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diff --git a/dom/canvas/TiedFields.h b/dom/canvas/TiedFields.h
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_CANVAS_TIED_FIELDS_H
+#define DOM_CANVAS_TIED_FIELDS_H
+
+#include "TupleUtils.h"
+
+namespace mozilla {
+
+// -
+
+/**
+ * TiedFields(T&) -> std::tuple<Fields&...>
+ * TiedFields(const T&) -> std::tuple<const Fields&...>
+ *
+ * You can also overload TiedFields without adding T::MutTiedFields:
+ * template<>
+ * inline auto TiedFields<gfx::IntSize>(gfx::IntSize& a) {
+ *   return std::tie(a.width, a.height);
+ * }
+ */
+template <class T>
+constexpr auto TiedFields(T& t) {
+  const auto fields = t.MutTiedFields();
+  return fields;
+}
+template <class T, class... Args, class Tup = std::tuple<Args&...>>
+constexpr auto TiedFields(const T& t) {
+  // Uncast const to get mutable-fields tuple, but reapply const to tuple args.
+  // We should do better than this when C++ gets a solution other than macros.
+  const auto mutFields = TiedFields(const_cast<T&>(t));
+  return ToTupleOfConstRefs(mutFields);
+}
+
+/**
+ * Returns true if all bytes in T are accounted for via size of all tied fields.
+ * Returns false if there's bytes unaccounted for, which might indicate either
+ * unaccounted-for padding or missing fields.
+ * The goal is to check that TiedFields returns every field in T, and this
+ * returns false if it suspects there are bytes that are not accounted for by
+ * TiedFields.
+ *
+ * `constexpr` effectively cannot do math on pointers, so it's not possible to
+ * figure out via `constexpr` whether fields are consecutive or dense.
+ * However, we can at least compare `sizeof(T)` to the sum of `sizeof(Args...)`
+ * for `TiedFields(T) -> std::tuple<Args...>`.
+ *
+ * See TiedFieldsExamples.
+ */
+template <class T>
+constexpr bool AreAllBytesTiedFields() {
+  using fieldsT = decltype(TiedFields(std::declval<T>()));
+  const auto fields_size_sum = SizeofTupleArgs<fieldsT>::value;
+  const auto t_size = sizeof(T);
+  return fields_size_sum == t_size;
+}
+
+// It's also possible to determine AreAllBytesRecursiveTiedFields:
+// https://hackmd.io/@jgilbert/B16qa0Fa9
+
+// -
+
+/**
+ * Padding<T> can be used to pad out a struct so that it's not implicitly
+ * padded by struct rules.
+ * You can also just add your padding to TiedFields, but by explicitly typing
+ * padding like this, serialization can make a choice whether to copy Padding,
+ * or instead to omit the copy.
+ *
+ * Omitting the copy isn't always faster.
+ * struct Entry {
+ *   uint16_t key;
+ *   Padding<uint16_t> padding;
+ *   uint32_t val;
+ *   auto MutTiedFields() { return std::tie(key, padding, val); }
+ * };
+ * If you serialize Padding, the serialized size is 8, and the compiler will
+ * optimize serialization to a single 8-byte memcpy.
+ * If your serialization omits Padding, the serialized size of Entry shrinks
+ * by 25%. If you have a big list of Entrys, maybe this is a big savings!
+ * However, by optimizing for size here you sacrifice speed, because this splits
+ * the single memcpy into two: a 2-byte memcpy and a 4-byte memcpy.
+ *
+ * Explicitly marking padding gives callers the option of choosing.
+ */
+template <class T>
+struct Padding {
+  T ignored;
+
+  friend constexpr bool operator==(const Padding&, const Padding&) {
+    return true;
+  }
+  friend constexpr bool operator<(const Padding&, const Padding&) {
+    return false;
+  }
+};
+static_assert(sizeof(Padding<bool>) == 1);
+static_assert(sizeof(Padding<bool[2]>) == 2);
+static_assert(sizeof(Padding<int>) == 4);
+
+// -
+
+namespace TiedFieldsExamples {
+
+struct Cat {
+  int i;
+  bool b;
+
+  constexpr auto MutTiedFields() { return std::tie(i, b); }
+};
+static_assert(sizeof(Cat) == 8);
+static_assert(!AreAllBytesTiedFields<Cat>());
+
+struct Dog {
+  bool b;
+  int i;
+
+  constexpr auto MutTiedFields() { return std::tie(i, b); }
+};
+static_assert(sizeof(Dog) == 8);
+static_assert(!AreAllBytesTiedFields<Dog>());
+
+struct Fish {
+  bool b;
+  bool padding[3];
+  int i;
+
+  constexpr auto MutTiedFields() { return std::tie(i, b, padding); }
+};
+static_assert(sizeof(Fish) == 8);
+static_assert(AreAllBytesTiedFields<Fish>());
+
+struct Eel {  // Like a Fish, but you can skip serializing the padding.
+  bool b;
+  Padding<bool> padding[3];
+  int i;
+
+  constexpr auto MutTiedFields() { return std::tie(i, b, padding); }
+};
+static_assert(sizeof(Eel) == 8);
+static_assert(AreAllBytesTiedFields<Eel>());
+
+// -
+
+// #define LETS_USE_BIT_FIELDS
+#ifdef LETS_USE_BIT_FIELDS
+#  undef LETS_USE_BIT_FIELDS
+
+struct Platypus {
+  short s : 1;
+  short s2 : 1;
+  int i;
+
+  constexpr auto MutTiedFields() {
+    return std::tie(s, s2, i);  // Error: Can't take reference to bit-field.
+  }
+};
+
+#endif
+
+// -
+
+struct FishTank {
+  Fish f;
+  int i2;
+
+  constexpr auto MutTiedFields() { return std::tie(f, i2); }
+};
+static_assert(sizeof(FishTank) == 12);
+static_assert(AreAllBytesTiedFields<FishTank>());
+
+struct CatCarrier {
+  Cat c;
+  int i2;
+
+  constexpr auto MutTiedFields() { return std::tie(c, i2); }
+};
+static_assert(sizeof(CatCarrier) == 12);
+static_assert(AreAllBytesTiedFields<CatCarrier>());
+static_assert(
+    !AreAllBytesTiedFields<decltype(CatCarrier::c)>());  // BUT BEWARE THIS!
+// For example, if we had AreAllBytesRecursiveTiedFields:
+// static_assert(!AreAllBytesRecursiveTiedFields<CatCarrier>());
+
+}  // namespace TiedFieldsExamples
+}  // namespace mozilla
+
+#endif  // DOM_CANVAS_TIED_FIELDS_H