Rust isn't hashcode based. You don't define how to hash a type in that way. You define hashing algorithms which operate on arrays of bytes. All a type has to do

Rust isn't hashcode based. You don't define how to hash a type in that way. You define hashing algorithms which operate on arrays of bytes. All a type has to do to implement hashing is specify how to feed its bytes into a hasher. Hashers provide conveniences for feeding in a u8, u16, u32, u64, etc., but most hashers just implement this as casting the value to an array of bytes and using the generic implementation. This is because most algorithms are defined in terms of bytes. Evidently there isn't any interesting optimization to do by statically knowing you have a u32 vs a u64 for these algorithms (appears to be the case for SipHash, XXHash, and Fnv). And indeed, u128 continues this tradition in the PR: https://github.com/rust-lang/rust/pull/37900/files#diff-232730be2a8dd8895c3ab4fe291ff4d1R194 https://github.com/rust-lang/rust/pull/37900/files#diff-2327...