I think that you should have a second think about whether what you're saying makes sense. To "save" some code means that an application writes it to hard drive

I think that you should have a second think about whether what you're saying makes sense. To "save" some code means that an application writes it to hard drive. But writing to a hard drive is entirely orthogonal to the operation of a compiler. For example, a compiler should be able to compile code that is never written to hard drive -- exists only in memory. The hard drive is just one possible source of code to be compiled. I do agree with you though that autoformatting code on save is a fantastic experience. It works beautifully with Rust, and so now I do it (less beautifully) with Python/black. I just think that although it's a great developer experience, it's several steps removed from anything that anyone would want tied to the compiler.