ef5a4bb4cb
...to make it slightly clearer that there's not much point in boxing a vec. On a different note, I read the contribution guidelines (https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#pull-request-procedure) which say I should update the copyright date for this file. But I can see that nobody else has done this so far this year, despite there being a fair number of commits. Does that instruction need removing?