Then use a dynamic language like ruby or python and enjoy not only data type freedom, but also features like easy introspection and code manipulation at runtime. Protyping and experimentation in a REPL is much faster, easier, and memory safe. If you are just experimenting, why are you wasting time and effort on e.g. C++'s complex type system? If you don't care about memory leaks, why waste any effort on memory management when the garbage collector can handle that for you?
> starting from Rust but setting a flag to turn borrow checker errors into warnings
Experience suggests the "we'll do it right in the next version" argument leads to "prototypes" being shipped as the "next" (only) version.
The difficulties imposed by Rust is the goal. Actually writing correct and safe code is inherently a difficult problem. Rust is trying to force programmers into actually addressing that problem. Writing similarly correct and safe C++ is also time consuming and difficult. The difference is C++ lets the programmer shift that work and expense onto the user in the form of time-wasting crashes and expensive security vulnerabilities.