addressing Niko's comments

src/librustc_mir/build/mod.rs

@@ -647,9 +647,15 @@ fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
     let fn_def_id = tcx_hir.local_def_id(fn_id);
 
     // Gather the upvars of a closure, if any.
-    let upvar_decls: Vec<_> = match hir_tables.upvar_list.get(&fn_def_id) {
-        Some(upvars) => upvars
-            .iter()
+    // In analyze_closure() in upvar.rs we gathered a list of upvars used by a
+    // closure and we stored in a map called upvar_list in TypeckTables indexed
+    // with the closure's DefId. Here, we run through that vec of UpvarIds for
+    // the given closure and use the necessary information to create UpvarDecl.
+    let upvar_decls: Vec<_> = hir_tables
+        .upvar_list
+        .get(&fn_def_id)
+        .into_iter()
+        .flatten()
         .map(|upvar_id| {
             let var_hir_id = upvar_id.var_path.hir_id;
             let var_node_id = tcx_hir.hir_to_node_id(var_hir_id);
@@ -680,9 +686,7 @@ fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
             }
             decl
         })
-            .collect(),
-        _ => vec![],
-    };
+        .collect();
 
     let mut builder = Builder::new(hir,
         span,

src/librustc_typeck/check/writeback.rs

@@ -20,16 +20,16 @@ use syntax_pos::Span;
 
 ///////////////////////////////////////////////////////////////////////////
 // Entry point
-/// During type inference, partially inferred types are
-/// represented using Type variables (ty::Infer). These don't appear in
-/// the final TypeckTables since all of the types should have been
-/// inferred once typeck_tables_of is done.
-/// When type inference is running however, having to update the typeck
-/// tables every time a new type is inferred would be unreasonably slow,
-/// so instead all of the replacement happens at the end in
-/// resolve_type_vars_in_body, which creates a new TypeTables which
-/// doesn't contain any inference types.
 
+// During type inference, partially inferred types are
+// represented using Type variables (ty::Infer). These don't appear in
+// the final TypeckTables since all of the types should have been
+// inferred once typeck_tables_of is done.
+// When type inference is running however, having to update the typeck
+// tables every time a new type is inferred would be unreasonably slow,
+// so instead all of the replacement happens at the end in
+// resolve_type_vars_in_body, which creates a new TypeTables which
+// doesn't contain any inference types.
 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     pub fn resolve_type_vars_in_body(&self, body: &'gcx hir::Body) -> &'gcx ty::TypeckTables<'gcx> {
         let item_id = self.tcx.hir().body_owner(body.id());