[josephg]

If everything is in arenas, lifetimes get much easier.

But, the borrow checker doesn't just check lifetimes. It also checks ownership, and that variables either have a single mutable reference or immutable references. The optimizer assumes those invariants are maintained in the code. Many of its optimizations wouldn't be sound otherwise.

So, if you could compile code which fails the borrow checker, there's all sorts of weird and wonderful sources of UB eagerly waiting to give you a really bad day - from aliasing issues to thread safety problems to use-after-free bugs. The borrow checker has been around forever in rust. So I don't think anyone has any idea what the implications would be of compiling "bad" code.

[throwawaymaths]

Point being, there are many many individual programs where none of those things you talk about exist. So why not have a programming system where you can actually turn those things off for development velocity.

I'm rejecting the idea that "opt-in" is bad. Opt-out is of course better, but "no choice" is not good.

[josephg]

> there are many many individual programs where none of those things you talk about exist

Which things? Programs without mutable references and aliasing concerns? Can you give an example?

Having worked in rust for a few years now, I’m not convinced you’d gain much velocity by disabling the borrow checker. Some code would be a little simpler without lifetime annotations, but you’d also end up spending a lot more time debugging your code. The borrow checker and rust type system are insanely good at finding bugs at compile time. It just takes awhile of working in rust before you stop stubbing your toe on the borrow checker’s (sometimes silly) rules.

If you want easy to write rust, you can always just lean heavily on Box and Rc, and .clone() everywhere. The trade off is your code won’t be as performant - but that doesn’t matter much if you’re prototyping.

If you care that much, rust is opensource. Fork it and turn the borrow checker off when compiling. It’s probably not even that hard to do. I’d love to hear about the experience - and what it’s like using rust without the borrow checker.