Change try_define_child to return a Result instead of an Option

src/librustc_resolve/build_reduced_graph.rs

@@ -100,7 +100,7 @@ impl<'a, 'b:'a, 'tcx:'b> GraphBuilder<'a, 'b, 'tcx> {
     fn try_define<T>(&self, parent: Module<'b>, name: Name, ns: Namespace, def: T)
         where T: ToNameBinding<'b>
     {
-        parent.try_define_child(name, ns, self.new_name_binding(def.to_name_binding()));
+        let _ = parent.try_define_child(name, ns, self.new_name_binding(def.to_name_binding()));
     }
 
     /// Defines `name` in namespace `ns` of module `parent` to be `def` if it is not yet defined;
@@ -108,8 +108,8 @@ impl<'a, 'b:'a, 'tcx:'b> GraphBuilder<'a, 'b, 'tcx> {
     fn define<T: ToNameBinding<'b>>(&self, parent: Module<'b>, name: Name, ns: Namespace, def: T) {
         let binding = self.new_name_binding(def.to_name_binding());
         let old_binding = match parent.try_define_child(name, ns, binding) {
-            Some(old_binding) => old_binding,
-            None => return,
+            Ok(()) => return,
+            Err(old_binding) => old_binding,
         };
 
         let span = binding.span.unwrap_or(DUMMY_SP);

src/librustc_resolve/lib.rs

@@ -870,10 +870,10 @@ impl<'a> ModuleS<'a> {
     // If the name is not yet defined, define the name and return None.
     // Otherwise, return the existing definition.
     fn try_define_child(&self, name: Name, ns: Namespace, binding: &'a NameBinding<'a>)
-                        -> Option<&'a NameBinding<'a>> {
+                        -> Result<(), &'a NameBinding<'a>> {
         match self.children.borrow_mut().entry((name, ns)) {
-            hash_map::Entry::Vacant(entry) => { entry.insert(binding); None }
-            hash_map::Entry::Occupied(entry) => { Some(entry.get()) },
+            hash_map::Entry::Vacant(entry) => { entry.insert(binding); Ok(()) }
+            hash_map::Entry::Occupied(entry) => { Err(entry.get()) },
         }
     }