rustdoc: Fix intra-doc links for cross-crate re-exports of traits

#58972 ignored extern_traits because before #65983 was fixed, they
would always fail to resolve, giving spurious warnings.
This undoes that change, so extern traits are now seen by the
`collect_intra_doc_links` pass. There are also some minor changes in
librustdoc/fold.rs to avoid borrowing the extern_traits RefCell more
than once at a time.

src/librustdoc/clean/inline.rs

@@ -628,7 +628,9 @@ pub fn record_extern_trait(cx: &DocContext<'_>, did: DefId) {
         }
     }
 
+    {
         cx.active_extern_traits.borrow_mut().insert(did);
+    }
 
     debug!("record_extern_trait: {:?}", did);
     let trait_ = build_external_trait(cx, did);

src/librustdoc/fold.rs

@@ -93,15 +93,11 @@ pub trait DocFolder: Sized {
         c.module = c.module.take().and_then(|module| self.fold_item(module));
 
         {
-            let mut guard = c.external_traits.borrow_mut();
-            let external_traits = std::mem::replace(&mut *guard, Default::default());
-            *guard = external_traits
-                .into_iter()
-                .map(|(k, mut v)| {
+            let external_traits = { std::mem::take(&mut *c.external_traits.borrow_mut()) };
+            for (k, mut v) in external_traits {
                 v.items = v.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
-                    (k, v)
-                })
-                .collect();
+                c.external_traits.borrow_mut().insert(k, v);
+            }
         }
         c
     }

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -971,15 +971,6 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
             self.fold_item_recur(item)
         }
     }
-
-    // FIXME: if we can resolve intra-doc links from other crates, we can use the stock
-    // `fold_crate`, but until then we should avoid scanning `krate.external_traits` since those
-    // will never resolve properly
-    fn fold_crate(&mut self, mut c: Crate) -> Crate {
-        c.module = c.module.take().and_then(|module| self.fold_item(module));
-
-        c
-    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]

src/test/rustdoc/intra-doc-crate/traits.rs

@@ -1,5 +1,3 @@
-// ignore-test
-// ^ this is https://github.com/rust-lang/rust/issues/73829
 // aux-build:traits.rs
 // build-aux-docs
 // ignore-tidy-line-length