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| <h1 id="diagnostic-items"><a class="header" href="#diagnostic-items">Diagnostic Items</a></h1> |
| <p>While writing lints it’s common to check for specific types, traits and |
| functions. This raises the question on how to check for these. Types can be |
| checked by their complete type path. However, this requires hard coding paths |
| and can lead to misclassifications in some edge cases. To counteract this, |
| rustc has introduced diagnostic items that are used to identify types via |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html"><code>Symbol</code></a>s.</p> |
| <h2 id="finding-diagnostic-items"><a class="header" href="#finding-diagnostic-items">Finding diagnostic items</a></h2> |
| <p>Diagnostic items are added to items inside <code>rustc</code>/<code>std</code>/<code>core</code>/<code>alloc</code> with the |
| <code>rustc_diagnostic_item</code> attribute. The item for a specific type can be found by |
| opening the source code in the documentation and looking for this attribute. |
| Note that it’s often added with the <code>cfg_attr</code> attribute to avoid compilation |
| errors during tests. A definition often looks like this:</p> |
| <pre><code class="language-rs">// This is the diagnostic item for this type vvvvvvv |
| #[cfg_attr(not(test), rustc_diagnostic_item = "Penguin")] |
| struct Penguin; |
| </code></pre> |
| <p>Diagnostic items are usually only added to traits, |
| types, |
| and standalone functions. |
| If the goal is to check for an associated type or method, |
| please use the diagnostic item of the item and reference |
| <a href="#using-diagnostic-items"><em>Using Diagnostic Items</em></a>.</p> |
| <h2 id="adding-diagnostic-items"><a class="header" href="#adding-diagnostic-items">Adding diagnostic items</a></h2> |
| <p>A new diagnostic item can be added with these two steps:</p> |
| <ol> |
| <li> |
| <p>Find the target item inside the Rust repo. Now add the diagnostic item as a |
| string via the <code>rustc_diagnostic_item</code> attribute. This can sometimes cause |
| compilation errors while running tests. These errors can be avoided by using |
| the <code>cfg_attr</code> attribute with the <code>not(test)</code> condition (it’s fine adding |
| then for all <code>rustc_diagnostic_item</code> attributes as a preventive manner). At |
| the end, it should look like this:</p> |
| <pre><code class="language-rs">// This will be the new diagnostic item vvv |
| #[cfg_attr(not(test), rustc_diagnostic_item = "Cat")] |
| struct Cat; |
| </code></pre> |
| <p>For the naming conventions of diagnostic items, please refer to |
| <a href="#naming-conventions"><em>Naming Conventions</em></a>.</p> |
| </li> |
| <li><!-- date-check: Feb 2023 --> |
| <p>Diagnostic items in code are accessed via symbols in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/sym/index.html"><code>rustc_span::symbol::sym</code></a>. |
| To add your newly-created diagnostic item, |
| simply open the module file, |
| and add the name (In this case <code>Cat</code>) at the correct point in the list.</p> |
| </li> |
| </ol> |
| <p>Now you can create a pull request with your changes. :tada:</p> |
| <blockquote> |
| <p>NOTE: |
| When using diagnostic items in other projects like Clippy, |
| it might take some time until the repos get synchronized.</p> |
| </blockquote> |
| <h2 id="naming-conventions"><a class="header" href="#naming-conventions">Naming conventions</a></h2> |
| <p>Diagnostic items don’t have a naming convention yet. |
| Following are some guidelines that should be used in future, |
| but might differ from existing names:</p> |
| <ul> |
| <li>Types, traits, and enums are named using UpperCamelCase |
| (Examples: <code>Iterator</code> and <code>HashMap</code>)</li> |
| <li>For type names that are used multiple times, |
| like <code>Writer</code>, |
| it’s good to choose a more precise name, |
| maybe by adding the module to it |
| (Example: <code>IoWriter</code>)</li> |
| <li>Associated items should not get their own diagnostic items, |
| but instead be accessed indirectly by the diagnostic item |
| of the type they’re originating from.</li> |
| <li>Freestanding functions like <code>std::mem::swap()</code> should be named using |
| <code>snake_case</code> with one important (export) module as a prefix |
| (Examples: <code>mem_swap</code> and <code>cmp_max</code>)</li> |
| <li>Modules should usually not have a diagnostic item attached to them. |
| Diagnostic items were added to avoid the usage of paths, |
| and using them on modules would therefore most likely be counterproductive.</li> |
| </ul> |
| <h2 id="using-diagnostic-items"><a class="header" href="#using-diagnostic-items">Using diagnostic items</a></h2> |
| <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 |
| <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 |
| 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> |
| 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 |
| 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 |
| either the symbol isn’t a diagnostic item or the type is not registered, for |
| instance when compiling with <code>#[no_std]</code>. |
| 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> |
| <h3 id="example-checking-for-a-type"><a class="header" href="#example-checking-for-a-type">Example: Checking for a type</a></h3> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>use rustc_span::symbol::sym; |
| |
| /// This example checks if the given type (`ty`) has the type `HashMap` using |
| /// `TyCtxt::is_diagnostic_item()` |
| fn example_1(cx: &LateContext<'_>, ty: Ty<'_>) -> bool { |
| match ty.kind() { |
| ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::HashMap, adt.did()), |
| _ => false, |
| } |
| } |
| <span class="boring">}</span></code></pre> |
| <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> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>/// This example checks if a given [`DefId`] from a method is part of a trait |
| /// implementation defined by a diagnostic item. |
| fn is_diag_trait_item( |
| cx: &LateContext<'_>, |
| def_id: DefId, |
| diag_item: Symbol |
| ) -> bool { |
| if let Some(trait_did) = cx.tcx.trait_of_item(def_id) { |
| return cx.tcx.is_diagnostic_item(diag_item, trait_did); |
| } |
| false |
| } |
| <span class="boring">}</span></code></pre> |
| <h3 id="associated-types"><a class="header" href="#associated-types">Associated Types</a></h3> |
| <p>Associated types of diagnostic items can be accessed indirectly by first |
| 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 |
| <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 |
| be used for further checks. Checkout |
| <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> |
| <h3 id="usage-in-clippy"><a class="header" href="#usage-in-clippy">Usage in Clippy</a></h3> |
| <p>Clippy tries to use diagnostic items where possible and has developed some |
| wrapper and utility functions. Please also refer to its documentation when |
| 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 |
| lints</em></a>.)</p> |
| <h2 id="related-issues"><a class="header" href="#related-issues">Related issues</a></h2> |
| <p>These are probably only interesting to people |
| who really want to take a deep dive into the topic :)</p> |
| <ul> |
| <li><a href="https://github.com/rust-lang/rust/pull/60966">rust#60966</a>: The Rust PR that introduced diagnostic items</li> |
| <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 |
| diagnostic item</li> |
| </ul> |
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