Comment by Tipewryter

Here is how to reproduce it in a fresh docker container, so we have the same environment:

    docker run -it --rm debian:10-slim /bin/bash

    apt update
    apt install -y git python3-pip --no-install-recommends
    pip3 install git+https://github.com/bsdz/calcengine.git#master

Result: "This package requires Rust >=1.41.0."