Fix non-termination in resolve.

Author: jseyfried

src/librustc_resolve/lib.rs

@@ -2943,6 +2943,7 @@ impl<'a> Resolver<'a> {
 
         let mut lookup_results = Vec::new();
         let mut worklist = Vec::new();
+        let mut seen_modules = FxHashSet();
         worklist.push((self.graph_root, Vec::new(), false));
 
         while let Some((in_module,
@@ -2989,7 +2990,7 @@ impl<'a> Resolver<'a> {
                     if !in_module_is_extern || name_binding.vis == ty::Visibility::Public {
                         // add the module to the lookup
                         let is_extern = in_module_is_extern || name_binding.is_extern_crate();
-                        if !worklist.iter().any(|&(m, ..)| m.def() == module.def()) {
+                        if seen_modules.insert(module.def_id().unwrap()) {
                             worklist.push((module, path_segments, is_extern));
                         }
                     }

src/test/compile-fail/recursive-reexports.rs

@@ -10,6 +10,8 @@
 
 // aux-build:recursive_reexports.rs
 
-fn f() -> recursive_reexports::S {} //~ ERROR undeclared
+extern crate recursive_reexports;
+
+fn f() -> recursive_reexports::S {} //~ ERROR type name `recursive_reexports::S` is undefined
 
 fn main() {}