Mark an unreachable case as a bug instead of attempting to show an error to the user

diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index cd5ed0e70a323bf9297512e93b4f89384ac00a22..89bbc42df04991af5aee380384eef4383d0ef60e 100644 (file)
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -1853,7 +1853,7 @@ fn is_mutable<'d>(
                 }
             }
             // promoteds may never be mutated
-            Place::Promoted(_) => Err(place),
+            Place::Promoted(_) => bug!("encountered mutable promoted"),
             Place::Static(ref static_) => {
                 if self.tcx.is_static(static_.def_id) != Some(hir::Mutability::MutMutable) {
                     Err(place)