What causes "internal error: entered unreachable code" in swc-project/swc?

Fix the hue value: use a unitless number or one of deg/grad/rad/turn (e.g. `hsl(180deg, 50%, 50%)`)

internal error: entered unreachable code

Error message

internal error: entered unreachable code

What it means

The legacy rgb()/hsl() compat pass converts hue angles to plain numbers, accepting deg/grad/rad/turn after lowercasing the unit. Any other dimension unit in the hue position (px, %, pt, or an unknown unit) reaches unreachable!() — the parser accepted the dimension but the lowering has no conversion for it.

Source

Thrown at crates/swc_css_compat/src/compiler/legacy_rgb_and_hsl.rs:56

            } else if is_hsl {
                function.value = function
                    .value
                    .drain(..)
                    .map(|n| {
                        if let Some(Angle {
                            span,
                            value: Number { value, .. },
                            unit,
                            ..
                        }) = n.as_hue().and_then(|hue| hue.as_angle())
                        {
                            let value = match &*unit.value.to_ascii_lowercase() {
                                "deg" => *value,
                                "grad" => value * 180.0 / 200.0,
                                "rad" => value * 180.0 / PI,
                                "turn" => value * 360.0,
                                _ => {
                                    unreachable!();
                                }
                            };

                            ComponentValue::Number(Box::new(Number {
                                span: *span,
                                value: value.round(),
                                raw: None,
                            }))
                        } else {
                            n
                        }
                    })
                    .collect();
            }

            if is_rgb || is_hsl {
                if let Some(alpha_value) = function
                    .value

View on GitHub (pinned to 5176682b65)

Solutions

Fix the hue value: use a unitless number or one of deg/grad/rad/turn (e.g. `hsl(180deg, 50%, 50%)`)
Pre-validate color functions (scan hsl(/hsla( hue units) before running compat passes
Report the exact value to swc so the pass can emit a proper parse error instead of panicking

Example fix

/* before */
color: hsl(10px, 50%, 50%);

/* after */
color: hsl(10deg, 50%, 50%);

Defensive patterns

Strategy: validation

Validate before calling

// Reject legacy hsl()/hsla() whose hue has a non-angle unit
let bad_hue = regex::Regex::new(
    r#"(?i)hsla?\(\s*[+-]?[\d.]+(px|em|rem|ex|ch|vw|vh|vmin|vmax|cm|mm|in|pt|pc|q|fr|d|%)"#,
).unwrap();
if bad_hue.is_match(&css) {
    return Err("hsl() hue must be unitless or deg/grad/rad/turn".into());
}

Try / catch

match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| compile_css(&css))) {
    Ok(v) => v,
    Err(p) if panic_message(&p).contains("unreachable") => {
        // fallback: strip compat lowering for colors (raise target) and recompile
    }
    Err(p) => std::panic::resume_unwind(p),
}

Prevention

Validate color function syntax in CI lint for generated CSS
Normalize hues to unitless numbers or deg before compat passes
Fuzz color inputs in tests to catch unit typos early

When it happens

Trigger: Compiling CSS through swc_css_compat's legacy rgb/hsl lowering when a legacy hsl() hue component has a unit other than deg/grad/rad/turn — e.g. `hsl(10px, 50%, 50%)`, `hsl(2rem, ...)`, or a typo like `hsl(360d ...)`.

Common situations: Malformed hand-written CSS; CSS generated by tools that emit unusual units in the hue slot; values copied from design tools; fuzzing corpora.

Related errors

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/51dd82827923e88d. Report an issue: GitHub.