# Diagnostic Items

## Background

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
[`Symbol`]s.

## How To Find Diagnostic Items

Diagnostic items are added to items inside `rustc`/`std`/`core` 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:

```rs
// 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 [*How To Use Diagnostic Items*](#how-to-use-diagnostic-items).

## How To Add Diagnostic Items

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

1. 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:

    ```rs
    // 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*](#naming-conventions).

2. As of August 2021 <!-- date: 2021-08 --> 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 that 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 set in stone naming convention yet. These are some guidelines that
should be used for the future, but might differ from existing names:

* Types, traits and enums are named using UpperCamelCase (Examples: `Iterator`, `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 (Example: `mem_swap`, `cmp_max`)
* Modules should usually not have a diagnostic item attached to them. Diagnostic items were
  added to avoid the usage of paths, using them on modules would therefore most likely to be
  counterproductive.

## How To Use Diagnostic Items

In rustc, diagnostic items are looked up via [`Symbol`]s from inside the
[`rustc_span::symbol::sym`] module. These can then be mapped to [`DefId`]s 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 following
examples are based on [`DefId`]s and their usage.

### Check For A Type 

```rust
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,
    }
}
```

### Check For A Trait Implementation

```rust
/// 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*][clippy-Common-tools-for-writing-lints].)

## Related Issues

This lists some related issues. 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

<!-- Links -->

[`rustc_span::symbol::sym`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/sym/index.html
[`Symbol`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html
[`DefId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html
[`TyCtxt::get_diagnostic_item()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.get_diagnostic_item
[`TyCtxt::is_diagnostic_item()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.is_diagnostic_item
[`TyCtxt::associated_items()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.associated_items
[`AssocItems`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/assoc/struct.AssocItems.html
[`clippy_utils::ty::get_iterator_item_ty()`]: https://github.com/rust-lang/rust-clippy/blob/305177342fbc622c0b3cb148467bab4b9524c934/clippy_utils/src/ty.rs#L55-L72
[clippy-Common-tools-for-writing-lints]: https://github.com/rust-lang/rust-clippy/blob/master/doc/common_tools_writing_lints.md
[rust#60966]: https://github.com/rust-lang/rust/pull/60966
[rust-clippy#5393]: https://github.com/rust-lang/rust-clippy/issues/5393
