[core] Add feature state support to expression

include/mbgl/style/expression/expression.hpp

@@ -34,6 +34,9 @@ public:
     EvaluationContext(optional<mbgl::Value> accumulated_, GeometryTileFeature const * feature_) :
         accumulated(std::move(accumulated_)), feature(feature_)
     {}
+    EvaluationContext(float zoom_, GeometryTileFeature const * feature_, const FeatureState* state_) :
+        zoom(zoom_), feature(feature_), featureState(state_)
+    {}
     EvaluationContext(optional<float> zoom_, GeometryTileFeature const * feature_, optional<double> colorRampParameter_) :
         zoom(std::move(zoom_)), feature(feature_), colorRampParameter(std::move(colorRampParameter_))
     {}
@@ -43,12 +46,18 @@ public:
         return *this;
     };
 
+    EvaluationContext& withFeatureState(const FeatureState* featureState_) noexcept {
+        featureState = featureState_;
+        return *this;
+    };
+
     optional<float> zoom;
     optional<mbgl::Value> accumulated;
     GeometryTileFeature const * feature = nullptr;
     optional<double> colorRampParameter;
     // Contains formatted section object, std::unordered_map<std::string, Value>.
     const Value* formattedSection = nullptr;
+    const FeatureState* featureState = nullptr;
 };
 
 template <typename T>

include/mbgl/style/property_expression.hpp

@@ -61,6 +61,10 @@ public:
         return evaluate(expression::EvaluationContext(zoom, &feature), finalDefaultValue);
     }
 
+    T evaluate(float zoom, const GeometryTileFeature& feature, const FeatureState& state, T finalDefaultValue) const {
+        return evaluate(expression::EvaluationContext(zoom, &feature, &state), finalDefaultValue);
+    }
+
     std::vector<optional<T>> possibleOutputs() const {
         return expression::fromExpressionValues<T>(expression->possibleOutputs());
     }

src/mbgl/renderer/possibly_evaluated_property_value.hpp

@@ -49,6 +49,16 @@ public:
             }
         );
     }
+
+    template <class Feature>
+    T evaluate(const Feature& feature, float zoom, const FeatureState& featureState, T defaultValue) const {
+        return this->match(
+            [&] (const T& constant_) { return constant_; },
+            [&] (const style::PropertyExpression<T>& expression) {
+                return expression.evaluate(zoom, feature, featureState, defaultValue);
+            }
+        );
+    }
 };
 
 template <class T>

src/mbgl/style/properties.hpp

@@ -180,11 +180,28 @@ public:
                 });
         }
 
+        template <class T>
+        static T evaluate(float z, const GeometryTileFeature& feature, const FeatureState& state,
+                          const PossiblyEvaluatedPropertyValue<T>& v, const T& defaultValue) {
+            return v.match(
+                [&] (const T& t) {
+                    return t;
+                },
+                [&] (const PropertyExpression<T>& t) {
+                    return t.evaluate(z, feature, state, defaultValue);
+                });
+        }
+
         template <class P>
         auto evaluate(float z, const GeometryTileFeature& feature) const {
             return evaluate(z, feature, this->template get<P>(), P::defaultValue());
         }
 
+        template <class P>
+        auto evaluate(float z, const GeometryTileFeature& feature, const FeatureState& state) const {
+            return evaluate(z, feature, state, this->template get<P>(), P::defaultValue());
+        }
+
         Evaluated evaluate(float z, const GeometryTileFeature& feature) const {
             return Evaluated {
                 evaluate<Ps>(z, feature)...