alt Hacker News> HashMap implements Extend, so just h0.extend(h1) and you're done, the people who made your HashMap type are much better equipped to optimize this common operation.
Are you sure? I'm not very used to reading Rust stdlib, but this seems to be the implementation of the default HashMap extend [1]. It just calls self.base.extend. self.base seems to be hashbrown::hash_map, and this is the source for it's extend [2]. In other words, does exactly the same thing, just iterates through hash map and inserts it.
Maybe I'm misreading something going through the online docs, or Rust does the "random seed" thing that abseil does, but just blinding assuming something doesn't happen "because Rust" is a bit silly.
[1]: https://doc.rust-lang.org/src/std/collections/hash/map.rs.ht...
[2]: https://docs.rs/hashbrown/latest/src/hashbrown/map.rs.html#4...
Yes, HashMap will by default be randomly seeded in Rust, but also the code you linked intelligently reserves capacity. If h0 is empty, it reserves enough space for all of h1, and if it isn't then it reserves enough extra space for half of h1, which turns out to be a good compromise.
Note that the worst case is we ate a single unneeded growth, while the best case is that we avoided N - 1 grows where N may be quite large.