Rollup merge of #79182 - lochsh:78777-fix-extern-types-ref, r=jyn514

Fix links to extern types in rustdoc (fixes #78777)

 r? `@jyn514`
 Fixes #78777.
The initial fix we tried was:
```diff
diff --git a/src/librustdoc/passes/collect_intra_doc_links.rs b/src/librustdoc/passes/collect_intra_doc_links.rs
index 8be9482acff..c4b7086fdb1 100644
--- a/src/librustdoc/passes/collect_intra_doc_links.rs
+++ b/src/librustdoc/passes/collect_intra_doc_links.rs
`@@` -433,8 +433,9 `@@` impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
             Res::PrimTy(prim) => Some(
                 self.resolve_primitive_associated_item(prim, ns, module_id, item_name, item_str),
             ),
-            Res::Def(DefKind::Struct | DefKind::Union | DefKind::Enum | DefKind::TyAlias, did) => {
+            Res::Def(kind, did) if kind.ns() == Some(Namespace::TypeNS) => {
                 debug!("looking for associated item named {} for item {:?}", item_name, did);
+
                 // Checks if item_name belongs to `impl SomeItem`
                 let assoc_item = cx
                     .tcx
```

However, this caused traits to be matched, resulting in a panic when `resolve_associated_trait_item` is called further down in this function.

This PR also adds an error message for that panic. Currently it will look something like:
```rust
thread 'rustc' panicked at 'Not a type: DefIndex(8624)', compiler/rustc_metadata/src/rmeta/decoder.rs:951:32
```
I wasn't sure how to get a better debug output than `DefIndex(...)`, and am open to suggestions.

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -947,7 +947,12 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     }
 
     fn get_type(&self, id: DefIndex, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
-        self.root.tables.ty.get(self, id).unwrap().decode((self, tcx))
+        self.root
+            .tables
+            .ty
+            .get(self, id)
+            .unwrap_or_else(|| panic!("Not a type: {:?}", id))
+            .decode((self, tcx))
     }
 
     fn get_stability(&self, id: DefIndex) -> Option<attr::Stability> {

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -492,7 +492,14 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
             Res::PrimTy(prim) => Some(
                 self.resolve_primitive_associated_item(prim, ns, module_id, item_name, item_str),
             ),
-            Res::Def(DefKind::Struct | DefKind::Union | DefKind::Enum | DefKind::TyAlias, did) => {
+            Res::Def(
+                DefKind::Struct
+                | DefKind::Union
+                | DefKind::Enum
+                | DefKind::TyAlias
+                | DefKind::ForeignTy,
+                did,
+            ) => {
                 debug!("looking for associated item named {} for item {:?}", item_name, did);
                 // Checks if item_name belongs to `impl SomeItem`
                 let assoc_item = cx

src/test/rustdoc/intra-link-extern-type.rs

@@ -0,0 +1,18 @@
+#![feature(extern_types)]
+
+extern {
+    pub type ExternType;
+}
+
+impl ExternType {
+    pub fn f(&self) {
+
+    }
+}
+
+// @has 'intra_link_extern_type/foreigntype.ExternType.html'
+// @has 'intra_link_extern_type/fn.links_to_extern_type.html' \
+// 'href="../intra_link_extern_type/foreigntype.ExternType.html#method.f"'
+/// See also [ExternType::f]
+pub fn links_to_extern_type() {
+}