Bounds checking macro for builtin functions

src/builtin/color/hsl.rs

@@ -88,7 +88,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, _) => n / Number::from(100),
+            Value::Dimension(n, u) => bound!(n, u, 0, 100) / Number::from(100),
             v => return Err(format!("$amount: {} is not a number.", v).into())
         };
         Ok(Value::Color(color.lighten(amount)))

src/builtin/macros.rs

@@ -46,3 +46,17 @@ macro_rules! max_args {
         }
     };
 }
+
+macro_rules! bound {
+    ($arg:ident, $unit:ident, $low:literal, $high:literal) => {
+        if $arg > Number::from($high) || $arg < Number::from($low) {
+            return Err(format!(
+                "Expected {}{} to be within {}{} and {}{}.",
+                $arg, $unit, $low, $unit, $high, $unit,
+            )
+            .into());
+        } else {
+            $arg
+        }
+    };
+}