better error message for missing rgb/rgba args

src/args.rs

@@ -40,6 +40,10 @@ impl CallArgs {
         self.0.len()
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.0.len() == 0
+    }
+
     pub fn remove(&mut self, s: &str) -> Option<Value> {
         self.0.remove(s)
     }

src/builtin/color/hsl.rs

@@ -236,6 +236,10 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
         "saturate".to_owned(),
         Box::new(|args, _| {
             max_args!(args, 2);
+            if args.len() == 1 {
+                return Ok(Value::Ident(format!("saturate({})", arg!(args, 0, "amount")), QuoteKind::None));
+            }
+
             let amount = match arg!(args, 1, "amount") {
                 Value::Dimension(n, u) => bound!("amount", n, u, 0, 100) / Number::from(100),
                 v => return Err(format!("$amount: {} is not a number.", v).into()),

src/builtin/color/rgb.rs

@@ -11,6 +11,10 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "rgb".to_owned(),
         Box::new(|args, _| {
+            if args.is_empty() {
+                return Err("Missing argument $channels.".into());
+            }
+
             if args.len() == 1 {
                 let mut channels = match arg!(args, 0, "channels") {
                     Value::List(v, _) => v,
@@ -129,6 +133,10 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "rgba".to_owned(),
         Box::new(|args, _| {
+            if args.is_empty() {
+                return Err("Missing argument $channels.".into());
+            }
+
             if args.len() == 1 {
                 let mut channels = match arg!(args, 0, "channels") {
                     Value::List(v, _) => v,

tests/color.rs

@@ -126,6 +126,16 @@ test!(
     "a {\n  color: rgba(7.1%, 20.4%, 33.9%);\n}\n",
     "a {\n  color: #123456;\n}\n"
 );
+error!(
+    rgb_no_args,
+    "a {\n  color: rgb();\n}\n",
+    "Error: Missing argument $channels."
+);
+error!(
+    rgba_no_args,
+    "a {\n  color: rgba();\n}\n",
+    "Error: Missing argument $channels."
+);
 test!(
     hsl_basic,
     "a {\n  color: hsl(193, 67%, 99);\n}\n",
@@ -340,6 +350,11 @@ test!(
     "a {\n  color: saturate($color: hsl(25, 100%, 80%), $amount: 30%);\n}\n",
     "a {\n  color: #ffc499;\n}\n"
 );
+test!(
+    saturate_one_arg,
+    "a {\n  color: saturate($amount: 50%);\n}\n",
+    "a {\n  color: saturate(50%);\n}\n"
+);
 test!(
     saturate_basic,
     "a {\n  color: saturate(hsl(120, 30%, 90%), 20%);\n}\n",