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

 rust-lang#58972 ignored extern_traits because before rust-lang#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);
+ {
+ 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)| {
- v.items = v.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
- (k, v)
- })
- .collect();
+ 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();
+ 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