Idea: light weight rust as zero build scripting language with JIT runtime

Rust requires a complex build not because nobody had an idea to make it a lightweight scripting language, but because Rust's goals of maximum performance and correctness pushed it towards having features that depend on very specific compile-time information, with concrete types, and known type sizes. It requires the program being built in a particular way. You can't have "zero build" when the language needs to have concrete type information which is baked into data layouts. It needs to expand macros. It needs to see all relevant trait implementations to properly dispatch method calls. If you try to make the layouts magically polymorphic, it's either going to break a lot of unsafe code that won't be fooled by it, or require very very heavy hacks that will kill the idea of it being "lightweight".

Your proposal is like for a flying car. Why have 4 wheels when it could fly? It's exciting and it would be absolutely awesome if we made the cars fly. There are lots of great use-cases for flying cars. Just make the cars not heavy, with zero touching of asphalt, add easy lifting into the air.