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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"><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" class="active"><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/listing/allowed-by-default.html#allowed-by-default-lints" id="allowed-by-default-lints"><h1>Allowed-by-default lints</h1></a>
<p>These lints are all set to the 'allow' level by default. As such, they won't show up
unless you set them to a higher lint level with a flag or attribute.</p>
<a class="header" href="lints/listing/allowed-by-default.html#anonymous-parameters" id="anonymous-parameters"><h2>anonymous-parameters</h2></a>
<p>This lint detects anonymous parameters. Some example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Foo {
fn foo(usize);
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: use of deprecated anonymous parameter
--> src/lib.rs:5:11
|
5 | fn foo(usize);
| ^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>
</code></pre>
<p>This syntax is mostly a historical accident, and can be worked around quite
easily:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Foo {
fn foo(_: usize);
}
#}</code></pre></pre>
<a class="header" href="lints/listing/allowed-by-default.html#bare-trait-object" id="bare-trait-object"><h2>bare-trait-object</h2></a>
<p>This lint suggests using <code>dyn Trait</code> for trait objects. Some example code
that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#![feature(dyn_trait)]
#fn main() {
trait Trait { }
fn takes_trait_object(_: Box<Trait>) {
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: trait objects without an explicit `dyn` are deprecated
--> src/lib.rs:7:30
|
7 | fn takes_trait_object(_: Box<Trait>) {
| ^^^^^ help: use `dyn`: `dyn Trait`
|
</code></pre>
<p>To fix it, do as the help message suggests:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#![feature(dyn_trait)]
#![deny(bare_trait_objects)]
#fn main() {
trait Trait { }
fn takes_trait_object(_: Box<dyn Trait>) {
}
#}</code></pre></pre>
<a class="header" href="lints/listing/allowed-by-default.html#box-pointers" id="box-pointers"><h2>box-pointers</h2></a>
<p>This lints use of the Box type. Some example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Foo {
x: Box<isize>,
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: type uses owned (Box type) pointers: std::boxed::Box<isize>
--> src/lib.rs:6:5
|
6 | x: Box<isize> //~ ERROR type uses owned
| ^^^^^^^^^^^^^
|
</code></pre>
<p>This lint is mostly historical, and not particularly useful. <code>Box<T></code> used to
be built into the language, and the only way to do heap allocation. Today's
Rust can call into other allocators, etc.</p>
<a class="header" href="lints/listing/allowed-by-default.html#elided-lifetime-in-path" id="elided-lifetime-in-path"><h2>elided-lifetime-in-path</h2></a>
<p>This lint detects the use of hidden lifetime parameters. Some example code
that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Foo<'a> {
x: &'a u32
}
fn foo(x: &Foo) {
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: hidden lifetime parameters are deprecated, try `Foo<'_>`
--> src/lib.rs:5:12
|
5 | fn foo(x: &Foo) {
| ^^^
|
</code></pre>
<p>Lifetime elision elides this lifetime, but that is being deprecated.</p>
<a class="header" href="lints/listing/allowed-by-default.html#missing-copy-implementations" id="missing-copy-implementations"><h2>missing-copy-implementations</h2></a>
<p>This lint detects potentially-forgotten implementations of <code>Copy</code>. Some
example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub struct Foo {
pub field: i32
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: type could implement `Copy`; consider adding `impl Copy`
--> src/main.rs:3:1
|
3 | / pub struct Foo { //~ ERROR type could implement `Copy`; consider adding `impl Copy`
4 | | pub field: i32
5 | | }
| |_^
|
</code></pre>
<p>You can fix the lint by deriving <code>Copy</code>.</p>
<p>This lint is set to 'allow' because this code isn't bad; it's common to write
newtypes like this specifically so that a <code>Copy</code> type is no longer <code>Copy</code>.</p>
<a class="header" href="lints/listing/allowed-by-default.html#missing-debug-implementations" id="missing-debug-implementations"><h2>missing-debug-implementations</h2></a>
<p>This lint detects missing implementations of <code>fmt::Debug</code>. Some example code
that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub struct Foo;
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: type does not implement `fmt::Debug`; consider adding #[derive(Debug)] or a manual implementation
--> src/main.rs:3:1
|
3 | pub struct Foo;
| ^^^^^^^^^^^^^^^
|
</code></pre>
<p>You can fix the lint by deriving <code>Debug</code>.</p>
<a class="header" href="lints/listing/allowed-by-default.html#missing-docs" id="missing-docs"><h2>missing-docs</h2></a>
<p>This lint detects missing documentation for public items. Some example code
that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn foo() {}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: missing documentation for crate
--> src/main.rs:1:1
|
1 | / #![deny(missing_docs)]
2 | |
3 | | pub fn foo() {}
4 | |
5 | | fn main() {}
| |____________^
|
error: missing documentation for a function
--> src/main.rs:3:1
|
3 | pub fn foo() {}
| ^^^^^^^^^^^^
</code></pre>
<p>To fix the lint, add documentation to all items.</p>
<a class="header" href="lints/listing/allowed-by-default.html#single-use-lifetime" id="single-use-lifetime"><h2>single-use-lifetime</h2></a>
<p>This lint detects lifetimes that are only used once. Some example code that
triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct Foo<'x> {
x: &'x u32
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: lifetime name `'x` only used once
--> src/main.rs:3:12
|
3 | struct Foo<'x> {
| ^^
|
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#trivial-casts" id="trivial-casts"><h2>trivial-casts</h2></a>
<p>This lint detects trivial casts which could be removed. Some example code
that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let x: &u32 = &42;
let _ = x as *const u32;
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: trivial cast: `&u32` as `*const u32`. Cast can be replaced by coercion, this might require type ascription or a temporary variable
--> src/main.rs:5:13
|
5 | let _ = x as *const u32;
| ^^^^^^^^^^^^^^^
|
note: lint level defined here
--> src/main.rs:1:9
|
1 | #![deny(trivial_casts)]
| ^^^^^^^^^^^^^
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#trivial-numeric-casts" id="trivial-numeric-casts"><h2>trivial-numeric-casts</h2></a>
<p>This lint detects trivial casts of numeric types which could be removed. Some
example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let x = 42i32 as i32;
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: trivial numeric cast: `i32` as `i32`. Cast can be replaced by coercion, this might require type ascription or a temporary variable
--> src/main.rs:4:13
|
4 | let x = 42i32 as i32;
| ^^^^^^^^^^^^
|
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#unreachable-pub" id="unreachable-pub"><h2>unreachable-pub</h2></a>
<p>This lint triggers for <code>pub</code> items not reachable from the crate root. Some
example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
mod foo {
pub mod bar {
}
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: unreachable `pub` item
--> src/main.rs:4:5
|
4 | pub mod bar {
| ---^^^^^^^^
| |
| help: consider restricting its visibility: `pub(crate)`
|
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#unsafe-code" id="unsafe-code"><h2>unsafe-code</h2></a>
<p>This lint catches usage of <code>unsafe</code> code. Some example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
unsafe {
}
}
</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: usage of an `unsafe` block
--> src/main.rs:4:5
|
4 | / unsafe {
5 | |
6 | | }
| |_____^
|
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#unstable-features" id="unstable-features"><h2>unstable-features</h2></a>
<p>This lint is deprecated and no longer used.</p>
<a class="header" href="lints/listing/allowed-by-default.html#unused-extern-crates" id="unused-extern-crates"><h2>unused-extern-crates</h2></a>
<p>This lint guards against <code>extern crate</code> items that are never used. Some
example code that triggers this lint:</p>
<pre><code class="language-rust ignore">extern crate semver;
</code></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: unused extern crate
--> src/main.rs:3:1
|
3 | extern crate semver;
| ^^^^^^^^^^^^^^^^^^^^
|
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#unused-import-braces" id="unused-import-braces"><h2>unused-import-braces</h2></a>
<p>This lint catches unnecessary braces around an imported item. Some example
code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">use test::{A};
pub mod test {
pub struct A;
}
# fn main() {}
</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: braces around A is unnecessary
--> src/main.rs:3:1
|
3 | use test::{A};
| ^^^^^^^^^^^^^^
|
</code></pre>
<p>To fix it, <code>use test::A;</code></p>
<a class="header" href="lints/listing/allowed-by-default.html#unused-qualifications" id="unused-qualifications"><h2>unused-qualifications</h2></a>
<p>This lint detects unnecessarily qualified names. Some example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">mod foo {
pub fn bar() {}
}
fn main() {
use foo::bar;
foo::bar();
}
</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: unnecessary qualification
--> src/main.rs:9:5
|
9 | foo::bar();
| ^^^^^^^^
|
</code></pre>
<p>You can call <code>bar()</code> directly, without the <code>foo::</code>.</p>
<a class="header" href="lints/listing/allowed-by-default.html#unused-results" id="unused-results"><h2>unused-results</h2></a>
<p>This lint checks for the unused result of an expression in a statement. Some
example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust no_run">fn foo<T>() -> T { panic!() }
fn main() {
foo::<usize>();
}
</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: unused result
--> src/main.rs:6:5
|
6 | foo::<usize>();
| ^^^^^^^^^^^^^^^
|
</code></pre>
<a class="header" href="lints/listing/allowed-by-default.html#variant-size-differences" id="variant-size-differences"><h2>variant-size-differences</h2></a>
<p>This lint detects enums with widely varying variant sizes. Some example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum En {
V0(u8),
VBig([u8; 1024]),
}
#}</code></pre></pre>
<p>When set to 'deny', this will produce:</p>
<pre><code class="language-text">error: enum variant is more than three times larger (1024 bytes) than the next largest
--> src/main.rs:5:5
|
5 | VBig([u8; 1024]), //~ ERROR variant is more than three times larger
| ^^^^^^^^^^^^^^^^
|
</code></pre>
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