Fix rustdoc infinitely recursing when an external crate reexports itself

Previously, rustdoc's LibEmbargoVisitor unconditionally visited the
child modules of an external crate. If a module re-exported its parent
via 'pub use super::*', rustdoc would re-walk the parent, leading to
infinite recursion.

This commit makes LibEmbargoVisitor store already visited modules in an
FxHashSet, ensuring that each module is only walked once.

Fixes #40936

src/librustdoc/visit_lib.rs

@@ -13,6 +13,7 @@ use rustc::middle::privacy::{AccessLevels, AccessLevel};
 use rustc::hir::def::Def;
 use rustc::hir::def_id::{CrateNum, CRATE_DEF_INDEX, DefId};
 use rustc::ty::Visibility;
+use rustc::util::nodemap::FxHashSet;
 
 use std::cell::RefMut;
 
@@ -29,6 +30,8 @@ pub struct LibEmbargoVisitor<'a, 'b: 'a, 'tcx: 'b> {
     access_levels: RefMut<'a, AccessLevels<DefId>>,
     // Previous accessibility level, None means unreachable
     prev_level: Option<AccessLevel>,
+    // Keeps track of already visited modules, in case a module re-exports its parent
+    visited_mods: FxHashSet<DefId>,
 }
 
 impl<'a, 'b, 'tcx> LibEmbargoVisitor<'a, 'b, 'tcx> {
@@ -38,6 +41,7 @@ impl<'a, 'b, 'tcx> LibEmbargoVisitor<'a, 'b, 'tcx> {
             cstore: &*cx.sess().cstore,
             access_levels: cx.access_levels.borrow_mut(),
             prev_level: Some(AccessLevel::Public),
+            visited_mods: FxHashSet()
         }
     }
 
@@ -62,6 +66,10 @@ impl<'a, 'b, 'tcx> LibEmbargoVisitor<'a, 'b, 'tcx> {
     }
 
     pub fn visit_mod(&mut self, def_id: DefId) {
+        if !self.visited_mods.insert(def_id) {
+            return;
+        }
+
         for item in self.cstore.item_children(def_id) {
             self.visit_item(item.def);
         }