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.

0 files changed, 0 insertions, 0 deletions