swc-project/swc · error

unable to access unknown nodes

Error message

unable to access unknown nodes

What it means

babelify maps TS function-parameter kinds (Ident, Array, Rest, Object) to TsFnParamOutput for signatures in interfaces and type literals. The `_` arm is compiled only under the `swc_ast_unknown` cfg and panics on a TsFnParam variant this build of swc_estree_compat does not recognize.

Source

Thrown at crates/swc_estree_compat/src/babelify/typescript.rs:89

#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum TsFnParamOutput {
    Id(Identifier),
    Array(ArrayPattern),
    Rest(RestElement),
    Object(ObjectPattern),
}

impl Babelify for TsFnParam {
    type Output = TsFnParamOutput;

    fn babelify(self, ctx: &Context) -> Self::Output {
        match self {
            TsFnParam::Ident(i) => TsFnParamOutput::Id(i.babelify(ctx)),
            TsFnParam::Array(a) => TsFnParamOutput::Array(a.babelify(ctx)),
            TsFnParam::Rest(r) => TsFnParamOutput::Rest(r.babelify(ctx)),
            TsFnParam::Object(o) => TsFnParamOutput::Object(o.babelify(ctx)),
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        }
    }
}

impl From<TsFnParamOutput> for IdOrRest {
    fn from(o: TsFnParamOutput) -> Self {
        match o {
            TsFnParamOutput::Id(i) => IdOrRest::Id(i),
            TsFnParamOutput::Rest(r) => IdOrRest::Rest(r),
            _ => panic!("illegal conversion: Cannot convert {:?} to IdOrRest", &o),
        }
    }
}

impl From<TsFnParamOutput> for Identifier {
    fn from(o: TsFnParamOutput) -> Self {
        match o {
            TsFnParamOutput::Id(i) => i,

View on GitHub (pinned to 5176682b65)

Solutions

  1. Build the babelify path without `--cfg=swc_ast_unknown`
  2. Pin all swc crates to one swc_core release
  3. Cross boundaries via serialization so unknown variants error at decode
  4. Add the missing TsFnParam arm in typescript.rs upstream

Example fix

# before
RUSTFLAGS="--cfg=swc_ast_unknown" cargo build

# after
cargo build
Defensive patterns

Strategy: validation

Validate before calling

#[cfg(swc_ast_unknown)]
compile_error!("babelify panics on Unknown TsFnParam nodes; build without --cfg=swc_ast_unknown");

Try / catch

use std::panic::{catch_unwind, AssertUnwindSafe};

catch_unwind(AssertUnwindSafe(|| program.babelify()))
    .map_err(|p| anyhow!("babelify panicked: {:?}", p))?;

Prevention

When it happens

Trigger: Babelify a TypeScript interface/method signature whose parameter node carries an Unknown/newer TsFnParam variant while built with `--cfg=swc_ast_unknown` — e.g. converting an AST from a newer swc_ecma_ast inside a plugin host.

Common situations: Plugin development with the cfg auto-set; version drift between swc_ecma_ast and swc_estree_compat; converting deserialized ASTs across swc versions.

Related errors

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/5481bda67d318dd2. Report an issue: GitHub.