swc-project/swc · error

Unknown visitor type: {:?}

Error message

Unknown visitor type: {:?}

What it means

`#[parallel]` generates parallel visit helpers by inspecting the implemented trait path: only `Fold` and `VisitMut` are recognized (`p.is_ident("Fold")` / `VisitMut`); any other trait panics with `unimplemented!("Unknown visitor type: ...")`. An inherent impl with no trait panics even earlier on `trait_.as_ref().unwrap()`.

Source

Thrown at crates/swc_ecma_transforms_macros/src/parallel.rs:16

#![allow(non_snake_case)]

use proc_macro2::{Span, TokenStream};
use syn::{parse_quote, Expr, Ident, ImplItem, ImplItemFn, ItemImpl, Meta, Type};

use crate::common::Mode;

pub fn expand(attr: TokenStream, mut item: ItemImpl) -> ItemImpl {
    let mode = {
        let p = &item.trait_.as_ref().unwrap().1;
        if p.is_ident("Fold") {
            Mode::Fold
        } else if p.is_ident("VisitMut") {
            Mode::VisitMut
        } else {
            unimplemented!("Unknown visitor type: {:?}", p)
        }
    };
    let meta = if attr.is_empty() {
        None
    } else {
        Some(syn::parse2::<Meta>(attr).expect("failed to parse meta"))
    };
    let explode = meta
        .as_ref()
        .map(|v| v.path().is_ident("explode"))
        .unwrap_or(false);

    item.items.push(ImplItem::Fn(make_par_visit_method(
        mode,
        "module_items",
        explode,
    )));
    item.items

View on GitHub (pinned to d7d7434666)

Solutions

1. Remove `#[parallel]` from impl blocks of traits other than Fold/VisitMut
2. Keep the attribute strictly on the visitor impl blocks it was designed for

Example fix

// before
#[parallel]
impl MyPass for Visitor { ... }

// after
impl MyPass for Visitor { ... }

When it happens

Trigger: Apply `#[parallel]` to `impl MyTrait for X { ... }` or to an inherent `impl X { ... }` block.

Common situations: Attribute applied too broadly via copied boilerplate or editor macro expansion; refactoring a visitor impl into another trait without removing the attribute.

Related errors

• Fast-path injection for Fold / VisitMut where pattern is not
• generic parameter other than type
• Box() -> T or Box without a type parameter
• Unknown suffix `{}`
• union

AI-assisted analysis of swc-project/swc@d7d7434666 (2026-08-16). Data as JSON: /api/errors/12320d91354c58da. Report an issue: GitHub.