What causes "Expected 'deg', 'grad', 'rad' or 'turn' units" in swc-project/swc?

Use exactly deg, grad, rad, or turn as the unit (any casing; no plural forms).

swc-project/swc · error · Error

Expected 'deg', 'grad', 'rad' or 'turn' units

Error message

Expected 'deg', 'grad', 'rad' or 'turn' units

What it means

Thrown by Parse<Angle> in swc_css_parser when the token IS a dimension but its unit is not a CSS angle unit. After bumping the Dimension token it checks is_angle_unit(&unit), which accepts only deg, grad, rad, turn (ASCII case-insensitive via matches_eq_ignore_ascii_case!). Any other unit attached to the number (px, s, em, invented units) produces this error with the span of the whole dimension token.

Source

Thrown at crates/swc_css_parser/src/parser/values_and_units/mod.rs:2069

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

        match bump!(self) {
            Token::Dimension {
                dimension: dimension_token,
            } => {
                let DimensionToken {
                    value,
                    unit,
                    raw_value,
                    raw_unit,
                    ..
                } = *dimension_token;

                if !is_angle_unit(&unit) {
                    return Err(Error::new(
                        span,
                        ErrorKind::Expected("'deg', 'grad', 'rad' or 'turn' units"),
                    ));
                }

                let unit_len = raw_unit.len() as u32;

                Ok(Angle {
                    span,
                    value: Number {
                        span: Span::new(span.lo, span.hi - BytePos(unit_len)),
                        value,
                        raw: Some(raw_value),
                    },
                    unit: Ident {
                        span: Span::new(span.hi - BytePos(unit_len), span.hi),
                        value: unit,
                        raw: Some(raw_unit),

View on GitHub (pinned to 5176682b65)

Solutions

Use exactly deg, grad, rad, or turn as the unit (any casing; no plural forms).
Fix the source that generates the unit string (e.g. change a suffix constant from 'px'/'secs' to 'deg').
If the number is a plain hue in degrees, keep it a bare number ('hsl(120 50% 50%)') which Hue accepts without a unit.
Pre-validate with a unit whitelist before parsing.

Example fix

/* before */
a { transform: rotate(90segs); }

/* after */
a { transform: rotate(90deg); }

Defensive patterns

Strategy: validation

Validate before calling

const ANGLE_UNITS = new Set(["deg", "grad", "rad", "turn"]);
function hasValidAngleUnit(v: string): boolean {
  const i = [...v].findIndex(c => /[a-zA-Z]/.test(c));
  return i > 0 && ANGLE_UNITS.has(v.slice(i).toLowerCase());
}

Type guard

fn valid_angle_unit(unit: &str) -> bool {
    matches!(unit.to_ascii_lowercase().as_str(), "deg" | "grad" | "rad" | "turn")
}

Try / catch

use swc_css_parser::error::ErrorKind;
if let Err(e) = parse_input::<Stylesheet>(src, opts) {
    if matches!(e.kind(), ErrorKind::Expected("'deg', 'grad', 'rad' or 'turn' units")) {
        // unit typo: report span and reject the sheet rather than guessing a conversion
        return Err(format!("bad angle unit at {:?}", e.span()).into());
    }
    return Err(e.into());
}

Prevention

Whitelist angle units at the boundary where CSS is generated.
For hue positions prefer bare numbers (no unit) to sidestep unit checks entirely.
Add fixture tests for each angle unit in both lower and upper case.

When it happens

Trigger: Values like 'rotate(90segs)', 'rotate(100px)', 'transition-duration'-style values mistakenly used as angles, or '45degree'. Also reached via Parse<Hue>: 'hsl(30px 50% 50%)' passes the dimension check in Hue and then fails here because px is not an angle unit.

Common situations: Typos in angle units ('degree', 'degs'), confusion between animation duration ('s') and rotation angle ('deg') when generating CSS from JS, and data-driven CSS where a shared numeric suffix variable contains the wrong unit string.

Related errors

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/b0100e9cfeb71351. Report an issue: GitHub.