Diagnostic Items

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 Symbols.

Finding diagnostic items

Diagnostic items are added to items inside rustc/std/core/alloc with the rustc_diagnostic_item 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 cfg_attr attribute to avoid compilation errors during tests. A definition often looks like this:

// This is the diagnostic item for this type   vvvvvvv
#[cfg_attr(not(test), rustc_diagnostic_item = "Penguin")]
struct Penguin;

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 Using Diagnostic Items.

Adding diagnostic items

A new diagnostic item can be added with these two steps:

Find the target item inside the Rust repo. Now add the diagnostic item as a string via the rustc_diagnostic_item attribute. This can sometimes cause compilation errors while running tests. These errors can be avoided by using the cfg_attr attribute with the not(test) condition (it's fine adding then for all rustc_diagnostic_item attributes as a preventive manner). At the end, it should look like this:
```
// This will be the new diagnostic item        vvv
#[cfg_attr(not(test), rustc_diagnostic_item = "Cat")]
struct Cat;
```
For the naming conventions of diagnostic items, please refer to Naming Conventions.
Diagnostic items in code are accessed via symbols in rustc_span::symbol::sym. To add your newly-created diagnostic item, simply open the module file, and add the name (In this case Cat) at the correct point in the list.

Now you can create a pull request with your changes. :tada:

NOTE: When using diagnostic items in other projects like Clippy, it might take some time until the repos get synchronized.

Naming conventions

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:

Types, traits, and enums are named using UpperCamelCase (Examples: Iterator and HashMap)
For type names that are used multiple times, like Writer, it's good to choose a more precise name, maybe by adding the module to it (Example: IoWriter)
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.
Freestanding functions like std::mem::swap() should be named using snake_case with one important (export) module as a prefix (Examples: mem_swap and cmp_max)
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.

Using diagnostic items

In rustc, diagnostic items are looked up via Symbols from inside the rustc_span::symbol::sym module. These can then be mapped to DefIds using TyCtxt::get_diagnostic_item() or checked if they match a DefId using TyCtxt::is_diagnostic_item(). When mapping from a diagnostic item to a DefId, the method will return a Option<DefId>. This can be None if either the symbol isn't a diagnostic item or the type is not registered, for instance when compiling with #[no_std]. All the following examples are based on DefIds and their usage.

Example: Checking for a type

#![allow(unused)]
fn main() {
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,
    }
}
}

Example: Checking for a trait implementation

#![allow(unused)]
fn main() {
/// 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
}
}

Associated Types

Associated types of diagnostic items can be accessed indirectly by first getting the DefId of the trait and then calling TyCtxt::associated_items(). This returns an AssocItems object which can be used for further checks. Checkout clippy_utils::ty::get_iterator_item_ty() for an example usage of this.

Usage in Clippy

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 Common tools for writing lints.)

These are probably only interesting to people who really want to take a deep dive into the topic :)

rust#60966: The Rust PR that introduced diagnostic items
rust-clippy#5393: Clippy's tracking issue for moving away from hard coded paths to diagnostic item

Rust Compiler Development Guide