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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="contributing.html"><strong aria-hidden="true">6.</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-console">$ 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.</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() {}
| ^^^^^^^^^^^^
> 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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