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?

  • @Gobbel2000
    293 months ago

    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.

    • @arendjrOP
      93 months ago

      That was a super interesting and informative read! Exactly what I was hoping to find when I posted this, thanks!