We need to be able to clone AST nodes. But we cannot implement Clone on most AST types because oxc_allocator::Box is not Clone.

There is a good reason for that - Box stores the value it owns in arena allocator, so there is no Box::new method, only Box::new_in which takes an &Allocator param for the arena to allocate it in.

I propose a new trait CloneIn:

trait CloneIn<'alloc> {
    fn clone_in(&self, allocator: &'alloc Allocator) -> Self;
}

Impl for oxc_allocator::Box:

impl<'alloc, T> CloneIn<'alloc> for Box<'alloc, T>
where
    T: CloneIn<'alloc>,
{
    fn clone_in(&self, allocator: &'alloc Allocator) -> Self {
        Box::new_in(self.deref().clone_in(allocator), allocator)
    }
}

(I'm actually not sure if I have this right - maybe 'alloc lifetime should be on fn clone_in<'alloc> not CloneIn<'alloc>)

Same as Clone, CloneIn can be implemented on any type where all its fields/variants are also CloneIn.

We could make a derive macro for CloneIn so can just #[derive(CloneIn)] on AST types.

NB: clone_in should always clone into the arena of the provided allocator, which may be different from the arena that self is in. i.e. you can use clone_in to copy objects from one arena to another.