Rollup merge of #95316 - fmease:rustdoc-discr-req-prov-assoc-consts-tys, r=notriddle,GuillaumeGomez

Rustdoc: Discriminate required and provided associated constants and types

Currently, rustdoc merely separates required and provided associated _functions_ (i.e. methods). This PR extends this to constants (fixes #94652) and types. This makes the documentation of all three kinds of associated items more alike and consistent.

As an aside, associated types may actually be provided / have a default when users enable the unstable feature `associated_type_defaults`.

| Before | After |
|---|---|
| ![image](https://user-images.githubusercontent.com/14913065/160631832-d5862d13-b395-4d86-b45c-3873ffd4cd4e.png) | ![image](https://user-images.githubusercontent.com/14913065/160631903-33909a03-b6ee-4d75-9cbc-d188f7f8602e.png) |
| ![image](https://user-images.githubusercontent.com/14913065/160632173-040d4139-76f4-4410-851b-d8c1cef014d2.png) | ![image](https://user-images.githubusercontent.com/14913065/160632233-6fd3fe73-cadc-4291-b104-59d2e45366a6.png) |

### `clean::types::ItemKind` modification

* `ItemKind::TypedefItem(.., true)` → `ItemKind::AssocTypeItem(..)`
* `ItemKind::TypedefItem(.., false)` → `ItemKind::TypedefItem(..)`

Further, I added `ItemKind::TyAssoc{Const,Type}Item`, the “required” variant of `ItemKind::Assoc{Const,Type}Item`, analogous to `ItemKind::TyMethodItem` with `ItemKind::MethodItem`. These new variants don't contain new information really, they are just the result of me getting rid of the `Option<_>` field in `AssocConstItem` and `AssocTypeItem`.

**Goal**: Make associated items more consistent.
Originally I thought modifying `ItemKind` was necessary to achieve the new functionality of this PR but in retrospect, it does not. If you don't like the changes to `ItemKind`, I think I _can_ get rid of them.

This change is the root cause of those tiny changes in a lot of different files.

 ### Concerns and Open Questions

* **breaking changes** to hyperlinks: Some heading IDs change:
  * `associated-const` (sic!) -> `{provided,required}-associated-consts`
  * `associated-types` -> `{provided,required}-associated-types`
* **verbosity** of the headings _{Required,Provided} Associated {Constants,Types}_
* For some files, I am not sure if the changes I made are correct. So please take extra care when reviewing `conversions.rs` (conversion to JSON), `cache.rs`/`fold_item`, `stripper.rs`/`fold_item`, `check_doc_test_visibility.rs`/`should_have_doc_example`, `collect_intra_doc_links.rs`/`from_assoc_item`
* JSON output: I still map `AssocTypeItem`s to `Typedef` etc. (FIXME)

src/test/rustdoc/assoc-consts.rs

@@ -1,8 +1,8 @@
 pub trait Foo {
     // @has assoc_consts/trait.Foo.html '//*[@class="rust trait"]' \
-    //      'const FOO: usize;'
+    //      'const FOO: usize = 13usize;'
     // @has - '//*[@id="associatedconstant.FOO"]' 'const FOO: usize'
-    const FOO: usize = 12;
+    const FOO: usize = 12 + 1;
     // @has - '//*[@id="associatedconstant.FOO_NO_DEFAULT"]' 'const FOO_NO_DEFAULT: bool'
     const FOO_NO_DEFAULT: bool;
     // @!has - FOO_HIDDEN

src/test/rustdoc/extern-default-method.no_href_on_anchor.html

@@ -0,0 +1 @@
+<a class="fnname">provided</a>(&amp;self)

src/test/rustdoc/extern-default-method.rs

@@ -1,9 +1,23 @@
 // aux-build:rustdoc-extern-default-method.rs
 // ignore-cross-compile
+// ignore-tidy-linelength
 
 extern crate rustdoc_extern_default_method as ext;
 
+// For this test, the dependency is compiled but not documented.
+//
+// Still, the struct from the external crate and its impl should be documented since
+// the struct is re-exported from this crate.
+// However, the method in the trait impl should *not* have a link (an `href` attribute) to
+// its corresponding item in the trait declaration since it would otherwise be broken.
+//
+// In older versions of rustdoc, the impl item (`a[@class="fnname"]`) used to link to
+// `#method.provided` – i.e. "to itself". Put in quotes since that was actually incorrect in
+// general: If the type `Struct` also had an inherent method called `provided`, the impl item
+// would link to that one even though those two methods are distinct items!
+
 // @count extern_default_method/struct.Struct.html '//*[@id="method.provided"]' 1
-// @has extern_default_method/struct.Struct.html '//*[@id="method.provided"]//a[@class="fnname"]/@href' #method.provided
+// @count extern_default_method/struct.Struct.html '//*[@id="method.provided"]//a[@class="fnname"]' 1
+// @snapshot no_href_on_anchor - '//*[@id="method.provided"]//a[@class="fnname"]'
 // @has extern_default_method/struct.Struct.html '//*[@id="method.provided"]//a[@class="anchor"]/@href' #method.provided
 pub use ext::Struct;

src/test/rustdoc/intra-doc/prim-self.rs

@@ -5,6 +5,7 @@
 #![feature(no_core)]
 #![feature(rustdoc_internals)]
 #![feature(inherent_associated_types)]
+#![feature(lang_items)]
 #![no_core]
 
 /// [Self::f]
@@ -35,3 +36,6 @@ pub struct S;
 impl S {
     pub fn f() {}
 }
+
+#[lang = "sized"]
+pub trait Sized {}

src/test/rustdoc/sidebar-items.rs

@@ -1,3 +1,4 @@
+#![feature(associated_type_defaults)]
 #![crate_name = "foo"]
 
 // @has foo/trait.Foo.html
@@ -5,12 +6,18 @@
 // @has - '//*[@class="sidebar-elems"]//section//a' 'bar'
 // @has - '//*[@class="sidebar-title"]/a[@href="#provided-methods"]' 'Provided Methods'
 // @has - '//*[@class="sidebar-elems"]//section//a' 'foo'
-// @has - '//*[@class="sidebar-title"]/a[@href="#associated-const"]' 'Associated Constants'
+// @has - '//*[@class="sidebar-title"]/a[@href="#required-associated-consts"]' 'Required Associated Constants'
+// @has - '//*[@class="sidebar-elems"]//section//a' 'FOO'
+// @has - '//*[@class="sidebar-title"]/a[@href="#provided-associated-consts"]' 'Provided Associated Constants'
 // @has - '//*[@class="sidebar-elems"]//section//a' 'BAR'
-// @has - '//*[@class="sidebar-title"]/a[@href="#associated-types"]' 'Associated Types'
+// @has - '//*[@class="sidebar-title"]/a[@href="#required-associated-types"]' 'Required Associated Types'
 // @has - '//*[@class="sidebar-elems"]//section//a' 'Output'
+// @has - '//*[@class="sidebar-title"]/a[@href="#provided-associated-types"]' 'Provided Associated Types'
+// @has - '//*[@class="sidebar-elems"]//section//a' 'Extra'
 pub trait Foo {
+    const FOO: usize;
     const BAR: u32 = 0;
+    type Extra: Copy = ();
     type Output: ?Sized;
 
     fn foo() {}