swc-project/swc · error

json subdiagnostic: {d:?}

Error message

json subdiagnostic: {d:?}

What it means

An unreachable!() in the JSON diagnostic emit path (binding_nodejs_support_wasm lib.rs:173). While serializing a top-level diagnostic's sub-diagnostics (children), the code maps each child; the arm is reached only if the child list yields something the mapping code considers impossible for well-formed diagnostics from rustc/swc — i.e. a malformed sub-diagnostic structure. It marks a broken invariant in diagnostic construction rather than a user input error, and names the offending sub-diagnostic via {d:?}.

Source

Thrown at bindings/binding_nodejs_support_wasm/src/lib.rs:173

        let snippet = d.span.primary_span().and_then(|span| {
            let mut snippet = String::new();
            let reporter = self.reporter.get_or_insert_with(json_reporter);
            match reporter.render_report(
                &mut snippet,
                &Snippet {
                    source_code: &to_pretty_source_code(&self.cm, true),
                    span,
                },
            ) {
                Ok(()) => Some(snippet),
                Err(_) => None,
            }
        });

        let children = d
            .children
            .iter()
            .map(|d| todo!("json subdiagnostic: {d:?}"))
            .collect::<Vec<_>>();

        let error_code = match &d.code {
            Some(DiagnosticId::Error(s)) => Some(&**s),
            Some(DiagnosticId::Lint(s)) => Some(&**s),
            None => None,
        };

        let start = d
            .span
            .primary_span()
            .and_then(|span| self.cm.try_lookup_char_pos(span.lo()).ok());

        let end = d
            .span
            .primary_span()
            .and_then(|span| self.cm.try_lookup_char_pos(span.hi()).ok());

View on GitHub (pinned to 5176682b65)

Solutions

1. Log the full parent diagnostic to see which sub-diagnostic shape broke the invariant
2. Ensure diagnostics are constructed via the swc_common diagnostic builders so children are always well-formed
3. Return an error from emit instead of panicking so the wasm binding can surface a JS-side failure

When it happens

Trigger: Thrown at bindings/binding_nodejs_support_wasm/src/lib.rs:173 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/0b2e121b6302d534. Report an issue: GitHub.