rustc/infer: miscellaneous minor code improvements

Author: ljedrz

src/librustc/infer/error_reporting/mod.rs

@@ -147,9 +147,7 @@ impl<'cx, 'gcx, 'tcx> LexicalResolver<'cx, 'gcx, 'tcx> {
     fn construct_var_data(&self, tcx: TyCtxt<'_, '_, 'tcx>) -> LexicalRegionResolutions<'tcx> {
         LexicalRegionResolutions {
             error_region: tcx.types.re_static,
-            values: (0..self.num_vars())
-                .map(|_| VarValue::Value(tcx.types.re_empty))
-                .collect(),
+            values: IndexVec::from_elem_n(VarValue::Value(tcx.types.re_empty), self.num_vars())
         }
     }
 

src/librustc/infer/lexical_region_resolve/mod.rs

@@ -443,7 +443,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
         assert!(self.undo_log[snapshot.length] == OpenSnapshot);
 
         if snapshot.length == 0 {
-            self.undo_log.truncate(0);
+            self.undo_log.clear();
         } else {
             (*self.undo_log)[snapshot.length] = CommitedSnapshot;
         }