Add feature-exists() builtin function

src/builtin/macros.rs

@@ -0,0 +1,26 @@
+macro_rules! arg {
+    ($args:ident, $idx:literal, $name:literal) => {
+        match $args.get(stringify!($idx)) {
+            Some(v) => v,
+            None => match $args.get($name) {
+                Some(v) => v,
+                None => panic!("missing variable"),
+            },
+        };
+    };
+    ($args:ident, $idx:literal, $name:literal=$default:literal) => {
+        match $args.get(stringify!($idx)) {
+            Some(v) => v,
+            None => match $args.get($name) {
+                Some(v) => v,
+                None => $default,
+            },
+        };
+    };
+}
+
+macro_rules! decl {
+    ($f:ident $name:literal, $body:expr) => {
+        $f.insert($name.to_owned(), Box::new($body));
+    };
+}

src/builtin/meta.rs

@@ -14,4 +14,25 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             Some(if_false)
         }
     });
+    decl!(f "feature-exists", |args| {
+        let feature: &Value = arg!(args, 0, "feature");
+        match feature.to_string().as_str() {
+            // A local variable will shadow a global variable unless
+            // `!global` is used.
+            "global-variable-shadowing" => Some(Value::False),
+            // the @extend rule will affect selectors nested in pseudo-classes
+            // like :not()
+            "extend-selector-pseudoclass" => Some(Value::False),
+            // Full support for unit arithmetic using units defined in the
+            // [Values and Units Level 3][] spec.
+            "units-level-3" => Some(Value::False),
+            // The Sass `@error` directive is supported.
+            "at-error" => Some(Value::True),
+            // The "Custom Properties Level 1" spec is supported. This means
+            // that custom properties are parsed statically, with only
+            // interpolation treated as SassScript.
+            "custom-property" => Some(Value::False),
+            _ => Some(Value::False),
+        }
+    });
 }