Teach new error messages in MIR borrowck about promoted places

Author: oli-obk

src/librustc_mir/borrow_check/move_errors.rs

@@ -247,6 +247,7 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                                             Place::Projection(ref proj) => {
                                                 proj.base == Place::Local(Local::new(1))
                                             }
+                                            Place::Promoted(_) |
                                             Place::Local(_) | Place::Static(_) => unreachable!(),
                                         }
                                     } =>

src/librustc_mir/borrow_check/mutability_errors.rs

@@ -122,6 +122,8 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                 }
             }
 
+            Place::Promoted(_) => unreachable!(),
+
             Place::Static(box Static { def_id, ty: _ }) => {
                 if let Place::Static(_) = access_place {
                     item_msg = format!("immutable static item `{}`", access_place_desc.unwrap());