It'd probably work best in a 'notebook' format. Take something like Jupyter Notebooks, throw a rust backend on it with each cell being a compilation unit... either throw them all back into a global scope or make each cell a module, I'm not sure. But repls make sense for python or something where you can realistically do line-by-line programming. Rust... I very frequently want to have an expression extend across multiple lines or define structures, etc, that a notebook just makes more sense.