Give index temporaries a drop scope

Author: matthewjasper

src/librustc_mir/build/expr/as_place.rs

@@ -73,13 +73,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let (usize_ty, bool_ty) = (this.hir.usize_ty(), this.hir.bool_ty());
 
                 let slice = unpack!(block = this.as_place(block, lhs));
-                // region_scope=None so place indexes live forever. They are scalars so they
-                // do not need storage annotations, and they are often copied between
-                // places.
                 // Making this a *fresh* temporary also means we do not have to worry about
                 // the index changing later: Nothing will ever change this temporary.
                 // The "retagging" transformation (for Stacked Borrows) relies on this.
-                let idx = unpack!(block = this.as_temp(block, None, index, Mutability::Mut));
+                let idx = unpack!(block = this.as_temp(
+                    block,
+                    expr.temp_lifetime,
+                    index,
+                    Mutability::Not,
+                ));
 
                 // bounds check:
                 let (len, lt) = (