mfbt/Result.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* A type suitable for returning either a value or an error from a function. */
   8
   9 #ifndef mozilla_Result_h
  10 #define mozilla_Result_h
  11
  12 #include <algorithm>
  13 #include <cstdint>
  14 #include <cstring>
  15 #include <type_traits>
  16 #include "mozilla/Assertions.h"
  17 #include "mozilla/Attributes.h"
  18 #include "mozilla/CompactPair.h"
  19 #include "mozilla/MaybeStorageBase.h"
  20
  21 namespace mozilla {
  22
  23 /**
  24  * Empty struct, indicating success for operations that have no return value.
  25  * For example, if you declare another empty struct `struct OutOfMemory {};`,
  26  * then `Result<Ok, OutOfMemory>` represents either success or OOM.
  27  */
  28 struct Ok {};
  29
  30 /**
  31  * A tag used to differentiate between GenericErrorResult created by the Err
  32  * function (completely new error) and GenericErrorResult created by the
  33  * Result::propagateErr function (propagated error). This can be used to track
  34  * error propagation and eventually produce error stacks for logging/debugging
  35  * purposes.
  36  */
  37 struct ErrorPropagationTag {};
  38
  39 template <typename E>
  40 class GenericErrorResult;
  41 template <typename V, typename E>
  42 class Result;
  43
  44 namespace detail {
  45
  46 enum class PackingStrategy {
  47   Variant,
  48   NullIsOk,
  49   LowBitTagIsError,
  50   PackedVariant,
  51 };
  52
  53 template <typename T>
  54 struct UnusedZero;
  55
  56 template <typename V, typename E, PackingStrategy Strategy>
  57 class ResultImplementation;
  58
  59 template <typename V>
  60 struct EmptyWrapper : V {
  61   constexpr EmptyWrapper() = default;
  62   explicit constexpr EmptyWrapper(const V&) {}
  63   explicit constexpr EmptyWrapper(std::in_place_t) {}
  64
  65   constexpr V* addr() { return this; }
  66   constexpr const V* addr() const { return this; }
  67 };
  68
  69 // The purpose of AlignedStorageOrEmpty is to make an empty class look like
  70 // std::aligned_storage_t for the purposes of the PackingStrategy::NullIsOk
  71 // specializations of ResultImplementation below. We can't use
  72 // std::aligned_storage_t itself with an empty class, since it would no longer
  73 // be empty.
  74 template <typename V>
  75 using AlignedStorageOrEmpty =
  76     std::conditional_t<std::is_empty_v<V>, EmptyWrapper<V>,
  77                        MaybeStorageBase<V>>;
  78
  79 template <typename V, typename E>
  80 class ResultImplementationNullIsOkBase {
  81  protected:
  82   using ErrorStorageType = typename UnusedZero<E>::StorageType;
  83
  84   static constexpr auto kNullValue = UnusedZero<E>::nullValue;
  85
  86   static_assert(std::is_trivially_copyable_v<ErrorStorageType>);
  87
  88   // XXX This can't be statically asserted in general, if ErrorStorageType is
  89   // not a basic type. With C++20 bit_cast, we could probably re-add such as
  90   // assertion. static_assert(kNullValue == decltype(kNullValue)(0));
  91
  92   CompactPair<AlignedStorageOrEmpty<V>, ErrorStorageType> mValue;
  93
  94  public:
  95   explicit constexpr ResultImplementationNullIsOkBase(const V& aSuccessValue)
  96       : mValue(aSuccessValue, kNullValue) {}
  97   explicit constexpr ResultImplementationNullIsOkBase(V&& aSuccessValue)
  98       : mValue(std::move(aSuccessValue), kNullValue) {}
  99   template <typename... Args>
 100   explicit constexpr ResultImplementationNullIsOkBase(std::in_place_t,
 101                                                       Args&&... aArgs)
 102       : mValue(std::piecewise_construct,
 103                std::tuple(std::in_place, std::forward<Args>(aArgs)...),
 104                std::tuple(kNullValue)) {}
 105   explicit constexpr ResultImplementationNullIsOkBase(E aErrorValue)
 106       : mValue(std::piecewise_construct, std::tuple<>(),
 107                std::tuple(UnusedZero<E>::Store(std::move(aErrorValue)))) {
 108     MOZ_ASSERT(mValue.second() != kNullValue);
 109   }
 110
 111   constexpr ResultImplementationNullIsOkBase(
 112       ResultImplementationNullIsOkBase&& aOther)
 113       : mValue(std::piecewise_construct, std::tuple<>(),
 114                std::tuple(aOther.mValue.second())) {
 115     if constexpr (!std::is_empty_v<V>) {
 116       if (isOk()) {
 117         new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
 118       }
 119     }
 120   }
 121   ResultImplementationNullIsOkBase& operator=(
 122       ResultImplementationNullIsOkBase&& aOther) {
 123     if constexpr (!std::is_empty_v<V>) {
 124       if (isOk()) {
 125         mValue.first().addr()->~V();
 126       }
 127     }
 128     mValue.second() = std::move(aOther.mValue.second());
 129     if constexpr (!std::is_empty_v<V>) {
 130       if (isOk()) {
 131         new (mValue.first().addr()) V(std::move(*aOther.mValue.first().addr()));
 132       }
 133     }
 134     return *this;
 135   }
 136
 137   constexpr bool isOk() const { return mValue.second() == kNullValue; }
 138
 139   constexpr const V& inspect() const { return *mValue.first().addr(); }
 140   constexpr V unwrap() { return std::move(*mValue.first().addr()); }
 141
 142   constexpr decltype(auto) inspectErr() const {
 143     return UnusedZero<E>::Inspect(mValue.second());
 144   }
 145   constexpr E unwrapErr() { return UnusedZero<E>::Unwrap(mValue.second()); }
 146 };
 147
 148 template <typename V, typename E,
 149           bool IsVTriviallyDestructible = std::is_trivially_destructible_v<V>>
 150 class ResultImplementationNullIsOk;
 151
 152 template <typename V, typename E>
 153 class ResultImplementationNullIsOk<V, E, true>
 154     : public ResultImplementationNullIsOkBase<V, E> {
 155  public:
 156   using ResultImplementationNullIsOkBase<V,
 157                                          E>::ResultImplementationNullIsOkBase;
 158 };
 159
 160 template <typename V, typename E>
 161 class ResultImplementationNullIsOk<V, E, false>
 162     : public ResultImplementationNullIsOkBase<V, E> {
 163  public:
 164   using ResultImplementationNullIsOkBase<V,
 165                                          E>::ResultImplementationNullIsOkBase;
 166
 167   ResultImplementationNullIsOk(ResultImplementationNullIsOk&&) = default;
 168   ResultImplementationNullIsOk& operator=(ResultImplementationNullIsOk&&) =
 169       default;
 170
 171   ~ResultImplementationNullIsOk() {
 172     if (this->isOk()) {
 173       this->mValue.first().addr()->~V();
 174     }
 175   }
 176 };
 177
 178 /**
 179  * Specialization for when the success type is default-constructible and the
 180  * error type is a value type which can never have the value 0 (as determined by
 181  * UnusedZero<>).
 182  */
 183 template <typename V, typename E>
 184 class ResultImplementation<V, E, PackingStrategy::NullIsOk>
 185     : public ResultImplementationNullIsOk<V, E> {
 186  public:
 187   using ResultImplementationNullIsOk<V, E>::ResultImplementationNullIsOk;
 188 };
 189
 190 template <size_t S>
 191 using UnsignedIntType = std::conditional_t<
 192     S == 1, std::uint8_t,
 193     std::conditional_t<
 194         S == 2, std::uint16_t,
 195         std::conditional_t<S == 3 || S == 4, std::uint32_t,
 196                            std::conditional_t<S <= 8, std::uint64_t, void>>>>;
 197
 198 /**
 199  * Specialization for when alignment permits using the least significant bit
 200  * as a tag bit.
 201  */
 202 template <typename V, typename E>
 203 class ResultImplementation<V, E, PackingStrategy::LowBitTagIsError> {
 204   static_assert(std::is_trivially_copyable_v<V> &&
 205                 std::is_trivially_destructible_v<V>);
 206   static_assert(std::is_trivially_copyable_v<E> &&
 207                 std::is_trivially_destructible_v<E>);
 208
 209   static constexpr size_t kRequiredSize = std::max(sizeof(V), sizeof(E));
 210
 211   using StorageType = UnsignedIntType<kRequiredSize>;
 212
 213 #if defined(__clang__)
 214   alignas(std::max(alignof(V), alignof(E))) StorageType mBits;
 215 #else
 216   // Some gcc versions choke on using std::max with alignas, see
 217   // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929 (and this seems to have
 218   // regressed in some gcc 9.x version before being fixed again) Keeping the
 219   // code above since we would eventually drop this when we no longer support
 220   // gcc versions with the bug.
 221   alignas(alignof(V) > alignof(E) ? alignof(V) : alignof(E)) StorageType mBits;
 222 #endif
 223
 224  public:
 225   explicit constexpr ResultImplementation(V aValue) : mBits(0) {
 226     if constexpr (!std::is_empty_v<V>) {
 227       std::memcpy(&mBits, &aValue, sizeof(V));
 228       MOZ_ASSERT((mBits & 1) == 0);
 229     } else {
 230       (void)aValue;
 231     }
 232   }
 233   explicit constexpr ResultImplementation(E aErrorValue) : mBits(1) {
 234     if constexpr (!std::is_empty_v<E>) {
 235       std::memcpy(&mBits, &aErrorValue, sizeof(E));
 236       MOZ_ASSERT((mBits & 1) == 0);
 237       mBits |= 1;
 238     } else {
 239       (void)aErrorValue;
 240     }
 241   }
 242
 243   constexpr bool isOk() const { return (mBits & 1) == 0; }
 244
 245   constexpr V inspect() const {
 246     V res;
 247     std::memcpy(&res, &mBits, sizeof(V));
 248     return res;
 249   }
 250   constexpr V unwrap() { return inspect(); }
 251
 252   constexpr E inspectErr() const {
 253     const auto bits = mBits ^ 1;
 254     E res;
 255     std::memcpy(&res, &bits, sizeof(E));
 256     return res;
 257   }
 258   constexpr E unwrapErr() { return inspectErr(); }
 259 };
 260
 261 // Return true if any of the struct can fit in a word.
 262 template <typename V, typename E>
 263 struct IsPackableVariant {
 264   struct VEbool {
 265     V v;
 266     E e;
 267     bool ok;
 268   };
 269   struct EVbool {
 270     E e;
 271     V v;
 272     bool ok;
 273   };
 274
 275   using Impl =
 276       std::conditional_t<sizeof(VEbool) <= sizeof(EVbool), VEbool, EVbool>;
 277
 278   static const bool value = sizeof(Impl) <= sizeof(uintptr_t);
 279 };
 280
 281 /**
 282  * Specialization for when both type are not using all the bytes, in order to
 283  * use one byte as a tag.
 284  */
 285 template <typename V, typename E>
 286 class ResultImplementation<V, E, PackingStrategy::PackedVariant> {
 287   using Impl = typename IsPackableVariant<V, E>::Impl;
 288   Impl data;
 289
 290  public:
 291   explicit constexpr ResultImplementation(V aValue) {
 292     data.v = std::move(aValue);
 293     data.ok = true;
 294   }
 295   explicit constexpr ResultImplementation(E aErrorValue) {
 296     data.e = std::move(aErrorValue);
 297     data.ok = false;
 298   }
 299
 300   constexpr bool isOk() const { return data.ok; }
 301
 302   constexpr const V& inspect() const { return data.v; }
 303   constexpr V unwrap() { return std::move(data.v); }
 304
 305   constexpr const E& inspectErr() const { return data.e; }
 306   constexpr E unwrapErr() { return std::move(data.e); }
 307 };
 308
 309 // To use nullptr as a special value, we need the counter part to exclude zero
 310 // from its range of valid representations.
 311 //
 312 // By default assume that zero can be represented.
 313 template <typename T>
 314 struct UnusedZero {
 315   static const bool value = false;
 316 };
 317
 318 // This template can be used as a helper for specializing UnusedZero for scoped
 319 // enum types which never use 0 as an error value, e.g.
 320 //
 321 // namespace mozilla::detail {
 322 //
 323 // template <>
 324 // struct UnusedZero<MyEnumType> : UnusedZeroEnum<MyEnumType> {};
 325 //
 326 // }  // namespace mozilla::detail
 327 //
 328 template <typename T>
 329 struct UnusedZeroEnum {
 330   using StorageType = std::underlying_type_t<T>;
 331
 332   static constexpr bool value = true;
 333   static constexpr StorageType nullValue = 0;
 334
 335   static constexpr T Inspect(const StorageType& aValue) {
 336     return static_cast<T>(aValue);
 337   }
 338   static constexpr T Unwrap(StorageType aValue) {
 339     return static_cast<T>(aValue);
 340   }
 341   static constexpr StorageType Store(T aValue) {
 342     return static_cast<StorageType>(aValue);
 343   }
 344 };
 345
 346 // A bit of help figuring out which of the above specializations to use.
 347 //
 348 // We begin by safely assuming types don't have a spare bit, unless they are
 349 // empty.
 350 template <typename T>
 351 struct HasFreeLSB {
 352   static const bool value = std::is_empty_v<T>;
 353 };
 354
 355 // As an incomplete type, void* does not have a spare bit.
 356 template <>
 357 struct HasFreeLSB<void*> {
 358   static const bool value = false;
 359 };
 360
 361 // The lowest bit of a properly-aligned pointer is always zero if the pointee
 362 // type is greater than byte-aligned. That bit is free to use if it's masked
 363 // out of such pointers before they're dereferenced.
 364 template <typename T>
 365 struct HasFreeLSB<T*> {
 366   static const bool value = (alignof(T) & 1) == 0;
 367 };
 368
 369 // Select one of the previous result implementation based on the properties of
 370 // the V and E types.
 371 template <typename V, typename E>
 372 struct SelectResultImpl {
 373   static const PackingStrategy value =
 374       (HasFreeLSB<V>::value && HasFreeLSB<E>::value)
 375           ? PackingStrategy::LowBitTagIsError
 376       : (UnusedZero<E>::value && sizeof(E) <= sizeof(uintptr_t))
 377           ? PackingStrategy::NullIsOk
 378       : (std::is_default_constructible_v<V> &&
 379          std::is_default_constructible_v<E> && IsPackableVariant<V, E>::value)
 380           ? PackingStrategy::PackedVariant
 381           : PackingStrategy::Variant;
 382
 383   using Type = ResultImplementation<V, E, value>;
 384 };
 385
 386 template <typename T>
 387 struct IsResult : std::false_type {};
 388
 389 template <typename V, typename E>
 390 struct IsResult<Result<V, E>> : std::true_type {};
 391
 392 }  // namespace detail
 393
 394 template <typename V, typename E>
 395 constexpr auto ToResult(Result<V, E>&& aValue)
 396     -> decltype(std::forward<Result<V, E>>(aValue)) {
 397   return std::forward<Result<V, E>>(aValue);
 398 }
 399
 400 /**
 401  * Result<V, E> represents the outcome of an operation that can either succeed
 402  * or fail. It contains either a success value of type V or an error value of
 403  * type E.
 404  *
 405  * All Result methods are const, so results are basically immutable.
 406  * This is just like Variant<V, E> but with a slightly different API, and the
 407  * following cases are optimized so Result can be stored more efficiently:
 408  *
 409  * - If both the success and error types do not use their least significant bit,
 410  * are trivially copyable and destructible, Result<V, E> is guaranteed to be as
 411  * large as the larger type. This is determined via the HasFreeLSB trait. By
 412  * default, empty classes (in particular Ok) and aligned pointer types are
 413  * assumed to have a free LSB, but you can specialize this trait for other
 414  * types. If the success type is empty, the representation is guaranteed to be
 415  * all zero bits on success. Do not change this representation! There is JIT
 416  * code that depends on it. (Implementation note: The lowest bit is used as a
 417  * tag bit: 0 to indicate the Result's bits are a success value, 1 to indicate
 418  * the Result's bits (with the 1 masked out) encode an error value)
 419  *
 420  * - Else, if the error type can't have a all-zero bits representation and is
 421  * not larger than a pointer, a CompactPair is used to represent this rather
 422  * than a Variant. This has shown to be better optimizable, and the template
 423  * code is much simpler than that of Variant, so it should also compile faster.
 424  * Whether an error type can't be all-zero bits, is determined via the
 425  * UnusedZero trait. MFBT doesn't declare any public type UnusedZero, but
 426  * nsresult is declared UnusedZero in XPCOM.
 427  *
 428  * The purpose of Result is to reduce the screwups caused by using `false` or
 429  * `nullptr` to indicate errors.
 430  * What screwups? See <https://bugzilla.mozilla.org/show_bug.cgi?id=912928> for
 431  * a partial list.
 432  *
 433  * Result<const V, E> or Result<V, const E> are not meaningful. The success or
 434  * error values in a Result instance are non-modifiable in-place anyway. This
 435  * guarantee must also be maintained when evolving Result. They can be
 436  * unwrap()ped, but this loses const qualification. However, Result<const V, E>
 437  * or Result<V, const E> may be misleading and prevent movability. Just use
 438  * Result<V, E>. (Result<const V*, E> may make sense though, just Result<const
 439  * V* const, E> is not possible.)
 440  */
 441 template <typename V, typename E>
 442 class [[nodiscard]] Result final {
 443   // See class comment on Result<const V, E> and Result<V, const E>.
 444   static_assert(!std::is_const_v<V>);
 445   static_assert(!std::is_const_v<E>);
 446   static_assert(!std::is_reference_v<V>);
 447   static_assert(!std::is_reference_v<E>);
 448
 449   using Impl = typename detail::SelectResultImpl<V, E>::Type;
 450
 451   Impl mImpl;
 452
 453  public:
 454   using ok_type = V;
 455   using err_type = E;
 456
 457   /** Create a success result. */
 458   MOZ_IMPLICIT constexpr Result(V&& aValue) : mImpl(std::forward<V>(aValue)) {
 459     MOZ_ASSERT(isOk());
 460   }
 461
 462   /** Create a success result. */
 463   MOZ_IMPLICIT constexpr Result(const V& aValue) : mImpl(aValue) {
 464     MOZ_ASSERT(isOk());
 465   }
 466
 467   /** Create a success result in-place. */
 468   template <typename... Args>
 469   explicit constexpr Result(std::in_place_t, Args&&... aArgs)
 470       : mImpl(std::in_place, std::forward<Args>(aArgs)...) {
 471     MOZ_ASSERT(isOk());
 472   }
 473
 474   /** Create an error result. */
 475   explicit constexpr Result(E aErrorValue) : mImpl(std::move(aErrorValue)) {
 476     MOZ_ASSERT(isErr());
 477   }
 478
 479   /**
 480    * Create a (success/error) result from another (success/error) result with a
 481    * different but convertible error type. */
 482   template <typename E2,
 483             typename = std::enable_if_t<std::is_convertible_v<E2, E>>>
 484   MOZ_IMPLICIT constexpr Result(Result<V, E2>&& aOther)
 485       : mImpl(aOther.isOk() ? Impl{aOther.unwrap()}
 486                             : Impl{aOther.unwrapErr()}) {}
 487
 488   /**
 489    * Implementation detail of MOZ_TRY().
 490    * Create an error result from another error result.
 491    */
 492   template <typename E2>
 493   MOZ_IMPLICIT constexpr Result(GenericErrorResult<E2>&& aErrorResult)
 494       : mImpl(std::move(aErrorResult.mErrorValue)) {
 495     static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
 496     MOZ_ASSERT(isErr());
 497   }
 498
 499   /**
 500    * Implementation detail of MOZ_TRY().
 501    * Create an error result from another error result.
 502    */
 503   template <typename E2>
 504   MOZ_IMPLICIT constexpr Result(const GenericErrorResult<E2>& aErrorResult)
 505       : mImpl(aErrorResult.mErrorValue) {
 506     static_assert(std::is_convertible_v<E2, E>, "E2 must be convertible to E");
 507     MOZ_ASSERT(isErr());
 508   }
 509
 510   Result(const Result&) = delete;
 511   Result(Result&&) = default;
 512   Result& operator=(const Result&) = delete;
 513   Result& operator=(Result&&) = default;
 514
 515   /** True if this Result is a success result. */
 516   constexpr bool isOk() const { return mImpl.isOk(); }
 517
 518   /** True if this Result is an error result. */
 519   constexpr bool isErr() const { return !mImpl.isOk(); }
 520
 521   /** Take the success value from this Result, which must be a success result.
 522    */
 523   constexpr V unwrap() {
 524     MOZ_ASSERT(isOk());
 525     return mImpl.unwrap();
 526   }
 527
 528   /**
 529    * Take the success value from this Result, which must be a success result.
 530    * If it is an error result, then return the aValue.
 531    */
 532   constexpr V unwrapOr(V aValue) {
 533     return MOZ_LIKELY(isOk()) ? mImpl.unwrap() : std::move(aValue);
 534   }
 535
 536   /** Take the error value from this Result, which must be an error result. */
 537   constexpr E unwrapErr() {
 538     MOZ_ASSERT(isErr());
 539     return mImpl.unwrapErr();
 540   }
 541
 542   /** See the success value from this Result, which must be a success result. */
 543   constexpr decltype(auto) inspect() const {
 544     static_assert(!std::is_reference_v<
 545                       std::invoke_result_t<decltype(&Impl::inspect), Impl>> ||
 546                   std::is_const_v<std::remove_reference_t<
 547                       std::invoke_result_t<decltype(&Impl::inspect), Impl>>>);
 548     MOZ_ASSERT(isOk());
 549     return mImpl.inspect();
 550   }
 551
 552   /** See the error value from this Result, which must be an error result. */
 553   constexpr decltype(auto) inspectErr() const {
 554     static_assert(
 555         !std::is_reference_v<
 556             std::invoke_result_t<decltype(&Impl::inspectErr), Impl>> ||
 557         std::is_const_v<std::remove_reference_t<
 558             std::invoke_result_t<decltype(&Impl::inspectErr), Impl>>>);
 559     MOZ_ASSERT(isErr());
 560     return mImpl.inspectErr();
 561   }
 562
 563   /** Propagate the error value from this Result, which must be an error result.
 564    *
 565    * This can be used to propagate an error from a function call to the caller
 566    * with a different value type, but the same error type:
 567    *
 568    *    Result<T1, E> Func1() {
 569    *       Result<T2, E> res = Func2();
 570    *       if (res.isErr()) { return res.propagateErr(); }
 571    *    }
 572    */
 573   constexpr GenericErrorResult<E> propagateErr() {
 574     MOZ_ASSERT(isErr());
 575     return GenericErrorResult<E>{mImpl.unwrapErr(), ErrorPropagationTag{}};
 576   }
 577
 578   /**
 579    * Map a function V -> V2 over this result's success variant. If this result
 580    * is an error, do not invoke the function and propagate the error.
 581    *
 582    * Mapping over success values invokes the function to produce a new success
 583    * value:
 584    *
 585    *     // Map Result<int, E> to another Result<int, E>
 586    *     Result<int, E> res(5);
 587    *     Result<int, E> res2 = res.map([](int x) { return x * x; });
 588    *     MOZ_ASSERT(res.isOk());
 589    *     MOZ_ASSERT(res2.unwrap() == 25);
 590    *
 591    *     // Map Result<const char*, E> to Result<size_t, E>
 592    *     Result<const char*, E> res("hello, map!");
 593    *     Result<size_t, E> res2 = res.map(strlen);
 594    *     MOZ_ASSERT(res.isOk());
 595    *     MOZ_ASSERT(res2.unwrap() == 11);
 596    *
 597    * Mapping over an error does not invoke the function and propagates the
 598    * error:
 599    *
 600    *     Result<V, int> res(5);
 601    *     MOZ_ASSERT(res.isErr());
 602    *     Result<V2, int> res2 = res.map([](V v) { ... });
 603    *     MOZ_ASSERT(res2.isErr());
 604    *     MOZ_ASSERT(res2.unwrapErr() == 5);
 605    */
 606   template <typename F>
 607   constexpr auto map(F f) -> Result<std::result_of_t<F(V)>, E> {
 608     using RetResult = Result<std::result_of_t<F(V)>, E>;
 609     return MOZ_LIKELY(isOk()) ? RetResult(f(unwrap())) : RetResult(unwrapErr());
 610   }
 611
 612   /**
 613    * Map a function E -> E2 over this result's error variant. If this result is
 614    * a success, do not invoke the function and move the success over.
 615    *
 616    * Mapping over error values invokes the function to produce a new error
 617    * value:
 618    *
 619    *     // Map Result<V, int> to another Result<V, int>
 620    *     Result<V, int> res(5);
 621    *     Result<V, int> res2 = res.mapErr([](int x) { return x * x; });
 622    *     MOZ_ASSERT(res2.isErr());
 623    *     MOZ_ASSERT(res2.unwrapErr() == 25);
 624    *
 625    *     // Map Result<V, const char*> to Result<V, size_t>
 626    *     Result<V, const char*> res("hello, mapErr!");
 627    *     Result<V, size_t> res2 = res.mapErr(strlen);
 628    *     MOZ_ASSERT(res2.isErr());
 629    *     MOZ_ASSERT(res2.unwrapErr() == 14);
 630    *
 631    * Mapping over a success does not invoke the function and moves the success:
 632    *
 633    *     Result<int, E> res(5);
 634    *     MOZ_ASSERT(res.isOk());
 635    *     Result<int, E2> res2 = res.mapErr([](E e) { ... });
 636    *     MOZ_ASSERT(res2.isOk());
 637    *     MOZ_ASSERT(res2.unwrap() == 5);
 638    */
 639   template <typename F>
 640   constexpr auto mapErr(F f) {
 641     using RetResult = Result<V, std::result_of_t<F(E)>>;
 642     return MOZ_UNLIKELY(isErr()) ? RetResult(f(unwrapErr()))
 643                                  : RetResult(unwrap());
 644   }
 645
 646   /**
 647    * Map a function E -> Result<V, E2> over this result's error variant. If
 648    * this result is a success, do not invoke the function and move the success
 649    * over.
 650    *
 651    * `orElse`ing over error values invokes the function to produce a new
 652    * result:
 653    *
 654    *     // `orElse` Result<V, int> error variant to another Result<V, int>
 655    *     // error variant or Result<V, int> success variant
 656    *     auto orElse = [](int x) -> Result<V, int> {
 657    *       if (x != 6) {
 658    *         return Err(x * x);
 659    *       }
 660    *       return V(...);
 661    *     };
 662    *
 663    *     Result<V, int> res(5);
 664    *     auto res2 = res.orElse(orElse);
 665    *     MOZ_ASSERT(res2.isErr());
 666    *     MOZ_ASSERT(res2.unwrapErr() == 25);
 667    *
 668    *     Result<V, int> res3(6);
 669    *     auto res4 = res3.orElse(orElse);
 670    *     MOZ_ASSERT(res4.isOk());
 671    *     MOZ_ASSERT(res4.unwrap() == ...);
 672    *
 673    *     // `orElse` Result<V, const char*> error variant to Result<V, size_t>
 674    *     // error variant or Result<V, size_t> success variant
 675    *     auto orElse = [](const char* s) -> Result<V, size_t> {
 676    *       if (strcmp(s, "foo")) {
 677    *         return Err(strlen(s));
 678    *       }
 679    *       return V(...);
 680    *     };
 681    *
 682    *     Result<V, const char*> res("hello, orElse!");
 683    *     auto res2 = res.orElse(orElse);
 684    *     MOZ_ASSERT(res2.isErr());
 685    *     MOZ_ASSERT(res2.unwrapErr() == 14);
 686    *
 687    *     Result<V, const char*> res3("foo");
 688    *     auto res4 = ress.orElse(orElse);
 689    *     MOZ_ASSERT(res4.isOk());
 690    *     MOZ_ASSERT(res4.unwrap() == ...);
 691    *
 692    * `orElse`ing over a success does not invoke the function and moves the
 693    * success:
 694    *
 695    *     Result<int, E> res(5);
 696    *     MOZ_ASSERT(res.isOk());
 697    *     Result<int, E2> res2 = res.orElse([](E e) { ... });
 698    *     MOZ_ASSERT(res2.isOk());
 699    *     MOZ_ASSERT(res2.unwrap() == 5);
 700    */
 701   template <typename F>
 702   auto orElse(F f) -> Result<V, typename std::result_of_t<F(E)>::err_type> {
 703     return MOZ_UNLIKELY(isErr()) ? f(unwrapErr()) : unwrap();
 704   }
 705
 706   /**
 707    * Given a function V -> Result<V2, E>, apply it to this result's success
 708    * value and return its result. If this result is an error value, it is
 709    * propagated.
 710    *
 711    * This is sometimes called "flatMap" or ">>=" in other contexts.
 712    *
 713    * `andThen`ing over success values invokes the function to produce a new
 714    * result:
 715    *
 716    *     Result<const char*, Error> res("hello, andThen!");
 717    *     Result<HtmlFreeString, Error> res2 = res.andThen([](const char* s) {
 718    *       return containsHtmlTag(s)
 719    *         ? Result<HtmlFreeString, Error>(Error("Invalid: contains HTML"))
 720    *         : Result<HtmlFreeString, Error>(HtmlFreeString(s));
 721    *       }
 722    *     });
 723    *     MOZ_ASSERT(res2.isOk());
 724    *     MOZ_ASSERT(res2.unwrap() == HtmlFreeString("hello, andThen!");
 725    *
 726    * `andThen`ing over error results does not invoke the function, and just
 727    * propagates the error result:
 728    *
 729    *     Result<int, const char*> res("some error");
 730    *     auto res2 = res.andThen([](int x) { ... });
 731    *     MOZ_ASSERT(res2.isErr());
 732    *     MOZ_ASSERT(res.unwrapErr() == res2.unwrapErr());
 733    */
 734   template <typename F, typename = std::enable_if_t<detail::IsResult<
 735                             std::invoke_result_t<F, V&&>>::value>>
 736   constexpr auto andThen(F f) -> std::invoke_result_t<F, V&&> {
 737     return MOZ_LIKELY(isOk()) ? f(unwrap()) : propagateErr();
 738   }
 739 };
 740
 741 /**
 742  * A type that auto-converts to an error Result. This is like a Result without
 743  * a success type. It's the best return type for functions that always return
 744  * an error--functions designed to build and populate error objects. It's also
 745  * useful in error-handling macros; see MOZ_TRY for an example.
 746  */
 747 template <typename E>
 748 class [[nodiscard]] GenericErrorResult {
 749   E mErrorValue;
 750
 751   template <typename V, typename E2>
 752   friend class Result;
 753
 754  public:
 755   explicit constexpr GenericErrorResult(const E& aErrorValue)
 756       : mErrorValue(aErrorValue) {}
 757
 758   explicit constexpr GenericErrorResult(E&& aErrorValue)
 759       : mErrorValue(std::move(aErrorValue)) {}
 760
 761   constexpr GenericErrorResult(const E& aErrorValue, const ErrorPropagationTag&)
 762       : GenericErrorResult(aErrorValue) {}
 763
 764   constexpr GenericErrorResult(E&& aErrorValue, const ErrorPropagationTag&)
 765       : GenericErrorResult(std::move(aErrorValue)) {}
 766 };
 767
 768 template <typename E>
 769 inline constexpr auto Err(E&& aErrorValue) {
 770   return GenericErrorResult<std::decay_t<E>>(std::forward<E>(aErrorValue));
 771 }
 772
 773 }  // namespace mozilla
 774
 775 /**
 776  * MOZ_TRY(expr) is the C++ equivalent of Rust's `try!(expr);`. First, it
 777  * evaluates expr, which must produce a Result value. On success, it
 778  * discards the result altogether. On error, it immediately returns an error
 779  * Result from the enclosing function.
 780  */
 781 #define MOZ_TRY(expr)                                   \
 782   do {                                                  \
 783     auto mozTryTempResult_ = ::mozilla::ToResult(expr); \
 784     if (MOZ_UNLIKELY(mozTryTempResult_.isErr())) {      \
 785       return mozTryTempResult_.propagateErr();          \
 786     }                                                   \
 787   } while (0)
 788
 789 /**
 790  * MOZ_TRY_VAR(target, expr) is the C++ equivalent of Rust's `target =
 791  * try!(expr);`. First, it evaluates expr, which must produce a Result value. On
 792  * success, the result's success value is assigned to target. On error,
 793  * immediately returns the error result. |target| must be an lvalue.
 794  */
 795 #define MOZ_TRY_VAR(target, expr)                     \
 796   do {                                                \
 797     auto mozTryVarTempResult_ = (expr);               \
 798     if (MOZ_UNLIKELY(mozTryVarTempResult_.isErr())) { \
 799       return mozTryVarTempResult_.propagateErr();     \
 800     }                                                 \
 801     (target) = mozTryVarTempResult_.unwrap();         \
 802   } while (0)
 803
 804 #endif  // mozilla_Result_h