make sure miri never switches over an invalid char value

src/interpreter/mod.rs

@@ -406,11 +406,17 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
 
             SwitchInt { ref discr, ref values, ref targets, .. } => {
                 let discr_ptr = self.eval_lvalue(discr)?.to_ptr();
+                let discr_ty = self.lvalue_ty(discr);
                 let discr_size = self
-                    .type_layout(self.lvalue_ty(discr))
+                    .type_layout(discr_ty)
                     .size(&self.tcx.data_layout)
                     .bytes() as usize;
                 let discr_val = self.memory.read_uint(discr_ptr, discr_size)?;
+                if let ty::TyChar = discr_ty.sty {
+                    if ::std::char::from_u32(discr_val as u32).is_none() {
+                        return Err(EvalError::InvalidChar(discr_val as u32));
+                    }
+                }
 
                 // Branch to the `otherwise` case by default, if no match is found.
                 let mut target_block = targets[targets.len() - 1];