Can't use a library that doesn't exist.

If you want an additional Rust hashmap implementation then you need to persuade the Rust community to provide one in std::collections.

No one is allowed to -- in your words -- "write an entirely custom [hashmap] just for the benchmark".

> If you want an additional Rust hashmap implementation then you need to persuade the Rust community to provide one in std::collections.

Again, this means that for Rust, it _has_ to be in the standard library. But C doesn't have one in the standard library. So C gets to write their own, and Rust doesn't.