Make match in register_res easier to read

- Don't duplicate DefKind -> ItemType handling; that's a good way to get bugs
- Use exhaustive match
- Add comments

This found that register_res is very wrong in at least one way: if it
registers a Res for `Variant`, it should also register one for `Field`.
But I don't know whether the one for Variant should be removed or Field
added. Maybe someone has ideas?

src/librustdoc/clean/utils.rs

@@ -2,7 +2,7 @@ use crate::clean::auto_trait::AutoTraitFinder;
 use crate::clean::blanket_impl::BlanketImplFinder;
 use crate::clean::{
     inline, Clean, Crate, Generic, GenericArg, GenericArgs, ImportSource, Item, ItemKind, Lifetime,
-    MacroKind, Path, PathSegment, Primitive, PrimitiveType, ResolvedPath, Type, TypeBinding,
+    Path, PathSegment, Primitive, PrimitiveType, ResolvedPath, Type, TypeBinding,
 };
 use crate::core::DocContext;
 use crate::formats::item_type::ItemType;
@@ -431,35 +431,48 @@ crate fn get_auto_trait_and_blanket_impls(
     auto_impls.into_iter().chain(blanket_impls)
 }
 
+/// If `res` has a documentation page associated, store it in the cache.
+///
+/// This is later used by [`href()`] to determine the HTML link for the item.
+///
+/// [`href()`]: crate::html::format::href
 crate fn register_res(cx: &mut DocContext<'_>, res: Res) -> DefId {
+    use DefKind::*;
     debug!("register_res({:?})", res);
 
     let (did, kind) = match res {
-        Res::Def(DefKind::Fn, i) => (i, ItemType::Function),
-        Res::Def(DefKind::TyAlias, i) => (i, ItemType::Typedef),
-        Res::Def(DefKind::Enum, i) => (i, ItemType::Enum),
-        Res::Def(DefKind::Trait, i) => (i, ItemType::Trait),
         Res::Def(DefKind::AssocTy | DefKind::AssocFn | DefKind::AssocConst, i) => {
+            // associated items are documented, but on the page of their parent
             (cx.tcx.parent(i).unwrap(), ItemType::Trait)
         }
-        Res::Def(DefKind::Struct, i) => (i, ItemType::Struct),
-        Res::Def(DefKind::Union, i) => (i, ItemType::Union),
-        Res::Def(DefKind::Mod, i) => (i, ItemType::Module),
-        Res::Def(DefKind::ForeignTy, i) => (i, ItemType::ForeignType),
-        Res::Def(DefKind::Const, i) => (i, ItemType::Constant),
-        Res::Def(DefKind::Static, i) => (i, ItemType::Static),
         Res::Def(DefKind::Variant, i) => {
+            // variant items are documented, but on the page of their parent
             (cx.tcx.parent(i).expect("cannot get parent def id"), ItemType::Enum)
         }
-        Res::Def(DefKind::Macro(mac_kind), i) => match mac_kind {
-            MacroKind::Bang => (i, ItemType::Macro),
-            MacroKind::Attr => (i, ItemType::ProcAttribute),
-            MacroKind::Derive => (i, ItemType::ProcDerive),
-        },
-        Res::Def(DefKind::TraitAlias, i) => (i, ItemType::TraitAlias),
-        Res::SelfTy(Some(def_id), _) => (def_id, ItemType::Trait),
-        Res::SelfTy(_, Some((impl_def_id, _))) => return impl_def_id,
-        _ => return res.def_id(),
+        // Each of these have their own page.
+        Res::Def(
+            kind
+            @
+            (Fn | TyAlias | Enum | Trait | Struct | Union | Mod | ForeignTy | Const | Static
+            | Macro(..) | TraitAlias),
+            i,
+        ) => (i, kind.into()),
+        // This is part of a trait definition; document the trait.
+        Res::SelfTy(Some(trait_def_id), _) => (trait_def_id, ItemType::Trait),
+        // This is an inherent impl; it doesn't have its own page.
+        Res::SelfTy(None, Some((impl_def_id, _))) => return impl_def_id,
+        Res::SelfTy(None, None)
+        | Res::PrimTy(_)
+        | Res::ToolMod
+        | Res::SelfCtor(_)
+        | Res::Local(_)
+        | Res::NonMacroAttr(_)
+        | Res::Err => return res.def_id(),
+        Res::Def(
+            TyParam | ConstParam | Ctor(..) | ExternCrate | Use | ForeignMod | AnonConst | OpaqueTy
+            | Field | LifetimeParam | GlobalAsm | Impl | Closure | Generator,
+            id,
+        ) => return id,
     };
     if did.is_local() {
         return did;

src/test/rustdoc/intra-doc/field.rs

@@ -0,0 +1,4 @@
+// @has field/index.html '//a[@href="https://doc.rust-lang.org/nightly/core/ops/range/struct.Range.html#structfield.start"]' 'start'
+// @has field/index.html '//a[@href="https://doc.rust-lang.org/nightly/std/io/error/enum.ErrorKind.html#variant.NotFound"]' 'not_found'
+//! [start][std::ops::Range::start]
+//! [not_found][std::io::ErrorKind::NotFound]