Rust Optimization: Vec::into_iter().collect()
Rust Optimization: Vec::into_iter().collect()
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?
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_ain your case, but it ultimately is quite complicated.29That was a super interesting and informative read! Exactly what I was hoping to find when I posted this, thanks!
9
I think the better solution would be to use
Vec::retain().15I don't know and don't think so, but what you are doing is better done with retain anyways.
9I 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.5map()can still be used withVec::iter_mut(),filter_map()can be replaced withVec::retain_mut().8
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 returningimpl Iterator3I thought this was the point of
vec.drain(..).collect().2