I agree. The D program is cool, but pretty cryptic. A simple Rust version [0] is only 21 lines yet much more readable in my opinion. I'd take readability over a couple less loc anyday.
The `op.parse()` at line 7 was a little hard to understand for me at the beginning. Only a few seconds after, I understood it was `op.parse::<f64>()` with the type parameter inferred from the type of `stack`, but I think it's easier to understand with the explicit type annotation (the turbofish) than without it.
You miss the point of the original one. When I wrote it the idea was that it is functionally pure and guaranteed to be such by the compiler (the annotation was removed because readln is not pure for obvious reasons).
Most uses of loops break down to map, fold, or similar being able to use them cleanly can avoid a lot of bugs and API plasticity.
[0]: https://play.rust-lang.org/?version=stable&mode=debug&editio...