message: atomics wrap around on overflow
diff: @@ -134,7 +134,7 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for EvalContext<'a, 'mir, 'tcx, super:
  "xsub" => mir::BinOp::Sub,
  _ => bug!(),
  };
  - // FIXME: what do atomics do on overflow?
  + // Atomics wrap around on overflow.
  self.binop_ignore_overflow(op, old, rhs, ptr.into())?;
  }
  