swc-project/swc · error

unable to access unknown nodes

Error message

unable to access unknown nodes

What it means

This wildcard arm closes the Pat match inside Param::babelify (function parameters) and compiles only when `--cfg swc_ast_unknown` is set — the cfg SWC plugin builds use to make swc_ecma_ast enums non_exhaustive for cross-version tolerance. If the Pat value holds a variant beyond Ident/Array/Rest/Object/Assign/Expr/Invalid that the linked swc_ecma_ast version does not define, the code panics with 'unable to access unknown nodes'. Like the other unknown-node panics, it signals an AST produced by a different swc version than the one linked in.

Source

Thrown at crates/swc_estree_compat/src/babelify/function.rs:94

                base: ctx.base(self.span),
                decorators: Some(self.decorators.babelify(ctx)),
                ..o.babelify(ctx)
            })),
            Pat::Assign(a) => BabelParam::Pat(Pattern::Assignment(AssignmentPattern {
                base: ctx.base(self.span),
                decorators: Some(self.decorators.babelify(ctx)),
                ..a.babelify(ctx)
            })),
            Pat::Expr(_) => panic!(
                "illegal conversion: Cannot convert {:?} to BabelParam",
                &self.pat
            ),
            Pat::Invalid(_) => panic!(
                "illegal conversion: Cannot convert {:?} to BabelParam",
                &self.pat
            ),
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        }
    }
}

impl Babelify for ParamOrTsParamProp {
    type Output = BabelParam;

    fn babelify(self, ctx: &Context) -> Self::Output {
        match self {
            ParamOrTsParamProp::TsParamProp(p) => BabelParam::TSProp(p.babelify(ctx)),
            ParamOrTsParamProp::Param(p) => p.babelify(ctx),
            #[cfg(swc_ast_unknown)]
            _ => panic!("unable to access unknown nodes"),
        }
    }
}

View on GitHub (pinned to 5176682b65)

Solutions

  1. Unify swc versions: `cargo tree -i swc_ecma_ast` must show one version; update via swc_core
  2. Upgrade swc_estree_compat alongside swc_ecma_ast so the new Pat variant is covered
  3. Rebuild the plugin/wasm boundary against the host's swc release
  4. Use catch_unwind around babelify to degrade to a per-file error during migration
Defensive patterns

Strategy: try-catch

Type guard

fn param_pat_known(p: &Pat) -> bool {
    matches!(
        p,
        Pat::Ident(_) | Pat::Array(_) | Pat::Rest(_) | Pat::Object(_) | Pat::Assign(_) | Pat::Expr(_) | Pat::Invalid(_)
    )
}

Try / catch

let out = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| program.babelify(&ctx)))
    .map_err(|_| anyhow::anyhow!("babelify failed: swc_ecma_ast version mismatch (unknown Pat)"))?;

Prevention

When it happens

Trigger: A function parameter's Pat enum arriving from a newer/older swc_ecma_ast across a plugin ABI or mixed-version Cargo graph, carrying a variant this build cannot name.

Common situations: Plugin host/plugin version drift; partial swc upgrades leaving swc_estree_compat behind; wasm binaries cached from an older build.

Related errors

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/0cda59da45d8069b. Report an issue: GitHub.