So, what you're not seeing is that even though the Rust doesn't announce mutation the implementation may be mutation anyway if that's probably faster/ cheaper.

    unsafe impl<I, U, F> InPlaceIterable for FlatMap<I, U, F> where
    I: InPlaceIterable,
    U: BoundedSize + IntoIterator,

What you're seeing is the graceful goose on the water moving forward at pace. Beneath there is frantic action to make that happen. The Rust surface was more a maintainable immutable operations, but the implementation is a frantic whirling mutation like the Zig.

In Zig you end up spending a lot of time writing How to do a thing, where in Rust you only wrote What the thing is and the machine did it. There are edge cases where Zig's explicitness wins for the best programmers, but there just aren't enough of those cases or those programmers for this to net out IMNSHO.