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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"><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" class="active"><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/listing/deny-by-default.html#deny-by-default-lints" id="deny-by-default-lints"><h1>Deny-by-default lints</h1></a>
<p>These lints are all set to the 'deny' level by default.</p>
<a class="header" href="lints/listing/deny-by-default.html#exceeding-bitshifts" id="exceeding-bitshifts"><h2>exceeding-bitshifts</h2></a>
<p>This lint detects that a shift exceeds the type's number of bits. Some
example code that triggers this lint:</p>
<pre><code class="language-rust ignore">1_i32 << 32;
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: bitshift exceeds the type's number of bits
--> src/main.rs:2:5
|
2 | 1_i32 << 32;
| ^^^^^^^^^^^
|
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#invalid-type-param-default" id="invalid-type-param-default"><h2>invalid-type-param-default</h2></a>
<p>This lint detects type parameter default erroneously allowed in invalid location. Some
example code that triggers this lint:</p>
<pre><code class="language-rust ignore">fn foo<T=i32>(t: T) {}
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions.
--> src/main.rs:4:8
|
4 | fn foo<T=i32>(t: T) {}
| ^
|
= note: #[deny(invalid_type_param_default)] on by default
= 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 #36887 <https://github.com/rust-lang/rust/issues/36887>
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#legacy-constructor-visibility" id="legacy-constructor-visibility"><h2>legacy-constructor-visibility</h2></a>
<p><a href="https://github.com/rust-lang/rfcs/blob/master/text/1506-adt-kinds.md">RFC 1506</a> modified some
visibility rules, and changed the visibility of struct constructors. Some
example code that triggers this lint:</p>
<pre><code class="language-rust ignore">mod m {
pub struct S(u8);
fn f() {
// this is trying to use S from the 'use' line, but because the `u8` is
// not pub, it is private
::S;
}
}
use m::S;
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: private struct constructors are not usable through re-exports in outer modules
--> src/main.rs:5:9
|
5 | ::S;
| ^^^
|
= note: #[deny(legacy_constructor_visibility)] on by default
= 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 #39207 <https://github.com/rust-lang/rust/issues/39207>
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#legacy-directory-ownership" id="legacy-directory-ownership"><h2>legacy-directory-ownership</h2></a>
<p>The legacy_directory_ownership warning is issued when</p>
<ul>
<li>There is a non-inline module with a #[path] attribute (e.g. #[path = "foo.rs"] mod bar;),</li>
<li>The module's file ("foo.rs" in the above example) is not named "mod.rs", and</li>
<li>The module's file contains a non-inline child module without a #[path] attribute.</li>
</ul>
<p>The warning can be fixed by renaming the parent module to "mod.rs" and moving
it into its own directory if appropriate.</p>
<a class="header" href="lints/listing/deny-by-default.html#missing-fragment-specifier" id="missing-fragment-specifier"><h2>missing-fragment-specifier</h2></a>
<p>The missing_fragment_specifier warning is issued when an unused pattern in a
<code>macro_rules!</code> macro definition has a meta-variable (e.g. <code>$e</code>) that is not
followed by a fragment specifier (e.g. <code>:expr</code>).</p>
<p>This warning can always be fixed by removing the unused pattern in the
<code>macro_rules!</code> macro definition.</p>
<a class="header" href="lints/listing/deny-by-default.html#mutable-transmutes" id="mutable-transmutes"><h2>mutable-transmutes</h2></a>
<p>This lint catches transmuting from <code>&T</code> to <code>&mut T</code> because it is undefined
behavior. Some example code that triggers this lint:</p>
<pre><code class="language-rust ignore">unsafe {
let y = std::mem::transmute::<&i32, &mut i32>(&5);
}
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: mutating transmuted &mut T from &T may cause undefined behavior, consider instead using an UnsafeCell
--> src/main.rs:3:17
|
3 | let y = std::mem::transmute::<&i32, &mut i32>(&5);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#no-mangle-const-items" id="no-mangle-const-items"><h2>no-mangle-const-items</h2></a>
<p>This lint detects any <code>const</code> items with the <code>#[no_mangle]</code> attribute.
Constants do not have their symbols exported, and therefore, this probably
means you meant to use a <code>static</code>, not a <code>const</code>. Some example code that
triggers this lint:</p>
<pre><code class="language-rust ignore">#[no_mangle]
const FOO: i32 = 5;
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: const items should never be #[no_mangle]
--> src/main.rs:3:1
|
3 | const FOO: i32 = 5;
| -----^^^^^^^^^^^^^^
| |
| help: try a static value: `pub static`
|
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#overflowing-literals" id="overflowing-literals"><h2>overflowing-literals</h2></a>
<p>This lint detects literal out of range for its type. Some
example code that triggers this lint:</p>
<pre><pre class="playpen"><code class="language-rust compile_fail">
# #![allow(unused_variables)]
#fn main() {
let x: u8 = 1000;
#}</code></pre></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: literal out of range for u8
--> src/main.rs:2:17
|
2 | let x: u8 = 1000;
| ^^^^
|
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#parenthesized-params-in-types-and-modules" id="parenthesized-params-in-types-and-modules"><h2>parenthesized-params-in-types-and-modules</h2></a>
<p>This lint detects incorrect parentheses. Some example code that triggers this
lint:</p>
<pre><code class="language-rust ignore">let x = 5 as usize();
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: parenthesized parameters may only be used with a trait
--> src/main.rs:2:21
|
2 | let x = 5 as usize();
| ^^
|
= note: #[deny(parenthesized_params_in_types_and_modules)] on by default
= 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 #42238 <https://github.com/rust-lang/rust/issues/42238>
</code></pre>
<p>To fix it, remove the <code>()</code>s.</p>
<a class="header" href="lints/listing/deny-by-default.html#pub-use-of-private-extern-crate" id="pub-use-of-private-extern-crate"><h2>pub-use-of-private-extern-crate</h2></a>
<p>This lint detects a specific situation of re-exporting a private <code>extern crate</code>;</p>
<a class="header" href="lints/listing/deny-by-default.html#safe-extern-statics" id="safe-extern-statics"><h2>safe-extern-statics</h2></a>
<p>In older versions of Rust, there was a soundness issue where <code>extern static</code>s were allowed
to be accessed in safe code. This lint now catches and denies this kind of code.</p>
<a class="header" href="lints/listing/deny-by-default.html#unknown-crate-types" id="unknown-crate-types"><h2>unknown-crate-types</h2></a>
<p>This lint detects an unknown crate type found in a <code>#[crate_type]</code> directive. Some
example code that triggers this lint:</p>
<pre><code class="language-rust ignore">#![crate_type="lol"]
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: invalid `crate_type` value
--> src/lib.rs:1:1
|
1 | #![crate_type="lol"]
| ^^^^^^^^^^^^^^^^^^^^
|
</code></pre>
<a class="header" href="lints/listing/deny-by-default.html#incoherent-fundamental-impls" id="incoherent-fundamental-impls"><h2>incoherent-fundamental-impls</h2></a>
<p>This lint detects potentially-conflicting impls that were erroneously allowed. Some
example code that triggers this lint:</p>
<pre><code class="language-rust ignore">pub trait Trait1<X> {
type Output;
}
pub trait Trait2<X> {}
pub struct A;
impl<X, T> Trait1<X> for T where T: Trait2<X> {
type Output = ();
}
impl<X> Trait1<Box<X>> for A {
type Output = i32;
}
</code></pre>
<p>This will produce:</p>
<pre><code class="language-text">error: conflicting implementations of trait `Trait1<std::boxed::Box<_>>` for type `A`: (E0119)
--> src/main.rs:13:1
|
9 | impl<X, T> Trait1<X> for T where T: Trait2<X> {
| --------------------------------------------- first implementation here
...
13 | impl<X> Trait1<Box<X>> for A {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `A`
|
= note: #[deny(incoherent_fundamental_impls)] on by default
= 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 #46205 <https://github.com/rust-lang/rust/issues/46205>
= note: downstream crates may implement trait `Trait2<std::boxed::Box<_>>` for type `A`
</code></pre>
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