swc-project/swc · error · Error

Expected valid dashed, '--' is not valid custom property nam

Error message

Expected valid dashed, '--' is not valid custom property name

What it means

Fired by the CustomPropertyName parser: the ident is exactly '--' with nothing after the prefix. Per spec, '--' alone is not a valid custom property name; at least one character must follow the two hyphens.

Source

Thrown at crates/swc_css_parser/src/parser/values_and_units/mod.rs:1912

where
    I: ParserInput,
{
    fn parse(&mut self) -> PResult<CustomPropertyName> {
        let span = self.input.cur_span();

        if !is!(self, Ident) {
            return Err(Error::new(span, ErrorKind::Expected("Ident")));
        }

        match bump!(self) {
            Token::Ident { value, raw, .. } => {
                if !value.starts_with("--") {
                    return Err(Error::new(
                        span,
                        ErrorKind::Expected("'--' at the start of custom property name"),
                    ));
                } else if &*value == "--" {
                    return Err(Error::new(
                        span,
                        ErrorKind::Expected("valid dashed, '--' is not valid custom property name"),
                    ));
                }

                Ok(CustomPropertyName {
                    span,
                    value,
                    raw: Some(raw),
                })
            }
            _ => {
                unreachable!()
            }
        }
    }
}

View on GitHub (pinned to 5176682b65)

Solutions

1. Add a name after the '--' prefix (e.g. '--spacing')
2. Remove the empty custom property declaration

When it happens

Trigger: Thrown at crates/swc_css_parser/src/parser/values_and_units/mod.rs:1912 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/769c80619b5fa8fe. Report an issue: GitHub.