Support first class mixins

lib/sass/embedded/host/value_protofier.rb

@@ -96,6 +96,12 @@ def to_proto(obj)
                 )
               )
             end
+          when Sass::Value::Mixin
+            EmbeddedProtocol::Value.new(
+              compiler_mixin: EmbeddedProtocol::Value::CompilerMixin.new(
+                id: obj.id
+              )
+            )
           when Sass::Value::Calculation
             EmbeddedProtocol::Value.new(
               calculation: Calculation.to_proto(obj)
@@ -176,6 +182,8 @@ def from_proto(proto)
             Sass::Value::Function.new(obj.id)
           when :host_function
             raise Sass::ScriptError, 'The compiler may not send Value.host_function to host'
+          when :compiler_mixin
+            Sass::Value::Mixin.new(obj.id)
           when :calculation
             Calculation.from_proto(obj)
           when :singleton

lib/sass/value.rb

@@ -91,6 +91,12 @@ def assert_map(name = nil)
       raise Sass::ScriptError.new("#{self} is not a map", name)
     end
 
+    # @return [Mixin]
+    # @raise [ScriptError]
+    def assert_mixin(name = nil)
+      raise Sass::ScriptError.new("#{self} is not a mixin", name)
+    end
+
     # @return [Number]
     # @raise [ScriptError]
     def assert_number(name = nil)
@@ -132,6 +138,7 @@ def to_a_length
 require_relative 'value/function'
 require_relative 'value/fuzzy_math'
 require_relative 'value/map'
+require_relative 'value/mixin'
 require_relative 'value/null'
 require_relative 'value/number'
 require_relative 'value/string'

lib/sass/value/mixin.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module Sass
+  module Value
+    # Sass's mixin type.
+    #
+    # @see https://sass-lang.com/documentation/js-api/classes/sassmixin/
+    class Mixin
+      include Value
+
+      # @!visibility private
+      def initialize(id)
+        @id = id
+      end
+
+      # @return [Integer]
+      attr_reader :id
+
+      # @return [::Boolean]
+      def ==(other)
+        other.is_a?(Sass::Value::Mixin) && other.id == id
+      end
+
+      # @return [Integer]
+      def hash
+        @hash ||= id.hash
+      end
+
+      # @return [Mixin]
+      def assert_mixin(_name = nil)
+        self
+      end
+    end
+  end
+end