The ChaCha20Legacy construction, i.e. the djb variant, is supposed to use a 64-bit counter but currently uses a 32-bit counter because it shares its core implementation with the IETF construction which uses a 32-bit counter.

This results in a counter overflow after generating 256 GiB of keystream. Compatible implementations are able to generate larger keystreams.

I'm not sure how much of a practical concern this actually is, but it did come up in discussions here: rust-random/rand#934 (comment)

We can probably make the counter type generic between u32/u64 in the core implementation if need be.