I just had a random thought: a common pattern in Rust is to things such as:
let vec_a: Vec<String> = /* ... */;
let vec_b: Vec<String> = vec_a.into_iter().filter(some_filter).collect();
Usually, we need to be aware of the fact that Iterator::collect() allocates for the container we are collecting into. But in the snippet above, we’ve consumed a container of the same type. And since Rust has full ownership of the vector, in theory the memory allocated by vec_a could be reused to store the collected results of vec_b, meaning everything could be done in-place and no additional allocation is necessary.
It’s a highly specific optimization though, so I wonder if such a thing has been implemented in the Rust compiler. Anybody who has an idea about this?
I don’t know and don’t think so, but what you are doing is better done with retain anyways.
I mean, the actual operation is just an example, of course. Feel free to make it a .map() operation instead. The strings couldn’t be reused then, but the vector’s allocation still could… in theory.
map() can still be used with Vec::iter_mut(), filter_map() can be replaced with Vec::retain_mut().
Yeah, that’s helpful if I would be currently optimizing a hot loop now. But I was really just using it as an example. Also, retain_mut() doesn’t compose as well.
I’d much rather write:
let vec_a: Vec<String> = /* ... */;
let vec_b: Vec<String> = vec_a
.into_iter()
.filter(some_filter)
.map(some_map_fn)
.collect();
Over:
let mut vec_a: Vec<String> = /* ... */;
vec_a.retain_mut(|x| if some_filter(x) {
*x = some_map_fn(*x); // Yikes, cannot move out of reference.
true
} else {
false
});
And it would be nice if that would be optimized the same. After all, the point of Rust’s iterators is to provide zero-cost abstractions. In my opinion, functions like retain_mut() represent a leakiness to that abstraction, because the alternative turns out to not be zero cost.
The standard library does have some specialisation internally for certain iterators and collection combinations. Not sure if it will optimise that one specifically, but Vec::into_iter().collect::<Vec>() is optimised (it may look silly, but it comes up with functions returning impl Iterator
I think the better solution would be to use Vec::retain().
This blog post goes into some specifics of Rust reusing Vec allocations and some of the consequences. I think it’s really worth a read to better understand Vecs. From what I understand, it is possible that Rust will reuse the allocation of vec_a in your case, but it ultimately is quite complicated.
I thought this was the point of vec.drain(..).collect().
