Cannot use or_insert_with for HashMap::<usize, HashMap<usize, i64>> - is this supposed to be supported?

[FelixZY]

I have a program like this:

use hashbrown::HashMap;

fn main() {
    let mut value_by_col_by_row = HashMap::<usize, HashMap<usize, i64>>::new();
    value_by_col_by_row.entry_ref(&0).or_insert_with(HashMap::new).insert(0, 1);
}

Unfortunately, cargo build produces the following error:

   Compiling rust v0.1.0 (/my/project/path)
error[E0277]: the trait bound `usize: From<&usize>` is not satisfied
    --> src/main.rs:5:39
     |
5    |     value_by_col_by_row.entry_ref(&0).or_insert_with(HashMap::new).insert(0, 1);
     |                                       ^^^^^^^^^^^^^^ the trait `From<&usize>` is not implemented for `usize`
     |
note: required by a bound in `EntryRef::<'a, 'b, K, Q, V, S, A>::or_insert_with`
    --> /my/home/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.15.2/src/map.rs:4197:19
     |
4195 |     pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V
     |            -------------- required by a bound in this associated function
4196 |     where
4197 |         K: Hash + From<&'b Q>,
     |                   ^^^^^^^^^^^ required by this bound in `EntryRef::<'a, 'b, K, Q, V, S, A>::or_insert_with`
help: consider borrowing here
     |
5    |     (&value_by_col_by_row.entry_ref(&0)).or_insert_with(HashMap::new).insert(0, 1);
     |     ++                                 +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rust` (bin "rust") due to 1 previous error

I'm not sure if I'm doing something wrong or if this is not supposed to be a supported use case?

$ cargo --version
cargo 1.83.0 (5ffbef321 2024-10-29)
$ rustc --version
rustc 1.83.0 (90b35a623 2024-11-26)

#####

[dependencies]
hashbrown = "0.15.2"

[clarfonthey]

So, the EntryRef API is very weird, and the fact that it requires K: From<&Q> is very weird, but this is intended for cases like &str, where you're converting a borrowed objecting into an owned copy.

For integer keys, you should be using entry instead, and passing the integer key directly, since it can be copied. The or_insert_with method on entry(0) should work, even though entry_ref(&0) won't.

[FelixZY]

Alright, thank you @clarfonthey ! I'm fine with closing this unless anyone wants to e.g. add an extra snippet of documentation somewhere or further discuss the "weirdness" of the EntryRef API.

[clarfonthey]

Up to you— I think that documentation can always be improved, although I also was able to figure this out mostly by just checking the docs for EntryRef::or_insert_with to see the weird From bound more explicitly.

[FelixZY]

I guess I would have liked to see a reference to entry() in the entry_ref() docs (and vice versa) with a short "use X instead if ...".

To me, it's not very clear what the difference between these two are, going only by the description text:

entry

Gets the given key’s corresponding entry in the map for in-place manipulation.

entry_ref

Gets the given key’s corresponding entry by reference in the map for in-place manipulation.

In what way isn't entry providing a reference? I mean, the docs clearly state that it operates in-place! (Question I would have liked the docs to make more clear).