[Wrinting Plugins] How bad .clone() is and how to avoid it?

[timofei-iatsenko]

This is kinda more Rust-generic, or Plugin writing patterns question:

I often face a situation when I have only a reference to some part of AST and trying to create a new nodes which want to own the value, the most common example:

    fn fold_expr(&mut self, expr: Expr) -> Expr {
        if let Expr::TaggedTpl(tagged_tpl /* &TaggedTpl */) = &expr {
            create_new_node(tagged_tpl /* TaggedTpl */) // Type mismatch &TaggedTpl -> TaggedTpl
        }

        expr.fold_children_with(self)
    }

So i've forced to clone the tagged_tpl to be able to get non-referenced value. How bad is that? And how to avoid this?
How to do pattern matching while still keeping ownership of value.

If I write it like that, I get another error:

    fn fold_expr(&mut self, expr: Expr) -> Expr {
        if let Expr::TaggedTpl(tagged_tpl /* TaggedTpl */) = expr {
            create_new_node(tagged_tpl /* TaggedTpl */) 
        }

        expr.fold_children_with(self) // use of partially moved value, value was moved in pattern matching expr
    }

The same may happen with Visitor pattern as well, where I always getting a reference to a node in visit_{expr} method