Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 873 insertions, 0 deletions

diff --git a/mfbt/Result.h b/mfbt/Result.h
new file mode 100644
index 0000000000..052920fdbf
--- /dev/null
+++ b/mfbt/Result.h
@@ -0,0 +1,873 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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/. */
+
+/* A type suitable for returning either a value or an error from a function. */
+
+#ifndef mozilla_Result_h
+#define mozilla_Result_h
+
+#include <algorithm>
+#include <cstdint>
+#include <cstring>
+#include <type_traits>
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/CompactPair.h"
+#include "mozilla/MaybeStorageBase.h"
+
+namespace mozilla {
+
+/**
+ * Empty struct, indicating success for operations that have no return value.
+ * For example, if you declare another empty struct `struct OutOfMemory {};`,
+ * then `Result<Ok, OutOfMemory>` represents either success or OOM.
+ */
+struct Ok {};
+
+/**
+ * A tag used to differentiate between GenericErrorResult created by the Err
+ * function (completely new error) and GenericErrorResult created by the
+ * Result::propagateErr function (propagated error). This can be used to track
+ * error propagation and eventually produce error stacks for logging/debugging
+ * purposes.
+ */
+struct ErrorPropagationTag {};
+
+template <typename E>
+class GenericErrorResult;
+template <typename V, typename E>
+class Result;
+
+namespace detail {
+
+enum class PackingStrategy {
+  Variant,
+  NullIsOk,
+  LowBitTagIsError,
+  PackedVariant,
+  ZeroIsEmptyError,
+};
+
+template <typename T>
+struct UnusedZero;
+
+template <typename V, typename E, PackingStrategy Strategy>
+class ResultImplementation;
+
+template <typename V>
+struct EmptyWrapper : V {
+  constexpr EmptyWrapper() = default;
+  explicit constexpr EmptyWrapper(const V&) {}
+  explicit constexpr EmptyWrapper(std::in_place_t) {}
+
+  constexpr V* addr() { return this; }
+  constexpr const V* addr() const { return this; }
+};
+
+// The purpose of AlignedStorageOrEmpty is to make an empty class look like
+// std::aligned_storage_t for the purposes of the PackingStrategy::NullIsOk
+// specializations of ResultImplementation below. We can't use
+// std::aligned_storage_t itself with an empty class, since it would no longer
+// be empty.
+template <typename V>
+using AlignedStorageOrEmpty =
+    std::conditional_t<std::is_empty_v<V>, EmptyWrapper<V>,
+                       MaybeStorageBase<V>>;
+
+template <typename V, typename E>
+class ResultImplementationNullIsOkBase {
+ protected:
+  using ErrorStorageType = typename UnusedZero<E>::StorageType;
+
+  static constexpr auto kNullValue = UnusedZero<E>::nullValue;
+
+  static_assert(std::is_trivially_copyable_v<ErrorStorageType>);
+
+  // XXX This can't be statically asserted in general, if ErrorStorageType is
+  // not a basic type. With C++20 bit_cast, we could probably re-add such as
+  // assertion. static_assert(kNullValue == decltype(kNullValue)(0));
+
+  CompactPair<AlignedStorageOrEmpty<V>, ErrorStorageType> mValue;
+
+ public:
+  explicit constexpr ResultImplementationNullIsOkBase(const V& aSuccessValue)
+      : mValue(aSuccessValue, kNullValue) {}
+  explicit constexpr ResultImplementationNullIsOkBase(V&& aSuccessValue)
+      : mValue(std::move(aSuccessValue), kNullValue) {}
+  template <typename... Args>
+  explicit constexpr ResultImplementationNullIsOkBase(std::in_place_t,
+                                                      Args&&... aArgs)
+      : mValue(std::piecewise_construct,
+               std::tuple(std::in_place, std::forward<Args>(aArgs)...),
+               std::tuple(kNullValue)) {}
+  explicit constexpr ResultImplementationNullIsOkBase(E aErrorValue)
+      : mValue(std::piecewise_construct, std::tuple<>(),
+               std::tuple(UnusedZero<E>::Store(std::move(aErrorValue)))) {
+    MOZ_ASSERT(mValue.second() != kNullValue);
+  }
+
+  constexpr ResultImplementationNullIsOkBase(
+      ResultImplementationNullIsOkBase&& aOther)
+      : mValue(std::piecewise_construct, std::tuple<>(),
+               std::tuple(aOther.mValue.second())) {
+    if constexpr (!std::is_empty_v<V>) {
+      if (isOk()) {
+        new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
+      }
+    }
+  }
+  ResultImplementationNullIsOkBase& operator=(
+      ResultImplementationNullIsOkBase&& aOther) {
+    if constexpr (!std::is_empty_v<V>) {
+      if (isOk()) {
+        mValue.first().addr()->~V();
+      }
+    }
+    mValue.second() = std::move(aOther.mValue.second());
+    if constexpr (!std::is_empty_v<V>) {
+      if (isOk()) {
+        new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
+      }
+    }
+    return *this;
+  }
+
+  constexpr bool isOk() const { return mValue.second() == kNullValue; }
+
+  constexpr const V& inspect() const { return *mValue.first().addr(); }
+  constexpr V unwrap() { return std::move(*mValue.first().addr()); }
+  constexpr void updateAfterTracing(V&& aValue) {
+    MOZ_ASSERT(isOk());
+    if (!std::is_empty_v<V>) {
+      mValue.first().addr()->~V();
+      new (mValue.first().addr()) V(std::move(aValue));
+    }
+  }
+
+  constexpr decltype(auto) inspectErr() const {
+    return UnusedZero<E>::Inspect(mValue.second());
+  }
+  constexpr E unwrapErr() { return UnusedZero<E>::Unwrap(mValue.second()); }
+  constexpr void updateErrorAfterTracing(E&& aErrorValue) {
+    mValue.second() = UnusedZero<E>::Store(std::move(aErrorValue));
+  }
+};
+
+template <typename V, typename E,
+          bool IsVTriviallyDestructible = std::is_trivially_destructible_v<V>>
+class ResultImplementationNullIsOk;
+
+template <typename V, typename E>
+class ResultImplementationNullIsOk<V, E, true>
+    : public ResultImplementationNullIsOkBase<V, E> {
+ public:
+  using ResultImplementationNullIsOkBase<V,
+                                         E>::ResultImplementationNullIsOkBase;
+};
+
+template <typename V, typename E>
+class ResultImplementationNullIsOk<V, E, false>
+    : public ResultImplementationNullIsOkBase<V, E> {
+ public:
+  using ResultImplementationNullIsOkBase<V,
+                                         E>::ResultImplementationNullIsOkBase;
+
+  ResultImplementationNullIsOk(ResultImplementationNullIsOk&&) = default;
+  ResultImplementationNullIsOk& operator=(ResultImplementationNullIsOk&&) =
+      default;
+
+  ~ResultImplementationNullIsOk() {
+    if (this->isOk()) {
+      this->mValue.first().addr()->~V();
+    }
+  }
+};
+
+/**
+ * Specialization for when the success type is one of integral, pointer, or
+ * enum, where 0 is unused, and the error type is an empty struct.
+ */
+template <typename V, typename E>
+class ResultImplementation<V, E, PackingStrategy::ZeroIsEmptyError> {
+  static_assert(std::is_integral_v<V> || std::is_pointer_v<V> ||
+                std::is_enum_v<V>);
+  static_assert(std::is_empty_v<E>);
+
+  V mValue;
+
+ public:
+  static constexpr PackingStrategy Strategy = PackingStrategy::ZeroIsEmptyError;
+
+  explicit constexpr ResultImplementation(V aValue) : mValue(aValue) {}
+  explicit constexpr ResultImplementation(E aErrorValue) : mValue(V(0)) {}
+
+  constexpr bool isOk() const { return mValue != V(0); }
+
+  constexpr V inspect() const { return mValue; }
+  constexpr V unwrap() { return inspect(); }
+
+  constexpr E inspectErr() const { return E(); }
+  constexpr E unwrapErr() { return inspectErr(); }
+
+  constexpr void updateAfterTracing(V&& aValue) {
+    this->~ResultImplementation();
+    new (this) ResultImplementation(std::move(aValue));
+  }
+  constexpr void updateErrorAfterTracing(E&& aErrorValue) {
+    this->~ResultImplementation();
+    new (this) ResultImplementation(std::move(aErrorValue));
+  }
+};
+
+/**
+ * Specialization for when the success type is default-constructible and the
+ * error type is a value type which can never have the value 0 (as determined by
+ * UnusedZero<>).
+ */
+template <typename V, typename E>
+class ResultImplementation<V, E, PackingStrategy::NullIsOk>
+    : public ResultImplementationNullIsOk<V, E> {
+ public:
+  static constexpr PackingStrategy Strategy = PackingStrategy::NullIsOk;
+  using ResultImplementationNullIsOk<V, E>::ResultImplementationNullIsOk;
+};
+
+template <size_t S>
+using UnsignedIntType = std::conditional_t<
+    S == 1, std::uint8_t,
+    std::conditional_t<
+        S == 2, std::uint16_t,
+        std::conditional_t<S == 3 || S == 4, std::uint32_t,
+                           std::conditional_t<S <= 8, std::uint64_t, void>>>>;
+
+/**
+ * Specialization for when alignment permits using the least significant bit
+ * as a tag bit.
+ */
+template <typename V, typename E>
+class ResultImplementation<V, E, PackingStrategy::LowBitTagIsError> {
+  static_assert(std::is_trivially_copyable_v<V> &&
+                std::is_trivially_destructible_v<V>);
+  static_assert(std::is_trivially_copyable_v<E> &&
+                std::is_trivially_destructible_v<E>);
+
+  static constexpr size_t kRequiredSize = std::max(sizeof(V), sizeof(E));
+
+  using StorageType = UnsignedIntType<kRequiredSize>;
+
+#if defined(__clang__)
+  alignas(std::max(alignof(V), alignof(E))) StorageType mBits;
+#else
+  // Some gcc versions choke on using std::max with alignas, see
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929 (and this seems to have
+  // regressed in some gcc 9.x version before being fixed again) Keeping the
+  // code above since we would eventually drop this when we no longer support
+  // gcc versions with the bug.
+  alignas(alignof(V) > alignof(E) ? alignof(V) : alignof(E)) StorageType mBits;
+#endif
+
+ public:
+  static constexpr PackingStrategy Strategy = PackingStrategy::LowBitTagIsError;
+
+  explicit constexpr ResultImplementation(V aValue) : mBits(0) {
+    if constexpr (!std::is_empty_v<V>) {
+      std::memcpy(&mBits, &aValue, sizeof(V));
+      MOZ_ASSERT((mBits & 1) == 0);
+    } else {
+      (void)aValue;
+    }
+  }
+  explicit constexpr ResultImplementation(E aErrorValue) : mBits(1) {
+    if constexpr (!std::is_empty_v<E>) {
+      std::memcpy(&mBits, &aErrorValue, sizeof(E));
+      MOZ_ASSERT((mBits & 1) == 0);
+      mBits |= 1;
+    } else {
+      (void)aErrorValue;
+    }
+  }
+
+  constexpr bool isOk() const { return (mBits & 1) == 0; }
+
+  constexpr V inspect() const {
+    V res;
+    std::memcpy(&res, &mBits, sizeof(V));
+    return res;
+  }
+  constexpr V unwrap() { return inspect(); }
+
+  constexpr E inspectErr() const {
+    const auto bits = mBits ^ 1;
+    E res;
+    std::memcpy(&res, &bits, sizeof(E));
+    return res;
+  }
+  constexpr E unwrapErr() { return inspectErr(); }
+
+  constexpr void updateAfterTracing(V&& aValue) {
+    this->~ResultImplementation();
+    new (this) ResultImplementation(std::move(aValue));
+  }
+  constexpr void updateErrorAfterTracing(E&& aErrorValue) {
+    this->~ResultImplementation();
+    new (this) ResultImplementation(std::move(aErrorValue));
+  }
+};
+
+// Return true if any of the struct can fit in a word.
+template <typename V, typename E>
+struct IsPackableVariant {
+  struct VEbool {
+    explicit constexpr VEbool(V&& aValue) : v(std::move(aValue)), ok(true) {}
+    explicit constexpr VEbool(E&& aErrorValue)
+        : e(std::move(aErrorValue)), ok(false) {}
+    V v;
+    E e;
+    bool ok;
+  };
+  struct EVbool {
+    explicit constexpr EVbool(V&& aValue) : v(std::move(aValue)), ok(true) {}
+    explicit constexpr EVbool(E&& aErrorValue)
+        : e(std::move(aErrorValue)), ok(false) {}
+    E e;
+    V v;
+    bool ok;
+  };
+
+  using Impl =
+      std::conditional_t<sizeof(VEbool) <= sizeof(EVbool), VEbool, EVbool>;
+
+  static const bool value = sizeof(Impl) <= sizeof(uintptr_t);
+};
+
+/**
+ * Specialization for when both type are not using all the bytes, in order to
+ * use one byte as a tag.
+ */
+template <typename V, typename E>
+class ResultImplementation<V, E, PackingStrategy::PackedVariant> {
+  using Impl = typename IsPackableVariant<V, E>::Impl;
+  Impl data;
+
+ public:
+  static constexpr PackingStrategy Strategy = PackingStrategy::PackedVariant;
+
+  explicit constexpr ResultImplementation(V aValue) : data(std::move(aValue)) {}
+  explicit constexpr ResultImplementation(E aErrorValue)
+      : data(std::move(aErrorValue)) {}
+
+  constexpr bool isOk() const { return data.ok; }
+
+  constexpr const V& inspect() const { return data.v; }
+  constexpr V unwrap() { return std::move(data.v); }
+
+  constexpr const E& inspectErr() const { return data.e; }
+  constexpr E unwrapErr() { return std::move(data.e); }
+
+  constexpr void updateAfterTracing(V&& aValue) {
+    MOZ_ASSERT(data.ok);
+    this->~ResultImplementation();
+    new (this) ResultImplementation(std::move(aValue));
+  }
+  constexpr void updateErrorAfterTracing(E&& aErrorValue) {
+    MOZ_ASSERT(!data.ok);
+    this->~ResultImplementation();
+    new (this) ResultImplementation(std::move(aErrorValue));
+  }
+};
+
+// To use nullptr as a special value, we need the counter part to exclude zero
+// from its range of valid representations.
+//
+// By default assume that zero can be represented.
+template <typename T>
+struct UnusedZero {
+  static const bool value = false;
+};
+
+// This template can be used as a helper for specializing UnusedZero for scoped
+// enum types which never use 0 as an error value, e.g.
+//
+// namespace mozilla::detail {
+//
+// template <>
+// struct UnusedZero<MyEnumType> : UnusedZeroEnum<MyEnumType> {};
+//
+// }  // namespace mozilla::detail
+//
+template <typename T>
+struct UnusedZeroEnum {
+  using StorageType = std::underlying_type_t<T>;
+
+  static constexpr bool value = true;
+  static constexpr StorageType nullValue = 0;
+
+  static constexpr T Inspect(const StorageType& aValue) {
+    return static_cast<T>(aValue);
+  }
+  static constexpr T Unwrap(StorageType aValue) {
+    return static_cast<T>(aValue);
+  }
+  static constexpr StorageType Store(T aValue) {
+    return static_cast<StorageType>(aValue);
+  }
+};
+
+// A bit of help figuring out which of the above specializations to use.
+//
+// We begin by safely assuming types don't have a spare bit, unless they are
+// empty.
+template <typename T>
+struct HasFreeLSB {
+  static const bool value = std::is_empty_v<T>;
+};
+
+// As an incomplete type, void* does not have a spare bit.
+template <>
+struct HasFreeLSB<void*> {
+  static const bool value = false;
+};
+
+// The lowest bit of a properly-aligned pointer is always zero if the pointee
+// type is greater than byte-aligned. That bit is free to use if it's masked
+// out of such pointers before they're dereferenced.
+template <typename T>
+struct HasFreeLSB<T*> {
+  static const bool value = (alignof(T) & 1) == 0;
+};
+
+// Select one of the previous result implementation based on the properties of
+// the V and E types.
+template <typename V, typename E>
+struct SelectResultImpl {
+  static const PackingStrategy value =
+      (UnusedZero<V>::value && std::is_empty_v<E>)
+          ? PackingStrategy::ZeroIsEmptyError
+      : (HasFreeLSB<V>::value && HasFreeLSB<E>::value)
+          ? PackingStrategy::LowBitTagIsError
+      : (UnusedZero<E>::value && sizeof(E) <= sizeof(uintptr_t))
+          ? PackingStrategy::NullIsOk
+      : (std::is_default_constructible_v<V> &&
+         std::is_default_constructible_v<E> && IsPackableVariant<V, E>::value)
+          ? PackingStrategy::PackedVariant
+          : PackingStrategy::Variant;
+
+  using Type = ResultImplementation<V, E, value>;
+};
+
+template <typename T>
+struct IsResult : std::false_type {};
+
+template <typename V, typename E>
+struct IsResult<Result<V, E>> : std::true_type {};
+
+}  // namespace detail
+
+template <typename V, typename E>
+constexpr auto ToResult(Result<V, E>&& aValue)
+    -> decltype(std::forward<Result<V, E>>(aValue)) {
+  return std::forward<Result<V, E>>(aValue);
+}
+
+/**
+ * Result<V, E> represents the outcome of an operation that can either succeed
+ * or fail. It contains either a success value of type V or an error value of
+ * type E.
+ *
+ * All Result methods are const, so results are basically immutable.
+ * This is just like Variant<V, E> but with a slightly different API, and the
+ * following cases are optimized so Result can be stored more efficiently:
+ *
+ * - If both the success and error types do not use their least significant bit,
+ * are trivially copyable and destructible, Result<V, E> is guaranteed to be as
+ * large as the larger type. This is determined via the HasFreeLSB trait. By
+ * default, empty classes (in particular Ok) and aligned pointer types are
+ * assumed to have a free LSB, but you can specialize this trait for other
+ * types. If the success type is empty, the representation is guaranteed to be
+ * all zero bits on success. Do not change this representation! There is JIT
+ * code that depends on it. (Implementation note: The lowest bit is used as a
+ * tag bit: 0 to indicate the Result's bits are a success value, 1 to indicate
+ * the Result's bits (with the 1 masked out) encode an error value)
+ *
+ * - Else, if the error type can't have a all-zero bits representation and is
+ * not larger than a pointer, a CompactPair is used to represent this rather
+ * than a Variant. This has shown to be better optimizable, and the template
+ * code is much simpler than that of Variant, so it should also compile faster.
+ * Whether an error type can't be all-zero bits, is determined via the
+ * UnusedZero trait. MFBT doesn't declare any public type UnusedZero, but
+ * nsresult is declared UnusedZero in XPCOM.
+ *
+ * The purpose of Result is to reduce the screwups caused by using `false` or
+ * `nullptr` to indicate errors.
+ * What screwups? See <https://bugzilla.mozilla.org/show_bug.cgi?id=912928> for
+ * a partial list.
+ *
+ * Result<const V, E> or Result<V, const E> are not meaningful. The success or
+ * error values in a Result instance are non-modifiable in-place anyway. This
+ * guarantee must also be maintained when evolving Result. They can be
+ * unwrap()ped, but this loses const qualification. However, Result<const V, E>
+ * or Result<V, const E> may be misleading and prevent movability. Just use
+ * Result<V, E>. (Result<const V*, E> may make sense though, just Result<const
+ * V* const, E> is not possible.)
+ */
+template <typename V, typename E>
+class [[nodiscard]] Result final {
+  // See class comment on Result<const V, E> and Result<V, const E>.
+  static_assert(!std::is_const_v<V>);
+  static_assert(!std::is_const_v<E>);
+  static_assert(!std::is_reference_v<V>);
+  static_assert(!std::is_reference_v<E>);
+
+  using Impl = typename detail::SelectResultImpl<V, E>::Type;
+
+  Impl mImpl;
+  // Are you getting this error?
+  // > error: implicit instantiation of undefined template
+  // > 'mozilla::detail::ResultImplementation<$V,$E,
+  // >                      mozilla::detail::PackingStrategy::Variant>'
+  // You need to include "ResultVariant.h"!
+
+ public:
+  static constexpr detail::PackingStrategy Strategy = Impl::Strategy;
+  using ok_type = V;
+  using err_type = E;
+
+  /** Create a success result. */
+  MOZ_IMPLICIT constexpr Result(V&& aValue) : mImpl(std::move(aValue)) {
+    MOZ_ASSERT(isOk());
+  }
+
+  /** Create a success result. */
+  MOZ_IMPLICIT constexpr Result(const V& aValue) : mImpl(aValue) {
+    MOZ_ASSERT(isOk());
+  }
+
+  /** Create a success result in-place. */
+  template <typename... Args>
+  explicit constexpr Result(std::in_place_t, Args&&... aArgs)
+      : mImpl(std::in_place, std::forward<Args>(aArgs)...) {
+    MOZ_ASSERT(isOk());
+  }
+
+  /** Create an error result. */
+  explicit constexpr Result(const E& aErrorValue) : mImpl(aErrorValue) {
+    MOZ_ASSERT(isErr());
+  }
+  explicit constexpr Result(E&& aErrorValue) : mImpl(std::move(aErrorValue)) {
+    MOZ_ASSERT(isErr());
+  }
+
+  /**
+   * Create a (success/error) result from another (success/error) result with
+   * different but convertible value and error types.
+   */
+  template <typename V2, typename E2,
+            typename = std::enable_if_t<std::is_convertible_v<V2, V> &&
+                                        std::is_convertible_v<E2, E>>>
+  MOZ_IMPLICIT constexpr Result(Result<V2, E2>&& aOther)
+      : mImpl(aOther.isOk() ? Impl{aOther.unwrap()}
+                            : Impl{aOther.unwrapErr()}) {}
+
+  /**
+   * Implementation detail of MOZ_TRY().
+   * Create an error result from another error result.
+   */
+  template <typename E2>
+  MOZ_IMPLICIT constexpr Result(GenericErrorResult<E2>&& aErrorResult)
+      : mImpl(std::move(aErrorResult.mErrorValue)) {
+    static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
+    MOZ_ASSERT(isErr());
+  }
+
+  /**
+   * Implementation detail of MOZ_TRY().
+   * Create an error result from another error result.
+   */
+  template <typename E2>
+  MOZ_IMPLICIT constexpr Result(const GenericErrorResult<E2>& aErrorResult)
+      : mImpl(aErrorResult.mErrorValue) {
+    static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
+    MOZ_ASSERT(isErr());
+  }
+
+  Result(const Result&) = delete;
+  Result(Result&&) = default;
+  Result& operator=(const Result&) = delete;
+  Result& operator=(Result&&) = default;
+
+  /** True if this Result is a success result. */
+  constexpr bool isOk() const { return mImpl.isOk(); }
+
+  /** True if this Result is an error result. */
+  constexpr bool isErr() const { return !mImpl.isOk(); }
+
+  /** Take the success value from this Result, which must be a success result.
+   */
+  constexpr V unwrap() {
+    MOZ_ASSERT(isOk());
+    return mImpl.unwrap();
+  }
+
+  /**
+   * Take the success value from this Result, which must be a success result.
+   * If it is an error result, then return the aValue.
+   */
+  constexpr V unwrapOr(V aValue) {
+    return MOZ_LIKELY(isOk()) ? mImpl.unwrap() : std::move(aValue);
+  }
+
+  /** Take the error value from this Result, which must be an error result. */
+  constexpr E unwrapErr() {
+    MOZ_ASSERT(isErr());
+    return mImpl.unwrapErr();
+  }
+
+  /** Used only for GC tracing. If used in Rooted<Result<...>>, V must have a
+   * GCPolicy for tracing it. */
+  constexpr void updateAfterTracing(V&& aValue) {
+    mImpl.updateAfterTracing(std::move(aValue));
+  }
+
+  /** Used only for GC tracing. If used in Rooted<Result<...>>, E must have a
+   * GCPolicy for tracing it. */
+  constexpr void updateErrorAfterTracing(E&& aErrorValue) {
+    mImpl.updateErrorAfterTracing(std::move(aErrorValue));
+  }
+
+  /** See the success value from this Result, which must be a success result. */
+  constexpr decltype(auto) inspect() const {
+    static_assert(!std::is_reference_v<
+                      std::invoke_result_t<decltype(&Impl::inspect), Impl>> ||
+                  std::is_const_v<std::remove_reference_t<
+                      std::invoke_result_t<decltype(&Impl::inspect), Impl>>>);
+    MOZ_ASSERT(isOk());
+    return mImpl.inspect();
+  }
+
+  /** See the error value from this Result, which must be an error result. */
+  constexpr decltype(auto) inspectErr() const {
+    static_assert(
+        !std::is_reference_v<
+            std::invoke_result_t<decltype(&Impl::inspectErr), Impl>> ||
+        std::is_const_v<std::remove_reference_t<
+            std::invoke_result_t<decltype(&Impl::inspectErr), Impl>>>);
+    MOZ_ASSERT(isErr());
+    return mImpl.inspectErr();
+  }
+
+  /** Propagate the error value from this Result, which must be an error result.
+   *
+   * This can be used to propagate an error from a function call to the caller
+   * with a different value type, but the same error type:
+   *
+   *    Result<T1, E> Func1() {
+   *       Result<T2, E> res = Func2();
+   *       if (res.isErr()) { return res.propagateErr(); }
+   *    }
+   */
+  constexpr GenericErrorResult<E> propagateErr() {
+    MOZ_ASSERT(isErr());
+    return GenericErrorResult<E>{mImpl.unwrapErr(), ErrorPropagationTag{}};
+  }
+
+  /**
+   * Map a function V -> V2 over this result's success variant. If this result
+   * is an error, do not invoke the function and propagate the error.
+   *
+   * Mapping over success values invokes the function to produce a new success
+   * value:
+   *
+   *     // Map Result<int, E> to another Result<int, E>
+   *     Result<int, E> res(5);
+   *     Result<int, E> res2 = res.map([](int x) { return x * x; });
+   *     MOZ_ASSERT(res.isOk());
+   *     MOZ_ASSERT(res2.unwrap() == 25);
+   *
+   *     // Map Result<const char*, E> to Result<size_t, E>
+   *     Result<const char*, E> res("hello, map!");
+   *     Result<size_t, E> res2 = res.map(strlen);
+   *     MOZ_ASSERT(res.isOk());
+   *     MOZ_ASSERT(res2.unwrap() == 11);
+   *
+   * Mapping over an error does not invoke the function and propagates the
+   * error:
+   *
+   *     Result<V, int> res(5);
+   *     MOZ_ASSERT(res.isErr());
+   *     Result<V2, int> res2 = res.map([](V v) { ... });
+   *     MOZ_ASSERT(res2.isErr());
+   *     MOZ_ASSERT(res2.unwrapErr() == 5);
+   */
+  template <typename F>
+  constexpr auto map(F f) -> Result<std::invoke_result_t<F, V>, E> {
+    using RetResult = Result<std::invoke_result_t<F, V>, E>;
+    return MOZ_LIKELY(isOk()) ? RetResult(f(unwrap())) : RetResult(unwrapErr());
+  }
+
+  /**
+   * Map a function E -> E2 over this result's error variant. If this result is
+   * a success, do not invoke the function and move the success over.
+   *
+   * Mapping over error values invokes the function to produce a new error
+   * value:
+   *
+   *     // Map Result<V, int> to another Result<V, int>
+   *     Result<V, int> res(5);
+   *     Result<V, int> res2 = res.mapErr([](int x) { return x * x; });
+   *     MOZ_ASSERT(res2.isErr());
+   *     MOZ_ASSERT(res2.unwrapErr() == 25);
+   *
+   *     // Map Result<V, const char*> to Result<V, size_t>
+   *     Result<V, const char*> res("hello, mapErr!");
+   *     Result<V, size_t> res2 = res.mapErr(strlen);
+   *     MOZ_ASSERT(res2.isErr());
+   *     MOZ_ASSERT(res2.unwrapErr() == 14);
+   *
+   * Mapping over a success does not invoke the function and moves the success:
+   *
+   *     Result<int, E> res(5);
+   *     MOZ_ASSERT(res.isOk());
+   *     Result<int, E2> res2 = res.mapErr([](E e) { ... });
+   *     MOZ_ASSERT(res2.isOk());
+   *     MOZ_ASSERT(res2.unwrap() == 5);
+   */
+  template <typename F>
+  constexpr auto mapErr(F f) {
+    using RetResult = Result<V, std::invoke_result_t<F, E>>;
+    return MOZ_UNLIKELY(isErr()) ? RetResult(f(unwrapErr()))
+                                 : RetResult(unwrap());
+  }
+
+  /**
+   * Map a function E -> Result<V, E2> over this result's error variant. If
+   * this result is a success, do not invoke the function and move the success
+   * over.
+   *
+   * `orElse`ing over error values invokes the function to produce a new
+   * result:
+   *
+   *     // `orElse` Result<V, int> error variant to another Result<V, int>
+   *     // error variant or Result<V, int> success variant
+   *     auto orElse = [](int x) -> Result<V, int> {
+   *       if (x != 6) {
+   *         return Err(x * x);
+   *       }
+   *       return V(...);
+   *     };
+   *
+   *     Result<V, int> res(5);
+   *     auto res2 = res.orElse(orElse);
+   *     MOZ_ASSERT(res2.isErr());
+   *     MOZ_ASSERT(res2.unwrapErr() == 25);
+   *
+   *     Result<V, int> res3(6);
+   *     auto res4 = res3.orElse(orElse);
+   *     MOZ_ASSERT(res4.isOk());
+   *     MOZ_ASSERT(res4.unwrap() == ...);
+   *
+   *     // `orElse` Result<V, const char*> error variant to Result<V, size_t>
+   *     // error variant or Result<V, size_t> success variant
+   *     auto orElse = [](const char* s) -> Result<V, size_t> {
+   *       if (strcmp(s, "foo")) {
+   *         return Err(strlen(s));
+   *       }
+   *       return V(...);
+   *     };
+   *
+   *     Result<V, const char*> res("hello, orElse!");
+   *     auto res2 = res.orElse(orElse);
+   *     MOZ_ASSERT(res2.isErr());
+   *     MOZ_ASSERT(res2.unwrapErr() == 14);
+   *
+   *     Result<V, const char*> res3("foo");
+   *     auto res4 = ress.orElse(orElse);
+   *     MOZ_ASSERT(res4.isOk());
+   *     MOZ_ASSERT(res4.unwrap() == ...);
+   *
+   * `orElse`ing over a success does not invoke the function and moves the
+   * success:
+   *
+   *     Result<int, E> res(5);
+   *     MOZ_ASSERT(res.isOk());
+   *     Result<int, E2> res2 = res.orElse([](E e) { ... });
+   *     MOZ_ASSERT(res2.isOk());
+   *     MOZ_ASSERT(res2.unwrap() == 5);
+   */
+  template <typename F>
+  auto orElse(F f) -> Result<V, typename std::invoke_result_t<F, E>::err_type> {
+    return MOZ_UNLIKELY(isErr()) ? f(unwrapErr()) : unwrap();
+  }
+
+  /**
+   * Given a function V -> Result<V2, E>, apply it to this result's success
+   * value and return its result. If this result is an error value, it is
+   * propagated.
+   *
+   * This is sometimes called "flatMap" or ">>=" in other contexts.
+   *
+   * `andThen`ing over success values invokes the function to produce a new
+   * result:
+   *
+   *     Result<const char*, Error> res("hello, andThen!");
+   *     Result<HtmlFreeString, Error> res2 = res.andThen([](const char* s) {
+   *       return containsHtmlTag(s)
+   *         ? Result<HtmlFreeString, Error>(Error("Invalid: contains HTML"))
+   *         : Result<HtmlFreeString, Error>(HtmlFreeString(s));
+   *       }
+   *     });
+   *     MOZ_ASSERT(res2.isOk());
+   *     MOZ_ASSERT(res2.unwrap() == HtmlFreeString("hello, andThen!");
+   *
+   * `andThen`ing over error results does not invoke the function, and just
+   * propagates the error result:
+   *
+   *     Result<int, const char*> res("some error");
+   *     auto res2 = res.andThen([](int x) { ... });
+   *     MOZ_ASSERT(res2.isErr());
+   *     MOZ_ASSERT(res.unwrapErr() == res2.unwrapErr());
+   */
+  template <typename F, typename = std::enable_if_t<detail::IsResult<
+                            std::invoke_result_t<F, V&&>>::value>>
+  constexpr auto andThen(F f) -> std::invoke_result_t<F, V&&> {
+    return MOZ_LIKELY(isOk()) ? f(unwrap()) : propagateErr();
+  }
+};
+
+/**
+ * A type that auto-converts to an error Result. This is like a Result without
+ * a success type. It's the best return type for functions that always return
+ * an error--functions designed to build and populate error objects. It's also
+ * useful in error-handling macros; see MOZ_TRY for an example.
+ */
+template <typename E>
+class [[nodiscard]] GenericErrorResult {
+  E mErrorValue;
+
+  template <typename V, typename E2>
+  friend class Result;
+
+ public:
+  explicit constexpr GenericErrorResult(const E& aErrorValue)
+      : mErrorValue(aErrorValue) {}
+
+  explicit constexpr GenericErrorResult(E&& aErrorValue)
+      : mErrorValue(std::move(aErrorValue)) {}
+
+  constexpr GenericErrorResult(const E& aErrorValue, const ErrorPropagationTag&)
+      : GenericErrorResult(aErrorValue) {}
+
+  constexpr GenericErrorResult(E&& aErrorValue, const ErrorPropagationTag&)
+      : GenericErrorResult(std::move(aErrorValue)) {}
+};
+
+template <typename E>
+inline constexpr auto Err(E&& aErrorValue) {
+  return GenericErrorResult<std::decay_t<E>>(std::forward<E>(aErrorValue));
+}
+
+}  // namespace mozilla
+
+#endif  // mozilla_Result_h