$
</code></pre>
<p>This feature is used heavily by Cargo; it will pass <code>--cap-lints allow</code> when
compiling your dependencies, so that if they have any warnings, they do not
pollute the output of your build.</p>
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