The point of config/scripting languages is to make the user's life easier. I assume nobody wants to deal with the Rust borrow checker and lifetime annotations while tinkering with their configuration. So it depends - should it be an unchanging set of options/flags the user can switch around and then recompile? Probably fine though it'll never be as quick as e.g. recompiling programs from the suckless project. Want to expose interfaces for users to expand and modify the program's code? Not so much.

Rust is a fantastic language, but it's probably not the kind of language you want for this purpose. Same reason why game engines often have scripting languages; so people don't have to deal with C++ where it's not required for performance.

Imho the best compromise is embedding something like Lua which is simple and still flexible enough to allow more than JSON-style declarations.