How to walk SWC-generated AST in Rust

[PuruVJ]

I have this code

use std::{fs, sync::Arc};
use swc::ecmascript::ast::ModuleItem;
use swc_common::{
    errors::{ColorConfig, Handler},
    FileName, SourceMap,
};
use swc_ecma_parser::{Syntax, TsConfig};

fn main() {
    let cm = Arc::<SourceMap>::default();
    let handler = Arc::new(Handler::with_tty_emitter(
        ColorConfig::Auto,
        true,
        false,
        Some(cm.clone()),
    ));
    let c = swc::Compiler::new(cm.clone());
    let fm = cm.new_source_file(
        FileName::Custom("test.js".into()),
        include_str!("./main.ts").into(),
    );
    let result = c.parse_js(
        fm,
        &handler,
        swc::ecmascript::ast::EsVersion::Es2022,
        Syntax::Typescript(TsConfig::default()),
        swc::config::IsModule::Bool(true),
        false,
    );

    let res = result.unwrap();
    let body = res.module().unwrap().body;
}

I have the AST in body variable. Now, I want to walk this AST, akin to estree-walker by Rich Haris

How do I walk this in Rust in the best possible way?

[kwonoj]

You may able to provide custom visitor into transform chain, next.js have some reference example like

https://github.com/vercel/next.js/blob/a4159321b20148ff2f9f6fa847395a8c8162dbef/packages/next-swc/crates/napi/src/transform.rs#L97-L105

[PuruVJ]

Can you share a simpler example? This is too complex and I'm a Rust beginner

[RiESAEX]

use std::{fs, sync::Arc};
use swc::{config::Options, ecmascript::ast::ModuleItem};
use swc_common::{
    errors::{ColorConfig, Handler},
    FileName, SourceMap, DUMMY_SP,
};
use swc_ecma_parser::{Syntax, TsConfig};
use swc_ecmascript::{
    ast::Lit,
    transforms::pass::noop,
    visit::{as_folder, Fold},
};
use swc_ecmascript::{
    ast::{Expr, Number},
    visit::{VisitMut, VisitMutWith},
};
fn main() {
    let cm = Arc::<SourceMap>::default();
    let handler = Handler::with_tty_emitter(ColorConfig::Auto, true, false, Some(cm.clone()));
    let c = swc::Compiler::new(cm.clone());

    let fm = cm.new_source_file(
        FileName::Custom("test.js".into()),
        include_str!("./main.ts").into(),
    );
    let result = c.process_js_with_custom_pass(
        fm,
        None,
        &handler,
        &Options::default(),
        |_| noop(),
        |_| my_visitor(),
    );
    println!("{:#?}", result.unwrap());
}
fn my_visitor() -> impl Fold {
    as_folder(MyVisitor)
}
struct MyVisitor;
impl VisitMut for MyVisitor {
    fn visit_mut_expr(&mut self, expr: &mut Expr) {
        expr.visit_mut_children_with(self);
        *expr = Expr::Lit(Lit::Num(Number {  // here change expr to 0
            span: DUMMY_SP,
            value: 0.0,
        }))
    }
}

you can find more example of VisitMut and Fold in
https://github.com/swc-project/swc/tree/main/crates/swc_ecma_transforms_compat

[PuruVJ]

Thanks. Just one thing, import path of noop is wrong, my code won't compile. Can you specify where can I findit, or better, what can I replace it with?

[kdy1]

You need to enable cargo feature. Import path is correct.

[PuruVJ]

oh thanks, got it

[PuruVJ]

@kdy1 thank you for this amazing project, I'm currently using it rebuild svelte compiler in Rust, and SWC is the one driving it forward.

I wanna ask, where can I find documentation for rust based methods? I've been having trouble finding them and source code is pretty advanced for me 😅

[kdy1]

https://rustdoc.swc.rs/swc/
This page contains lots of useful links.