The function

  fn bits(&mut self, need: i32) -> i32 { ....

Put me in mind of one of my early experiments in Rust. It would be interesting to compare a iterator based form that just called .take(need)

I haven't written a lot of Rust, but one thing I did was to write an iterator that took an iterator of bytes as input and provided bits as output. Then used an iterator that gave bytes from a block of memory.

It was mostly as a test to see how much high level abstraction left an imprint on the compiled code.

The dissasembly showed it pulling in 32 bits at a time and shifting out the bits pretty much the same way I would have written in ASM.

I was quite impressed. Although I tested it was working by counting the bits and someone critizised it for not using popcount, so I guess you can't have everything.