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<main>
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<h1 id="diagnostic-items"><a class="header" href="#diagnostic-items">Diagnostic Items</a></h1>
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<p>While writing lints it's common to check for specific types, traits and
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functions. This raises the question on how to check for these. Types can be
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checked by their complete type path. However, this requires hard coding paths
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and can lead to misclassifications in some edge cases. To counteract this,
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rustc has introduced diagnostic items that are used to identify types via
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html"><code>Symbol</code></a>s.</p>
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<h2 id="finding-diagnostic-items"><a class="header" href="#finding-diagnostic-items">Finding diagnostic items</a></h2>
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<p>Diagnostic items are added to items inside <code>rustc</code>/<code>std</code>/<code>core</code>/<code>alloc</code> with the
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<code>rustc_diagnostic_item</code> attribute. The item for a specific type can be found by
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opening the source code in the documentation and looking for this attribute.
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Note that it's often added with the <code>cfg_attr</code> attribute to avoid compilation
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errors during tests. A definition often looks like this:</p>
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<pre><code class="language-rs">// This is the diagnostic item for this type vvvvvvv
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#[cfg_attr(not(test), rustc_diagnostic_item = "Penguin")]
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struct Penguin;
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</code></pre>
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<p>Diagnostic items are usually only added to traits,
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types,
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and standalone functions.
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If the goal is to check for an associated type or method,
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please use the diagnostic item of the item and reference
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<a href="#using-diagnostic-items"><em>Using Diagnostic Items</em></a>.</p>
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<h2 id="adding-diagnostic-items"><a class="header" href="#adding-diagnostic-items">Adding diagnostic items</a></h2>
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<p>A new diagnostic item can be added with these two steps:</p>
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<ol>
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<li>
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<p>Find the target item inside the Rust repo. Now add the diagnostic item as a
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string via the <code>rustc_diagnostic_item</code> attribute. This can sometimes cause
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compilation errors while running tests. These errors can be avoided by using
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the <code>cfg_attr</code> attribute with the <code>not(test)</code> condition (it's fine adding
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then for all <code>rustc_diagnostic_item</code> attributes as a preventive manner). At
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the end, it should look like this:</p>
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<pre><code class="language-rs">// This will be the new diagnostic item vvv
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#[cfg_attr(not(test), rustc_diagnostic_item = "Cat")]
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struct Cat;
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</code></pre>
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<p>For the naming conventions of diagnostic items, please refer to
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<a href="#naming-conventions"><em>Naming Conventions</em></a>.</p>
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</li>
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<li><!-- date-check: Feb 2023 -->
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<p>Diagnostic items in code are accessed via symbols in
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/sym/index.html"><code>rustc_span::symbol::sym</code></a>.
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To add your newly-created diagnostic item,
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simply open the module file,
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and add the name (In this case <code>Cat</code>) at the correct point in the list.</p>
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</li>
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</ol>
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<p>Now you can create a pull request with your changes. :tada:</p>
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<blockquote>
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<p>NOTE:
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When using diagnostic items in other projects like Clippy,
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it might take some time until the repos get synchronized.</p>
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</blockquote>
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<h2 id="naming-conventions"><a class="header" href="#naming-conventions">Naming conventions</a></h2>
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<p>Diagnostic items don't have a naming convention yet.
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Following are some guidelines that should be used in future,
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but might differ from existing names:</p>
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<ul>
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<li>Types, traits, and enums are named using UpperCamelCase
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(Examples: <code>Iterator</code> and <code>HashMap</code>)</li>
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<li>For type names that are used multiple times,
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like <code>Writer</code>,
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it's good to choose a more precise name,
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maybe by adding the module to it
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(Example: <code>IoWriter</code>)</li>
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<li>Associated items should not get their own diagnostic items,
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but instead be accessed indirectly by the diagnostic item
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of the type they're originating from.</li>
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<li>Freestanding functions like <code>std::mem::swap()</code> should be named using
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<code>snake_case</code> with one important (export) module as a prefix
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(Examples: <code>mem_swap</code> and <code>cmp_max</code>)</li>
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<li>Modules should usually not have a diagnostic item attached to them.
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Diagnostic items were added to avoid the usage of paths,
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and using them on modules would therefore most likely be counterproductive.</li>
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</ul>
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<h2 id="using-diagnostic-items"><a class="header" href="#using-diagnostic-items">Using diagnostic items</a></h2>
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<p>In rustc, diagnostic items are looked up via <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html"><code>Symbol</code></a>s from inside the
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/sym/index.html"><code>rustc_span::symbol::sym</code></a> module. These can then be mapped to <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a>s
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using <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.get_diagnostic_item"><code>TyCtxt::get_diagnostic_item()</code></a> or checked if they match a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a>
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using <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.is_diagnostic_item"><code>TyCtxt::is_diagnostic_item()</code></a>. When mapping from a diagnostic item to
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a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a>, the method will return a <code>Option<DefId></code>. This can be <code>None</code> if
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either the symbol isn't a diagnostic item or the type is not registered, for
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instance when compiling with <code>#[no_std]</code>.
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All the following examples are based on <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a>s and their usage.</p>
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<h3 id="example-checking-for-a-type"><a class="header" href="#example-checking-for-a-type">Example: Checking for a type</a></h3>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use rustc_span::symbol::sym;
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/// This example checks if the given type (`ty`) has the type `HashMap` using
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/// `TyCtxt::is_diagnostic_item()`
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fn example_1(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
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match ty.kind() {
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ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::HashMap, adt.did()),
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_ => false,
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<h3 id="example-checking-for-a-trait-implementation"><a class="header" href="#example-checking-for-a-trait-implementation">Example: Checking for a trait implementation</a></h3>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>/// This example checks if a given [`DefId`] from a method is part of a trait
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/// implementation defined by a diagnostic item.
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fn is_diag_trait_item(
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cx: &LateContext<'_>,
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def_id: DefId,
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diag_item: Symbol
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) -> bool {
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if let Some(trait_did) = cx.tcx.trait_of_item(def_id) {
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return cx.tcx.is_diagnostic_item(diag_item, trait_did);
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}
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false
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}
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<span class="boring">}</span></code></pre></pre>
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<h3 id="associated-types"><a class="header" href="#associated-types">Associated Types</a></h3>
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<p>Associated types of diagnostic items can be accessed indirectly by first
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getting the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a> of the trait and then calling
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.associated_items"><code>TyCtxt::associated_items()</code></a>. This returns an <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/assoc/struct.AssocItems.html"><code>AssocItems</code></a> object which can
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be used for further checks. Checkout
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<a href="https://github.com/rust-lang/rust-clippy/blob/305177342fbc622c0b3cb148467bab4b9524c934/clippy_utils/src/ty.rs#L55-L72"><code>clippy_utils::ty::get_iterator_item_ty()</code></a> for an example usage of this.</p>
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<h3 id="usage-in-clippy"><a class="header" href="#usage-in-clippy">Usage in Clippy</a></h3>
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<p>Clippy tries to use diagnostic items where possible and has developed some
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wrapper and utility functions. Please also refer to its documentation when
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using diagnostic items in Clippy. (See <a href="https://doc.rust-lang.org/nightly/clippy/development/common_tools_writing_lints.html"><em>Common tools for writing
|
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lints</em></a>.)</p>
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<h2 id="related-issues"><a class="header" href="#related-issues">Related issues</a></h2>
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<p>These are probably only interesting to people
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who really want to take a deep dive into the topic :)</p>
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<ul>
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<li><a href="https://github.com/rust-lang/rust/pull/60966">rust#60966</a>: The Rust PR that introduced diagnostic items</li>
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<li><a href="https://github.com/rust-lang/rust-clippy/issues/5393">rust-clippy#5393</a>: Clippy's tracking issue for moving away from hard coded paths to
|
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diagnostic item</li>
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</ul>
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