message: implement panic_if_uninhabited intrinsic
diff: @@ -286,6 +286,14 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
  this.write_scalar(result_ptr, dest)?;
  }
  + "panic_if_uninhabited" => {
  + let ty = substs.type_at(0);
  + let layout = this.layout_of(ty)?;
  + if layout.abi.is_uninhabited() {
  + return err!(Intrinsic(format!("Trying to instantiate uninhabited type {}", ty)))
  + }
  + }
  +
  "powf32" => {
  let f = this.read_scalar(args[0])?.to_f32()?;
  let f2 = this.read_scalar(args[1])?.to_f32()?;
  