make writes_through_immutable_pointer a hard error

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -22,6 +22,7 @@ pub enum ConstEvalErrKind {
     RecursiveStatic,
     AssertFailure(AssertKind<ConstInt>),
     Panic { msg: Symbol, line: u32, col: u32, file: Symbol },
+    WriteThroughImmutablePointer,
 }
 
 impl MachineStopType for ConstEvalErrKind {
@@ -35,12 +36,16 @@ impl MachineStopType for ConstEvalErrKind {
             Panic { .. } => const_eval_panic,
             RecursiveStatic => const_eval_recursive_static,
             AssertFailure(x) => x.diagnostic_message(),
+            WriteThroughImmutablePointer => const_eval_write_through_immutable_pointer,
         }
     }
     fn add_args(self: Box<Self>, adder: &mut dyn FnMut(DiagArgName, DiagArgValue)) {
         use ConstEvalErrKind::*;
         match *self {
-            RecursiveStatic | ConstAccessesMutGlobal | ModifiedGlobal => {}
+            RecursiveStatic
+            | ConstAccessesMutGlobal
+            | ModifiedGlobal
+            | WriteThroughImmutablePointer => {}
             AssertFailure(kind) => kind.add_args(adder),
             Panic { msg, line, col, file } => {
                 adder("msg".into(), msg.into_diag_arg());
@@ -159,6 +164,7 @@ where
 
 /// Emit a lint from a const-eval situation, with a backtrace.
 // Even if this is unused, please don't remove it -- chances are we will need to emit a lint during const-eval again in the future!
+#[allow(unused)]
 pub(super) fn lint<'tcx, L>(
     tcx: TyCtxtAt<'tcx>,
     machine: &CompileTimeMachine<'tcx>,