Visitor generator for the rust language.
There are three variants of visitor in swc. Those are `Fold`, `VisitMut`,
`Visit`.
# Comparisons
## `Fold` vs `VisitMut`
`Fold` and `VisitMut` do almost identical tasks, but `Fold` is easier to use
while being slower and weak to stack overflow for very deep asts. `Fold` is
fast enough for almost all cases so it would be better to start with `Fold`.
By very deep asts, I meant code like thousands of `a + a + a + a + ...`.
# `Fold`
> WARNING: `Fold` is slow, and it's recommended to use VisitMut if you are
experienced.
`Fold` takes ownership of value, which means you have to return the new
value. Returning new value means returning ownership of the value. But you
don't have to care about ownership or about managing memories while using
such visitors. `rustc` handles them automatically and all allocations will
be freed when it goes out of the scope.
You can invoke your `Fold` implementation like `node.fold_with(&mut
visitor)` where `visitor` is your visitor. Note that as it takes ownership
of value, you have to call `node.fold_children_with(self)` in e.g. `fn
fold_module(&mut self, m: Module) -> Module` if you override the default
behavior. Also you have to store return value from `fold_children_with`,
like `let node = node.fold_children_with(self)`. Order of execution can be
controlled using this. If there is some logic that should be applied to the
parent first, you can call `fold_children_with` after such logic.
# `VisitMut`
`VisitMut` uses a mutable reference to AST nodes (e.g. `&mut Expr`). You can
use `Take` from `swc_common::util::take::Take` to get owned value from a
mutable reference.
You will typically use code like
```ignore
*e = return_value.take();
```
where `e = &mut Expr` and `return_value` is also `&mut Expr`. `take()` is an
extension method defined on `MapWithMut`. It's almost identical to `Fold`,
so I'll skip memory management.
You can invoke your `VisitMut` implementation like `node.visit_mut_with(&mut
visitor)` where `visitor` is your visitor. Again, you need to call
`node.visit_mut_children_with(self)` in visitor implementation if you want
to modify children nodes. You don't need to store the return value in this
case.
# `Visit`
`Visit` uses non-mutable references to AST nodes. It can be used to see if
an AST node contains a specific node nested deeply in the AST. This is
useful for checking if AST node contains `this`. This is useful for lots of
cases - `this` in arrow expressions are special and we need to generate
different code if a `this` expression is used.
You can use your `Visit` implementation like `node.visit_with(&Invalid{
span: DUMMY_SP, }, &mut visitor`. I think API is mis-designed, but it works
and there are really lots of code using `Visit` already.