Auto merge of rust-lang#79539 - aDotInTheVoid:json-mvp, r=jyn514

Rustdoc: JSON backend experimental impl, with new tests.

Based on rust-lang#75114 by `@P1n3appl3`

The first commit is all of rust-lang#75114, but squased to 1 commit, as that was much easier to rebase onto master.

The git history is a mess, but I think I'll edit it after review, so it's obvious whats new.

## Still to do

- [ ] Update docs.
- [ ] Add bless option to tests.
- [ ] Add test option for multiple files in same crate.
- [ ] Decide if the tests should check for json to be equal or subset.
- [ ] Go through the rest of the review for the original pr. (This is open because the test system is done(ish), but stuff like [not using a hashmap](rust-lang#75114 (comment)) and [using `CRATE_DEF_INDEX` ](rust-lang#75114 (comment)) hasn't)

I'm also sure how many of these we need to do before landing on nightly, as it would be nice to get this in tree, so it isn't effected by churn like rust-lang#79125, rust-lang#79041, rust-lang#79061

r? `@jyn514`

src/bootstrap/builder.rs

@@ -425,6 +425,7 @@ impl<'a> Builder<'a> {
                 test::RustdocJSNotStd,
                 test::RustdocTheme,
                 test::RustdocUi,
+                test::RustdocJson,
                 // Run bootstrap close to the end as it's unlikely to fail
                 test::Bootstrap,
                 // Run run-make last, since these won't pass without make on Windows

src/bootstrap/test.rs

@@ -904,6 +904,12 @@ host_test!(UiFullDeps { path: "src/test/ui-fulldeps", mode: "ui", suite: "ui-ful
 host_test!(Rustdoc { path: "src/test/rustdoc", mode: "rustdoc", suite: "rustdoc" });
 host_test!(RustdocUi { path: "src/test/rustdoc-ui", mode: "ui", suite: "rustdoc-ui" });
 
+host_test!(RustdocJson {
+    path: "src/test/rustdoc-json",
+    mode: "rustdoc-json",
+    suite: "rustdoc-json"
+});
+
 host_test!(Pretty { path: "src/test/pretty", mode: "pretty", suite: "pretty" });
 
 default_test!(RunMake { path: "src/test/run-make", mode: "run-make", suite: "run-make" });
@@ -1001,6 +1007,7 @@ note: if you're sure you want to do this, please open an issue as to why. In the
             || (mode == "run-make" && suite.ends_with("fulldeps"))
             || (mode == "ui" && is_rustdoc)
             || mode == "js-doc-test"
+            || mode == "rustdoc-json"
         {
             cmd.arg("--rustdoc-path").arg(builder.rustdoc(compiler));
         }

src/librustdoc/clean/types.rs

@@ -337,6 +337,42 @@ crate enum ItemKind {
 }
 
 impl ItemKind {
+    /// Some items contain others such as structs (for their fields) and Enums
+    /// (for their variants). This method returns those contained items.
+    crate fn inner_items(&self) -> impl Iterator<Item = &Item> {
+        match self {
+            StructItem(s) => s.fields.iter(),
+            UnionItem(u) => u.fields.iter(),
+            VariantItem(Variant { kind: VariantKind::Struct(v) }) => v.fields.iter(),
+            EnumItem(e) => e.variants.iter(),
+            TraitItem(t) => t.items.iter(),
+            ImplItem(i) => i.items.iter(),
+            ModuleItem(m) => m.items.iter(),
+            ExternCrateItem(_, _)
+            | ImportItem(_)
+            | FunctionItem(_)
+            | TypedefItem(_, _)
+            | OpaqueTyItem(_)
+            | StaticItem(_)
+            | ConstantItem(_)
+            | TraitAliasItem(_)
+            | TyMethodItem(_)
+            | MethodItem(_, _)
+            | StructFieldItem(_)
+            | VariantItem(_)
+            | ForeignFunctionItem(_)
+            | ForeignStaticItem(_)
+            | ForeignTypeItem
+            | MacroItem(_)
+            | ProcMacroItem(_)
+            | PrimitiveItem(_)
+            | AssocConstItem(_, _)
+            | AssocTypeItem(_, _)
+            | StrippedItem(_)
+            | KeywordItem(_) => [].iter(),
+        }
+    }
+
     crate fn is_type_alias(&self) -> bool {
         match *self {
             ItemKind::TypedefItem(_, _) | ItemKind::AssocTypeItem(_, _) => true,
@@ -1613,6 +1649,11 @@ impl Path {
     crate fn last_name(&self) -> &str {
         self.segments.last().expect("segments were empty").name.as_str()
     }
+
+    crate fn whole_name(&self) -> String {
+        String::from(if self.global { "::" } else { "" })
+            + &self.segments.iter().map(|s| s.name.clone()).collect::<Vec<_>>().join("::")
+    }
 }
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]