Vec::truncate_par

[njaard]

I have some code that truncates a large Vec and I found that the drops were very costly. It would be nice to see a truncate_par in Rayon.

I use the following unsafe code to drop the tail of the vec in parallel:

self.par_iter_mut()
    .skip(len)
    .for_each(|r| std::ptr::drop_in_place(r));
let n = std::cmp::min(self.len(), len);
self.set_len( n );

[cuviper]

Note that skip is not as efficient as it could be (#352), and your example also has a problem with panic safety. If any of those drops panic, then the vector's length will not be adjusted, and the vector's own drop will try to re-drop some items!

You can see how rayon::vec::IntoIter deals with this here.

Parallel truncate seems kind of niche to me -- we should think if there may be a more general approach. A parallel drain would let you write something like vec.par_drain(len..), but we need stable RangeArgument to allow different range types. (Or we could create our own equivalent to RangeArgument.)

[njaard]

I agree that a parallel drain is a better and more general solution.