swc-project/swc · error

spread should be removed before applying generator

Error message

spread should be removed before applying generator

What it means

When the es2015 generator pass converts an object literal inside a generator body into a sequence of assignments to a temp object, it asserts that no spread properties remain (PropOrSpread::Spread). Object spread (`{...x}`) is an ES2018 feature that the es2018::object_rest_spread pass is expected to remove (rewriting to Object.assign/_extends) before generator lowering runs.

Source

Thrown at crates/swc_ecma_compat_es2015/src/generator.rs:740

                            .props
                            .iter_mut()
                            .take(num_initial_properties)
                            .map(|v| v.take())
                            .collect(),
                    }
                    .into(),
                    None,
                );

                let mut expressions = node
                    .props
                    .iter_mut()
                    .skip(num_initial_properties)
                    .map(|v| v.take())
                    .fold(Vec::<CompiledProp>::new(), |mut props, p| {
                        match p {
                            PropOrSpread::Spread(_) => {
                                unreachable!("spread should be removed before applying generator")
                            }
                            PropOrSpread::Prop(p) => match *p {
                                Prop::Getter(p) => {
                                    if let Some(CompiledProp::Accessor(g, _)) =
                                        props.iter_mut().find(|prev| match prev {
                                            CompiledProp::Accessor(_, Some(s)) => {
                                                s.key.eq_ignore_span(&p.key)
                                            }
                                            _ => false,
                                        })
                                    {
                                        *g = Some(p);
                                    } else {
                                        props.push(CompiledProp::Accessor(Some(p), None))
                                    }
                                }
                                Prop::Setter(p) => {
                                    if let Some(CompiledProp::Accessor(_, s)) =

View on GitHub (pinned to 5176682b65)

Solutions

1. Insert es2018::object_rest_spread (and the rest of the es2018 block) before es2015::generator in manual chains.
2. Use preset_env / CompatEnv so ordering is handled for you.
3. Verify all swc crates resolve to one swc_core version.
4. If ordering is right, report with a repro of `{...spread}` inside a generator.

Example fix

// before: object spread reaches the generator pass
let pass = Chain::new(swc_ecma_compat_es2015::generator());
// after
let pass = Chain::new(
    swc_ecma_compat_es2018::object_rest_spread(Default::default()),
    swc_ecma_compat_es2015::generator(),
);

// Reject object-spread before the generator pass runs
use swc_ecma_ast::*; use swc_ecma_visit::{Visit, VisitWith};
struct SpreadScan(bool); impl Visit for SpreadScan {
    fn visit_prop_or_spread(&mut self, p: &PropOrSpread) {
        self.0 |= matches!(p, PropOrSpread::Spread(_));
        p.visit_children_with(self);
    }
}
// if SpreadScan scan is true, run es2018 object_rest_spread before generator()

When it happens

Trigger: Running swc_ecma_compat_es2015 generator (or a chain containing it) on code with object-spread properties inside generator functions / async functions, when es2018::object_rest_spread has not executed first — typically a manually ordered pass chain or preset ordering broken by version skew.

Common situations: Custom Chain-of-passes drivers targeting ES5 that run the generator pass without the es2018 spread lowering; spack/bundlers embedding swc_core with partial compat pipelines; swc_core downgrades.

Related errors

• using declaration must be removed by previous pass
• object rest pattern should be removed by es2018::object_rest
• using declaration must be removed by previous pass
• unknown compound assignment operator
• assignment property be removed before generator pass

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/88a3f23a38e974c0. Report an issue: GitHub.