support single arg special color fns

src/builtin/functions/color/hsl.rs

@@ -23,6 +23,7 @@ fn inner_hsl(name: &'static str, mut args: CallArgs, parser: &mut Parser) -> Sas
     if len == 1 {
         let mut channels = match args.get_err(0, "channels")? {
             Value::List(v, ..) => v,
+            v if v.is_special_function() => vec![v],
             _ => return Err(("Missing argument $channels.", args.span()).into()),
         };
 

src/builtin/functions/color/rgb.rs

@@ -25,6 +25,7 @@ fn inner_rgb(name: &'static str, mut args: CallArgs, parser: &mut Parser) -> Sas
     if len == 1 {
         let mut channels = match args.get_err(0, "channels")? {
             Value::List(v, ..) => v,
+            v if v.is_special_function() => vec![v],
             _ => return Err(("Missing argument $channels.", args.span()).into()),
         };
 

tests/color.rs

@@ -757,6 +757,11 @@ test!(
     "a {\n  color: hsl(1, 0.02, max(0.4));\n}\n",
     "a {\n  color: hsl(1, 0.02, max(0.4));\n}\n"
 );
+test!(
+    hsla_special_fn_1_arg_is_not_list,
+    "a {\n  color: hsla(var(--foo));\n}\n",
+    "a {\n  color: hsla(var(--foo));\n}\n"
+);
 test!(
     #[ignore = "we do not check if interpolation occurred"]
     interpolated_named_color_is_not_color,