Improve the "Diagnostic items" chapter (#1427)

Author: tshepang

src/diagnostics/diagnostic-items.md

@@ -1,4 +1,5 @@
 # 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
@@ -7,7 +8,8 @@ rustc has introduced diagnostic items that are used to identify types via
 [`Symbol`]s.
 
 ## Finding diagnostic items
-Diagnostic items are added to items inside `rustc`/`std`/`core` with the
+
+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
@@ -19,12 +21,15 @@ errors during tests. A definition often looks like this:
 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).
+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*](#using-diagnostic-items).
 
 ## Adding 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
@@ -43,45 +48,55 @@ A new diagnostic item can be added with these two steps:
     For the naming conventions of diagnostic items, please refer to
     [*Naming Conventions*](#naming-conventions).
 
-2. As of <!-- date-check --> February 2022, 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.
+2. <!-- date-check: Aug 2022 -->
+   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.)
+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 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.
+
+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 (Example:
-  `mem_swap`, `cmp_max`)
+  `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, using them on
-  modules would therefore most likely to be counterproductive.
+  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 [`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.
+instance when compiling with `#[no_std]`.
+All the following examples are based on [`DefId`]s and their usage.
 
 ### Example: Checking for a type
+
 ```rust
 use rustc_span::symbol::sym;
 
@@ -96,6 +111,7 @@ fn example_1(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
 ```
 
 ### Example: Checking 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.
@@ -112,20 +128,23 @@ fn is_diag_trait_item(
 ```
 
 ### 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
+
+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