If you write some of the "clever" pop count algorithms in C or C++, good compilers will go "Oh you're doing bit counting" and on a modern platform with a bit counting CPU instruction your whole algorithm turns into one CPU instruction.

[In Rust the standard library provides count_ones() on integer types, and that's actually safely wrapping an intrinsic pop count feature]

In this case it's a combination of compiler magic and specializations in the rust standard library to make it easy for the compiler to understand that this particular iterator shape is a NOP.