Crate swc_visit

source ·
Expand description

Visitor generator for the rust language.

There are three variants of visitor in swc. Those are Fold, VisitMut, Visit.


§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 + ....


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 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

*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 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.

§Cargo features

You should add

path = []

If you want to allow using path-aware visitor.

§Path-aware visitor

Path-aware visitor is a visitor that can be used to visit AST nodes with current path from the entrypoint.

VisitMutAstPath and FoldAstPath can be used to transform AST nodes with the path to the node.


  • Some utilities for generated visitors.


  • Chains multiple visitor.
  • This creates Visit. This is extensible visitor generator, and it



  • The enum Either with variants Left and Right is a general purpose sum type with two cases.