The most equivalent Rust type to `Long` would be `Option<Box<u64>>`, which is represented as a nullable pointer to a u64. `Box<Option<u64>>` is represented as a

The most equivalent Rust type to `Long` would be `Option<Box<u64>>`, which is represented as a nullable pointer to a u64. `Box<Option<u64>>` is represented as a non-nullable pointer to a struct containing a u64 number and an aligned-to-u64 discriminant. Does it change the benchmarks? Not if you let the inner function get inlined: https://rust.godbolt.org/z/Ejc5c7sE8 https://rust.godbolt.org/z/Ejc5c7sE8