swc-project/swc · error

unable to access unknown nodes

Error message

unable to access unknown nodes

What it means

fast_dts's transform_decl matches Decl (Class/Fn/Var/Using/TsEnum/TsModule/TsInterface/TsTypeAlias) to rewrite top-level declarations for .d.ts output. Under the swc_ast_unknown cfg (wasm plugin builds pass `--cfg=swc_ast_unknown`, giving enums an Unknown(tag, payload) variant for kinds unknown to this swc_core) this trailing arm panics: an unknown declaration kind cannot be classified as value or type, so no correct .d.ts can be produced.

Source

Thrown at crates/swc_typescript/src/fast_dts/decl.rs:127

                }
                for type_element in ts_interface.body.body.iter() {
                    self.check_ts_signature(type_element);
                }
            }
            Decl::TsTypeAlias(ts_type_alias) => {
                if let Some(internal_annotations) = self.internal_annotations.as_ref() {
                    ts_type_alias.visit_mut_children_with(&mut InternalAnnotationTransformer::new(
                        internal_annotations,
                    ))
                }
                if let Some(ts_lit) = ts_type_alias.type_ann.as_ts_type_lit() {
                    for type_element in ts_lit.members.iter() {
                        self.check_ts_signature(type_element);
                    }
                }
            }
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        }
    }

    pub(crate) fn transform_variables_declarator(
        &mut self,
        kind: VarDeclKind,
        decl: &mut VarDeclarator,
        check_binding: bool,
    ) {
        let pat = match &decl.name {
            Pat::Assign(assign_pat) => &assign_pat.left,
            _ => &decl.name,
        };

        if matches!(pat, Pat::Array(_) | Pat::Object(_)) {
            pat.bound_names(&mut |ident| {
                if !check_binding || self.used_refs.used_as_value(&ident.to_id()) {
                    self.binding_element_export(ident.span);

View on GitHub (pinned to 5176682b65)

Solutions

1. Rebuild the plugin against the swc_core version that matches the host @swc/core
2. Bump swc_core to the latest release so the declaration decodes concretely
3. Pre-scan module items for Decl::Unknown and skip .d.ts emission for that file
4. Treat any Unknown-node panic as a signal to resync versions, not to patch the match

Example fix

// before
for item in module.body.iter_mut() {
    // fast_dts internally matches Decl; Decl::Unknown -> panic
}

// after (guard, only meaningful under the plugin cfg)
#[cfg(swc_ast_unknown)]
fn module_has_unknown_decl(m: &swc_ecma_ast::Module) -> bool {
    m.body.iter().any(|i| matches!(i, swc_ecma_ast::ModuleItem::Stmt(
        swc_ecma_ast::Stmt::Decl(swc_ecma_ast::Decl::Unknown(..)))))
}
if !module_has_unknown_decl(&module) { /* run fast_dts */ }

#[cfg(swc_ast_unknown)]
fn module_contains_unknown_decl(m: &swc_ecma_ast::Module) -> bool {
    use swc_ecma_ast::*;
    m.body.iter().any(|i| matches!(i, ModuleItem::Stmt(
        Stmt::Decl(Decl::Unknown(..)))))
}

#[cfg(swc_ast_unknown)]
fn decl_is_unknown(d: &swc_ecma_ast::Decl) -> bool {
    matches!(d, swc_ecma_ast::Decl::Unknown(..))
}

let out = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
    run_fast_dts(&mut program)
}));
if out.is_err() {
    handler.struct_err("fast_dts skipped file: unknown declaration (version mismatch)").emit();
}

When it happens

Trigger: Running fast_dts on a module whose top-level declaration deserialized as Unknown — a plugin built with --cfg=swc_ast_unknown receiving a Program from a host @swc/core newer than its pinned swc_core, where the host emitted a new Decl variant.

Common situations: Version skew between host and plugin; new declaration kinds added to swc_ecma_ast in a release; plugins distributed as prebuilt wasm not rebuilt after host upgrades.

Related errors

• unable to access unknown nodes
• unable to access unknown nodes
• unable to access unknown nodes
• Duplicated element (${element.key})
• Duplicated element (${elements[j].key})

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/66ebcd051eb2fb8b. Report an issue: GitHub.