optimize prefix method

Author: csmoe

src/librustc/mir/mod.rs

@@ -2222,7 +2222,7 @@ impl<'tcx> Debug for Place<'tcx> {
 }
 
 impl<'tcx> Debug for PlaceBase<'tcx> {
-    fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
         use self::PlaceBase::*;
 
         match self {

src/librustc/mir/tcx.rs

@@ -330,9 +330,7 @@ impl<'tcx> NeoPlace<'tcx> {
         } else {
             Self {
                 base: self.clone().base,
-                elems: tcx.mk_place_elems(
-                    self.elems.iter().cloned().take(elem_index)
-                )
+                elems: tcx.mk_place_elems(self.elems[..elem_index].iter()),
             }
         }
     }