Fabric: Fixing usage of Folly's hash_combine

Summary:
Accidentally I noticed that the signature of Folly's hash_combine is different from boost's one.
The Folly's one is:
`size_t hash_combine(const T& t, const Ts&... ts)`, so the first argument is immutable and the method returns the result normally.
It means that all hashes that we compute in Fabric using `hash_combine` were `0`.
So I fixed it.

I have no idea why this difference exists, but some modern papers suggest that folly's variant has good chances to be standardized.
E.g.: http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0814r0.pdf

Technically, it should improve performance, but I doubt that it can be more than 1-2 ms per screen TTI.

Reviewed By: JoshuaGross

Differential Revision: D14430380

fbshipit-source-id: 97da999ee5780b940bb789bc3eb5bf9f89c194ca

ReactCommon/fabric/attributedstring/AttributedString.h

@@ -95,14 +95,12 @@ template <>
 struct hash<facebook::react::AttributedString::Fragment> {
   size_t operator()(
       const facebook::react::AttributedString::Fragment &fragment) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed,
+    return folly::hash::hash_combine(
+        0,
         fragment.string,
         fragment.textAttributes,
         fragment.shadowView,
         fragment.parentShadowView);
-    return seed;
   }
 };
 
@@ -113,7 +111,7 @@ struct hash<facebook::react::AttributedString> {
     auto seed = size_t{0};
 
     for (const auto &fragment : attributedString.getFragments()) {
-      folly::hash::hash_combine(seed, fragment);
+      seed = folly::hash::hash_combine(seed, fragment);
     }
 
     return seed;

ReactCommon/fabric/attributedstring/ParagraphAttributes.h

@@ -73,15 +73,13 @@ template <>
 struct hash<facebook::react::ParagraphAttributes> {
   size_t operator()(
       const facebook::react::ParagraphAttributes &attributes) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed,
+    return folly::hash::hash_combine(
+        0,
         attributes.maximumNumberOfLines,
         attributes.ellipsizeMode,
         attributes.adjustsFontSizeToFit,
         attributes.minimumFontSize,
         attributes.maximumFontSize);
-    return seed;
   }
 };
 } // namespace std

ReactCommon/fabric/attributedstring/TextAttributes.h

@@ -98,9 +98,8 @@ template <>
 struct hash<facebook::react::TextAttributes> {
   size_t operator()(
       const facebook::react::TextAttributes &textAttributes) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed,
+    return folly::hash::hash_combine(
+        0,
         textAttributes.foregroundColor,
         textAttributes.backgroundColor,
         textAttributes.opacity,
@@ -124,7 +123,6 @@ struct hash<facebook::react::TextAttributes> {
         textAttributes.textShadowColor,
         textAttributes.isHighlighted,
         textAttributes.layoutDirection);
-    return seed;
   }
 };
 } // namespace std

ReactCommon/fabric/core/layout/LayoutConstraints.h

@@ -44,13 +44,11 @@ template <>
 struct hash<facebook::react::LayoutConstraints> {
   size_t operator()(
       const facebook::react::LayoutConstraints &constraints) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed,
+    return folly::hash::hash_combine(
+        0,
         constraints.minimumSize,
         constraints.maximumSize,
         constraints.layoutDirection);
-    return seed;
   }
 };
 } // namespace std

ReactCommon/fabric/graphics/Geometry.h

@@ -186,51 +186,41 @@ namespace std {
 template <>
 struct hash<facebook::react::Point> {
   size_t operator()(const facebook::react::Point &point) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(seed, point.x, point.y);
-    return seed;
+    return folly::hash::hash_combine(0, point.x, point.y);
   }
 };
 
 template <>
 struct hash<facebook::react::Size> {
   size_t operator()(const facebook::react::Size &size) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(seed, size.width, size.height);
-    return seed;
+    return folly::hash::hash_combine(0, size.width, size.height);
   }
 };
 
 template <>
 struct hash<facebook::react::Rect> {
   size_t operator()(const facebook::react::Rect &rect) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(seed, rect.origin, rect.size);
-    return seed;
+    return folly::hash::hash_combine(0, rect.origin, rect.size);
   }
 };
 
 template <typename T>
 struct hash<facebook::react::RectangleEdges<T>> {
   size_t operator()(const facebook::react::RectangleEdges<T> &edges) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed, edges.left, edges.right, edges.top, edges.bottom);
-    return seed;
+    return folly::hash::hash_combine(
+        0, edges.left, edges.right, edges.top, edges.bottom);
   }
 };
 
 template <typename T>
 struct hash<facebook::react::RectangleCorners<T>> {
   size_t operator()(const facebook::react::RectangleCorners<T> &corners) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed,
+    return folly::hash::hash_combine(
+        0,
         corners.topLeft,
         corners.bottomLeft,
         corners.topRight,
         corners.bottomRight);
-    return seed;
   }
 };
 

ReactCommon/fabric/mounting/ShadowView.h

@@ -71,15 +71,13 @@ namespace std {
 template <>
 struct hash<facebook::react::ShadowView> {
   size_t operator()(const facebook::react::ShadowView &shadowView) const {
-    auto seed = size_t{0};
-    folly::hash::hash_combine(
-        seed,
+    return folly::hash::hash_combine(
+        0,
         shadowView.componentHandle,
         shadowView.tag,
         shadowView.props,
         shadowView.eventEmitter,
         shadowView.localData);
-    return seed;
   }
 };