Fix `RefCell` `BorrowMut` error in `DocVisitor`

Until `external_traits` is cleaned up (i.e., no longer behind a
`RefCell`), `DocVisitor` will have to `take` `external_traits` -- just
like `DocFolder` -- to prevent `RefCell` runtime errors.

src/librustdoc/fold.rs

@@ -84,14 +84,12 @@ fn fold_mod(&mut self, m: Module) -> Module {
     fn fold_crate(&mut self, mut c: Crate) -> Crate {
         c.module = self.fold_item(c.module).unwrap();
 
-        {
-            let external_traits = { std::mem::take(&mut *c.external_traits.borrow_mut()) };
-            for (k, mut v) in external_traits {
-                v.trait_.items =
-                    v.trait_.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
-                c.external_traits.borrow_mut().insert(k, v);
-            }
+        let external_traits = { std::mem::take(&mut *c.external_traits.borrow_mut()) };
+        for (k, mut v) in external_traits {
+            v.trait_.items = v.trait_.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
+            c.external_traits.borrow_mut().insert(k, v);
         }
+
         c
     }
 }

src/librustdoc/visit.rs

@@ -43,9 +43,11 @@ fn visit_mod(&mut self, m: &Module) {
     fn visit_crate(&mut self, c: &Crate) {
         self.visit_item(&c.module);
 
-        let external_traits = c.external_traits.borrow();
-        for v in external_traits.values() {
-            v.trait_.items.iter().for_each(|i| self.visit_item(i))
+        // FIXME: make this a simple by-ref for loop once external_traits is cleaned up
+        let external_traits = { std::mem::take(&mut *c.external_traits.borrow_mut()) };
+        for (k, v) in external_traits {
+            v.trait_.items.iter().for_each(|i| self.visit_item(i));
+            c.external_traits.borrow_mut().insert(k, v);
         }
     }
 }