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<h1><a class="header" href="#denywarnings" id="denywarnings"><code>#![deny(warnings)]</code></a></h1>
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<h2><a class="header" href="#description" id="description">Description</a></h2>
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<p>A well-intentioned crate author wants to ensure their code builds without
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warnings. So they annotate their crate root with the following:</p>
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<h2><a class="header" href="#example" id="example">Example</a></h2>
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<pre><pre class="playground"><code class="language-rust edition2018">
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<span class="boring">#![allow(unused)]
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</span>#![deny(warnings)]
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<span class="boring">fn main() {
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</span>// All is well.
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<span class="boring">}
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</span></code></pre></pre>
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<h2><a class="header" href="#advantages" id="advantages">Advantages</a></h2>
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<p>It is short and will stop the build if anything is amiss.</p>
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<h2><a class="header" href="#drawbacks" id="drawbacks">Drawbacks</a></h2>
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<p>By disallowing the compiler to build with warnings, a crate author opts out of
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Rust's famed stability. Sometimes new features or old misfeatures need a change
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in how things are done, thus lints are written that <code>warn</code> for a certain grace
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period before being turned to <code>deny</code>.</p>
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<p>For example, it was discovered that a type could have two <code>impl</code>s with the same
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method. This was deemed a bad idea, but in order to make the transition smooth,
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the <code>overlapping-inherent-impls</code> lint was introduced to give a warning to those
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stumbling on this fact, before it becomes a hard error in a future release.</p>
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<p>Also sometimes APIs get deprecated, so their use will emit a warning where
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before there was none.</p>
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<p>All this conspires to potentially break the build whenever something changes.</p>
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<p>Furthermore, crates that supply additional lints (e.g. <a href="https://github.com/Manishearth/rust-clippy">rust-clippy</a>) can no
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longer be used unless the annotation is removed. This is mitigated with
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<a href="https://doc.rust-lang.org/rustc/lints/levels.html#capping-lints">--cap-lints</a>. The <code>--cap-lints=warn</code> command line argument, turns all <code>deny</code>
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lint errors into warnings. But be aware that <code>forbid</code> lints are stronger than
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<code>deny</code> hence the 'forbid' level cannot be overridden to be anything lower than
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an error. As a result <code>forbid</code> lints will still stop compilation.</p>
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<h2><a class="header" href="#alternatives" id="alternatives">Alternatives</a></h2>
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<p>There are two ways of tackling this problem: First, we can decouple the build
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setting from the code, and second, we can name the lints we want to deny
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explicitly.</p>
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<p>The following command line will build with all warnings set to <code>deny</code>:</p>
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<p><code>RUSTFLAGS="-D warnings" cargo build</code></p>
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<p>This can be done by any individual developer (or be set in a CI tool like
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Travis, but remember that this may break the build when something changes)
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without requiring a change to the code.</p>
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<p>Alternatively, we can specify the lints that we want to <code>deny</code> in the code.
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Here is a list of warning lints that is (hopefully) safe to deny:</p>
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<pre><code class="language-rust ignore">#[deny(bad-style,
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const-err,
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dead-code,
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extra-requirement-in-impl,
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improper-ctypes,
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legacy-directory-ownership,
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non-shorthand-field-patterns,
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no-mangle-generic-items,
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overflowing-literals,
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path-statements ,
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patterns-in-fns-without-body,
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plugin-as-library,
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private-in-public,
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private-no-mangle-fns,
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private-no-mangle-statics,
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raw-pointer-derive,
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safe-extern-statics,
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unconditional-recursion,
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unions-with-drop-fields,
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unused,
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unused-allocation,
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unused-comparisons,
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unused-parens,
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while-true)]
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</code></pre>
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<p>In addition, the following <code>allow</code>ed lints may be a good idea to <code>deny</code>:</p>
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<pre><code class="language-rust ignore">#[deny(missing-debug-implementations,
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missing-docs,
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trivial-casts,
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trivial-numeric-casts,
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unused-extern-crates,
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unused-import-braces,
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unused-qualifications,
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unused-results)]
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</code></pre>
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<p>Some may also want to add <code>missing-copy-implementations</code> to their list.</p>
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<p>Note that we explicitly did not add the <code>deprecated</code> lint, as it is fairly
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certain that there will be more deprecated APIs in the future.</p>
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<h2><a class="header" href="#see-also" id="see-also">See also</a></h2>
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<ul>
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<li><a href="https://doc.rust-lang.org/reference/attributes.html#deprecation">deprecate attribute</a> documentation</li>
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<li>Type <code>rustc -W help</code> for a list of lints on your system. Also type
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<code>rustc --help</code> for a general list of options</li>
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<li><a href="https://github.com/Manishearth/rust-clippy">rust-clippy</a> is a collection of lints for better Rust code</li>
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</ul>
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