wasmtime.hh

/**
 * \mainpage
 *
 * This project is a C++ API for
 * [Wasmtime](https://github.com/bytecodealliance/wasmtime). Support for the
 * C++ API is exclusively built on the [C API of
 * Wasmtime](https://docs.wasmtime.dev/c-api/), so the C++ support for this is
 * simply a single header file. To use this header file, though, it must be
 * combined with the header and binary of Wasmtime's C API. Note, though, that
 * while this header is built on top of the `wasmtime.h` header file you should
 * only need to use the contents of this header file to interact with Wasmtime.
 *
 * Examples can be [found
 * online](https://github.com/bytecodealliance/wasmtime-cpp/tree/main/examples)
 * and otherwise be sure to check out the
 * [README](https://github.com/bytecodealliance/wasmtime-cpp/blob/main/README.md)
 * for simple usage instructions. Otherwise you can dive right in to the
 * reference documentation of \ref wasmtime.hh
 *
 * \example hello.cc
 * \example gcd.cc
 * \example linking.cc
 * \example memory.cc
 * \example interrupt.cc
 * \example externref.cc
 */

/**
 * \file wasmtime.hh
 */

#ifndef WASMTIME_HH
#define WASMTIME_HH

#include <any>
#include <array>
#include <cstdio>
#include <initializer_list>
#include <iosfwd>
#include <limits>
#include <memory>
#include <optional>
#include <ostream>
#include <variant>
#include <vector>
#ifdef __has_include
#if __has_include(<span>)
#include <span>
#endif
#endif

#include "wasmtime.h"

namespace wasmtime {

#ifdef __cpp_lib_span

/// \brief Alias to C++20 std::span when it is available
template <typename T, std::size_t Extent = std::dynamic_extent>
using Span = std::span<T, Extent>;

#else

/// \brief Means number of elements determined at runtime
inline constexpr size_t dynamic_extent =
    std::numeric_limits<std::size_t>::max();

/**
 * \brief Span class used when c++20 is not available
 * @tparam T Type of data
 * @tparam Extent Static size of data referred by Span class
 */
template <typename T, std::size_t Extent = dynamic_extent> class Span;

/// \brief Check whether a type is `Span`
template <typename T> struct IsSpan : std::false_type {};

template <typename T, std::size_t Extent>
struct IsSpan<Span<T, Extent>> : std::true_type {};

template <typename T, std::size_t Extent> class Span {
  static_assert(Extent == dynamic_extent,
                "The current implementation supports dynamic-extent span only");

public:
  /// \brief Type used to iterate over this span (a raw pointer)
  using iterator = T *;

  /// \brief Constructor of Span class
  Span(T *t, std::size_t n) : ptr_{t}, size_{n} {}

  /// \brief Constructor of Span class for containers
  template <typename C,
            std::enable_if_t<
                !IsSpan<C>::value &&
                    std::is_pointer_v<decltype(std::declval<C &>().data())> &&
                    std::is_convertible_v<
                        std::remove_pointer_t<
                            decltype(std::declval<C &>().data())> (*)[],
                        T (*)[]> &&
                    std::is_convertible_v<decltype(std::declval<C>().size()),
                                          std::size_t>,
                int> = 0>
  Span(C &range) : ptr_{range.data()}, size_{range.size()} {}

  /// \brief Returns item by index
  T &operator[](ptrdiff_t idx) const {
    return ptr_[idx]; // NOLINT
  }

  /// \brief Returns pointer to data
  T *data() const { return ptr_; }

  /// \brief Returns number of data that referred by Span class
  std::size_t size() const { return size_; }

  /// \brief Returns begin iterator
  iterator begin() const { return ptr_; }

  /// \brief Returns end iterator
  iterator end() const {
    return ptr_ + size_; // NOLINT
  }

  /// \brief Returns size in bytes
  std::size_t size_bytes() const { return sizeof(T) * size_; }

private:
  T *ptr_;
  std::size_t size_;
};

#endif

class Trace;

/**
 * \brief Errors coming from Wasmtime
 *
 * This class represents an error that came from Wasmtime and contains a textual
 * description of the error that occurred.
 */
class Error {
  struct deleter {
    void operator()(wasmtime_error_t *p) const { wasmtime_error_delete(p); }
  };

  std::unique_ptr<wasmtime_error_t, deleter> ptr;

public:
  /// \brief Creates an error from the raw C API representation
  ///
  /// Takes ownership of the provided `error`.
  Error(wasmtime_error_t *error) : ptr(error) {}

  /// \brief Returns the error message associated with this error.
  std::string message() const {
    wasm_byte_vec_t msg_bytes;
    wasmtime_error_message(ptr.get(), &msg_bytes);
    auto ret = std::string(msg_bytes.data, msg_bytes.size);
    wasm_byte_vec_delete(&msg_bytes);
    return ret;
  }

  /// If this trap represents a call to `exit` for WASI, this will return the
  /// optional error code associated with the exit trap.
  std::optional<int32_t> i32_exit() const {
    int32_t status = 0;
    if (wasmtime_error_exit_status(ptr.get(), &status)) {
      return status;
    }
    return std::nullopt;
  }

  /// Returns the trace of WebAssembly frames associated with this error.
  ///
  /// Note that the `trace` cannot outlive this error object.
  Trace trace() const;
};

/// \brief Used to print an error.
inline std::ostream &operator<<(std::ostream &os, const Error &e) {
  os << e.message();
  return os;
}

/**
 * \brief Fallible result type used for Wasmtime.
 *
 * This type is used as the return value of many methods in the Wasmtime API.
 * This behaves similarly to Rust's `Result<T, E>` and will be replaced with a
 * C++ standard when it exists.
 */
template <typename T, typename E = Error> class [[nodiscard]] Result {
  std::variant<T, E> data;

public:
  /// \brief Creates a `Result` from its successful value.
  Result(T t) : data(std::move(t)) {}
  /// \brief Creates a `Result` from an error value.
  Result(E e) : data(std::move(e)) {}

  /// \brief Returns `true` if this result is a success, `false` if it's an
  /// error
  explicit operator bool() const { return data.index() == 0; }

  /// \brief Returns the error, if present, aborts if this is not an error.
  E &&err() { return std::get<E>(std::move(data)); }
  /// \brief Returns the error, if present, aborts if this is not an error.
  const E &&err() const { return std::get<E>(std::move(data)); }

  /// \brief Returns the success, if present, aborts if this is an error.
  T &&ok() { return std::get<T>(std::move(data)); }
  /// \brief Returns the success, if present, aborts if this is an error.
  const T &&ok() const { return std::get<T>(std::move(data)); }

  /// \brief Returns the success, if present, aborts if this is an error.
  T unwrap() {
    if (*this) {
      return this->ok();
    }
    unwrap_failed();
  }

private:
  [[noreturn]] void unwrap_failed() {
    fprintf(stderr, "error: %s\n", this->err().message().c_str()); // NOLINT
    std::abort();
  }
};

/// \brief Strategies passed to `Config::strategy`
enum class Strategy {
  /// Automatically selects the compilation strategy
  Auto = WASMTIME_STRATEGY_AUTO,
  /// Requires Cranelift to be used for compilation
  Cranelift = WASMTIME_STRATEGY_CRANELIFT,
};

/// \brief Values passed to `Config::cranelift_opt_level`
enum class OptLevel {
  /// No extra optimizations performed
  None = WASMTIME_OPT_LEVEL_NONE,
  /// Optimize for speed
  Speed = WASMTIME_OPT_LEVEL_SPEED,
  /// Optimize for speed and generated code size
  SpeedAndSize = WASMTIME_OPT_LEVEL_SPEED_AND_SIZE,
};

/// \brief Values passed to `Config::profiler`
enum class ProfilingStrategy {
  /// No profiling enabled
  None = WASMTIME_PROFILING_STRATEGY_NONE,
  /// Profiling hooks via perf's jitdump
  Jitdump = WASMTIME_PROFILING_STRATEGY_JITDUMP,
  /// Profiling hooks via VTune
  Vtune = WASMTIME_PROFILING_STRATEGY_VTUNE,
};

/**
 * \brief Configuration for Wasmtime.
 *
 * This class is used to configure Wasmtime's compilation and various other
 * settings such as enabled WebAssembly proposals.
 *
 * For more information be sure to consult the [rust
 * documentation](https://docs.wasmtime.dev/api/wasmtime/struct.Config.html).
 */
class Config {
  friend class Engine;

  struct deleter {
    void operator()(wasm_config_t *p) const { wasm_config_delete(p); }
  };

  std::unique_ptr<wasm_config_t, deleter> ptr;

public:
  /// \brief Creates configuration with all the default settings.
  Config() : ptr(wasm_config_new()) {}

  /// \brief Configures whether dwarf debuginfo is emitted for assisting
  /// in-process debugging.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.debug_info
  void debug_info(bool enable) {
    wasmtime_config_debug_info_set(ptr.get(), enable);
  }

  /// \brief Configures whether epochs are enabled which can be used to
  /// interrupt currently executing WebAssembly.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.epoch_interruption
  void epoch_interruption(bool enable) {
    wasmtime_config_epoch_interruption_set(ptr.get(), enable);
  }

  /// \brief Configures whether WebAssembly code will consume fuel and trap when
  /// it runs out.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.consume_fuel
  void consume_fuel(bool enable) {
    wasmtime_config_consume_fuel_set(ptr.get(), enable);
  }

  /// \brief Configures the maximum amount of native stack wasm can consume.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.max_wasm_stack
  void max_wasm_stack(size_t stack) {
    wasmtime_config_max_wasm_stack_set(ptr.get(), stack);
  }

  /// \brief Configures whether the WebAssembly threads proposal is enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_threads
  void wasm_threads(bool enable) {
    wasmtime_config_wasm_threads_set(ptr.get(), enable);
  }

  /// \brief Configures whether the WebAssembly reference types proposal is
  /// enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_reference_types
  void wasm_reference_types(bool enable) {
    wasmtime_config_wasm_reference_types_set(ptr.get(), enable);
  }

  /// \brief Configures whether the WebAssembly simd proposal is enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_simd
  void wasm_simd(bool enable) {
    wasmtime_config_wasm_simd_set(ptr.get(), enable);
  }

  /// \brief Configures whether the WebAssembly bulk memory proposal is enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_bulk_memory
  void wasm_bulk_memory(bool enable) {
    wasmtime_config_wasm_bulk_memory_set(ptr.get(), enable);
  }

  /// \brief Configures whether the WebAssembly multi value proposal is enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_multi_value
  void wasm_multi_value(bool enable) {
    wasmtime_config_wasm_multi_value_set(ptr.get(), enable);
  }

  /// \brief Configures whether the WebAssembly Garbage Collection proposal will be enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_gc
  void wasm_gc(bool enable) {
    wasmtime_config_wasm_gc_set(ptr.get(), enable);
  }

  /// \brief Configures whether the WebAssembly function references proposal will be enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.wasm_function_references
  void wasm_function_references(bool enable) {
    wasmtime_config_wasm_function_references_set(ptr.get(), enable);
  }

  /// \brief Configure whether wasmtime should compile a module using multiple threads.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.parallel_compilation
  void parallel_compilation(bool enable) {
    wasmtime_config_parallel_compilation_set(ptr.get(), enable);
  }

  /// \brief Configures compilation strategy for wasm code.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.strategy
  void strategy(Strategy strategy) {
    wasmtime_config_strategy_set(ptr.get(),
                                 static_cast<wasmtime_strategy_t>(strategy));
  }

  /// \brief Configures whether cranelift's debug verifier is enabled
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.cranelift_debug_verifier
  void cranelift_debug_verifier(bool enable) {
    wasmtime_config_cranelift_debug_verifier_set(ptr.get(), enable);
  }

  /// \brief Configures cranelift's optimization level
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.cranelift_opt_level
  void cranelift_opt_level(OptLevel level) {
    wasmtime_config_cranelift_opt_level_set(
        ptr.get(), static_cast<wasmtime_opt_level_t>(level));
  }

  /// \brief Configures an active wasm profiler
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.profiler
  void profiler(ProfilingStrategy profiler) {
    wasmtime_config_profiler_set(
        ptr.get(), static_cast<wasmtime_profiling_strategy_t>(profiler));
  }

  /// \brief Configures the size of the initial linear memory allocation.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.memory_reservation
  void memory_reservation(size_t size) {
    wasmtime_config_memory_reservation_set(ptr.get(), size);
  }

  /// \brief Configures the size of memory's guard region
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.memory_guard_size
  void memory_guard_size(size_t size) {
    wasmtime_config_memory_guard_size_set(ptr.get(), size);
  }

  /// \brief Loads the default cache configuration present on the system.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.cache_config_load_default
  Result<std::monostate> cache_load_default() {
    auto *error = wasmtime_config_cache_config_load(ptr.get(), nullptr);
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /// \brief Loads cache configuration from the specified filename.
  ///
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.cache_config_load
  Result<std::monostate> cache_load(const std::string &path) {
    auto *error = wasmtime_config_cache_config_load(ptr.get(), path.c_str());
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }
};

/**
 * \brief Global compilation state in Wasmtime.
 *
 * Created with either default configuration or with a specified instance of
 * configuration, an `Engine` is used as an umbrella "session" for all other
 * operations in Wasmtime.
 */
class Engine {
  friend class Store;
  friend class Module;
  friend class Linker;

  struct deleter {
    void operator()(wasm_engine_t *p) const { wasm_engine_delete(p); }
  };

  std::unique_ptr<wasm_engine_t, deleter> ptr;

public:
  /// \brief Creates an engine with default compilation settings.
  Engine() : ptr(wasm_engine_new()) {}
  /// \brief Creates an engine with the specified compilation settings.
  explicit Engine(Config config)
      : ptr(wasm_engine_new_with_config(config.ptr.release())) {}

  /// \brief Increments the current epoch which may result in interrupting
  /// currently executing WebAssembly in connected stores if the epoch is now
  /// beyond the configured threshold.
  void increment_epoch() const { wasmtime_engine_increment_epoch(ptr.get()); }
};

/**
 * \brief Converts the WebAssembly text format into the WebAssembly binary
 * format.
 *
 * This will parse the text format and attempt to translate it to the binary
 * format. Note that the text parser assumes that all WebAssembly features are
 * enabled and will parse syntax of future proposals. The exact syntax here
 * parsed may be tweaked over time.
 *
 * Returns either an error if parsing failed or the wasm binary.
 */
inline Result<std::vector<uint8_t>> wat2wasm(std::string_view wat) {
  wasm_byte_vec_t ret;
  auto *error = wasmtime_wat2wasm(wat.data(), wat.size(), &ret);
  if (error != nullptr) {
    return Error(error);
  }
  std::vector<uint8_t> vec;
  // NOLINTNEXTLINE TODO can this be done without triggering lints?
  Span<uint8_t> raw(reinterpret_cast<uint8_t *>(ret.data), ret.size);
  vec.assign(raw.begin(), raw.end());
  wasm_byte_vec_delete(&ret);
  return vec;
}

/// Different kinds of types accepted by Wasmtime.
enum class ValKind {
  /// WebAssembly's `i32` type
  I32,
  /// WebAssembly's `i64` type
  I64,
  /// WebAssembly's `f32` type
  F32,
  /// WebAssembly's `f64` type
  F64,
  /// WebAssembly's `v128` type from the simd proposal
  V128,
  /// WebAssembly's `externref` type from the reference types
  ExternRef,
  /// WebAssembly's `funcref` type from the reference types
  FuncRef,
};

/// Helper X macro to construct statement for each enumerator in `ValKind`.
/// X(enumerator in `ValKind`, name string, enumerator in `wasm_valkind_t`)
#define WASMTIME_FOR_EACH_VAL_KIND(X)                                          \
  X(I32, "i32", WASM_I32)                                                      \
  X(I64, "i64", WASM_I64)                                                      \
  X(F32, "f32", WASM_F32)                                                      \
  X(F64, "f64", WASM_F64)                                                      \
  X(ExternRef, "externref", WASM_EXTERNREF)                                    \
  X(FuncRef, "funcref", WASM_FUNCREF)                                          \
  X(V128, "v128", WASMTIME_V128)

/// \brief Used to print a ValKind.
inline std::ostream &operator<<(std::ostream &os, const ValKind &e) {
  switch (e) {
#define CASE_KIND_PRINT_NAME(kind, name, ignore)                               \
  case ValKind::kind:                                                          \
    os << name;                                                                \
    break;
    WASMTIME_FOR_EACH_VAL_KIND(CASE_KIND_PRINT_NAME)
#undef CASE_KIND_PRINT_NAME
  default:
    abort();
  }
  return os;
}

/**
 * \brief Type information about a WebAssembly value.
 *
 * Currently mostly just contains the `ValKind`.
 */
class ValType {
  friend class TableType;
  friend class GlobalType;
  friend class FuncType;

  struct deleter {
    void operator()(wasm_valtype_t *p) const { wasm_valtype_delete(p); }
  };

  std::unique_ptr<wasm_valtype_t, deleter> ptr;

  static wasm_valkind_t kind_to_c(ValKind kind) {
    switch (kind) {
#define CASE_KIND_TO_C(kind, ignore, ckind)                                    \
  case ValKind::kind:                                                          \
    return ckind;
      WASMTIME_FOR_EACH_VAL_KIND(CASE_KIND_TO_C)
#undef CASE_KIND_TO_C
    default:
      abort();
    }
  }

public:
  /// \brief Non-owning reference to a `ValType`, must not be used after the
  /// original `ValType` is deleted.
  class Ref {
    friend class ValType;

    const wasm_valtype_t *ptr;

  public:
    /// \brief Instantiates from the raw C API representation.
    Ref(const wasm_valtype_t *ptr) : ptr(ptr) {}
    /// Copy constructor
    Ref(const ValType &ty) : Ref(ty.ptr.get()) {}

    /// \brief Returns the corresponding "kind" for this type.
    ValKind kind() const {
      switch (wasm_valtype_kind(ptr)) {
#define CASE_C_TO_KIND(kind, ignore, ckind)                                    \
  case ckind:                                                                  \
    return ValKind::kind;
        WASMTIME_FOR_EACH_VAL_KIND(CASE_C_TO_KIND)
#undef CASE_C_TO_KIND
      }
      std::abort();
    }
  };

  /// \brief Non-owning reference to a list of `ValType` instances. Must not be
  /// used after the original owner is deleted.
  class ListRef {
    const wasm_valtype_vec_t *list;

  public:
    /// Creates a list from the raw underlying C API.
    ListRef(const wasm_valtype_vec_t *list) : list(list) {}

    /// This list iterates over a list of `ValType::Ref` instances.
    typedef const Ref *iterator;

    /// Pointer to the beginning of iteration
    iterator begin() const {
      return reinterpret_cast<Ref *>(&list->data[0]); // NOLINT
    }

    /// Pointer to the end of iteration
    iterator end() const {
      return reinterpret_cast<Ref *>(&list->data[list->size]); // NOLINT
    }

    /// Returns how many types are in this list.
    size_t size() const { return list->size; }
  };

private:
  Ref ref;
  ValType(wasm_valtype_t *ptr) : ptr(ptr), ref(ptr) {}

public:
  /// Creates a new type from its kind.
  ValType(ValKind kind) : ValType(wasm_valtype_new(kind_to_c(kind))) {}
  /// Copies a `Ref` to a new owned value.
  ValType(Ref other) : ValType(wasm_valtype_copy(other.ptr)) {}
  /// Copies one type to a new one.
  ValType(const ValType &other) : ValType(wasm_valtype_copy(other.ptr.get())) {}
  /// Copies the contents of another type into this one.
  ValType &operator=(const ValType &other) {
    ptr.reset(wasm_valtype_copy(other.ptr.get()));
    ref = other.ref;
    return *this;
  }
  ~ValType() = default;
  /// Moves the memory owned by another value type into this one.
  ValType(ValType &&other) = default;
  /// Moves the memory owned by another value type into this one.
  ValType &operator=(ValType &&other) = default;

  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator->() { return &ref; }
  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator*() { return &ref; }
};

/**
 * \brief Type information about a WebAssembly linear memory
 */
class MemoryType {
  friend class Memory;

  struct deleter {
    void operator()(wasm_memorytype_t *p) const { wasm_memorytype_delete(p); }
  };

  std::unique_ptr<wasm_memorytype_t, deleter> ptr;

public:
  /// \brief Non-owning reference to a `MemoryType`, must not be used after the
  /// original owner has been deleted.
  class Ref {
    friend class MemoryType;

    const wasm_memorytype_t *ptr;

  public:
    /// Creates a reference from the raw C API representation.
    Ref(const wasm_memorytype_t *ptr) : ptr(ptr) {}
    /// Creates a reference from an original `MemoryType`.
    Ref(const MemoryType &ty) : Ref(ty.ptr.get()) {}

    /// Returns the minimum size, in WebAssembly pages, of this memory.
    uint64_t min() const { return wasmtime_memorytype_minimum(ptr); }

    /// Returns the maximum size, in WebAssembly pages, of this memory, if
    /// specified.
    std::optional<uint64_t> max() const {
      uint64_t max = 0;
      auto present = wasmtime_memorytype_maximum(ptr, &max);
      if (present) {
        return max;
      }
      return std::nullopt;
    }

    /// Returns whether or not this is a 64-bit memory type.
    bool is_64() const { return wasmtime_memorytype_is64(ptr); }

    /// Returns whether or not this is a shared memory type.
    bool is_shared() const { return wasmtime_memorytype_isshared(ptr); }
  };

private:
  Ref ref;
  MemoryType(wasm_memorytype_t *ptr) : ptr(ptr), ref(ptr) {}

public:
  /// Creates a new 32-bit wasm memory type with the specified minimum number of
  /// pages for the minimum size. The created type will have no maximum memory
  /// size.
  explicit MemoryType(uint32_t min)
      : MemoryType(wasmtime_memorytype_new(min, false, 0, false, false)) {}
  /// Creates a new 32-bit wasm memory type with the specified minimum number of
  /// pages for the minimum size, and maximum number of pages for the max size.
  MemoryType(uint32_t min, uint32_t max)
      : MemoryType(wasmtime_memorytype_new(min, true, max, false, false)) {}

  /// Same as the `MemoryType` constructor, except creates a 64-bit memory.
  static MemoryType New64(uint64_t min) {
    return MemoryType(wasmtime_memorytype_new(min, false, 0, true, false));
  }

  /// Same as the `MemoryType` constructor, except creates a 64-bit memory.
  static MemoryType New64(uint64_t min, uint64_t max) {
    return MemoryType(wasmtime_memorytype_new(min, true, max, true, false));
  }

  /// Creates a new wasm memory type from the specified ref, making a fresh
  /// owned value.
  MemoryType(Ref other) : MemoryType(wasm_memorytype_copy(other.ptr)) {}
  /// Copies the provided type into a new type.
  MemoryType(const MemoryType &other)
      : MemoryType(wasm_memorytype_copy(other.ptr.get())) {}
  /// Copies the provided type into a new type.
  MemoryType &operator=(const MemoryType &other) {
    ptr.reset(wasm_memorytype_copy(other.ptr.get()));
    return *this;
  }
  ~MemoryType() = default;
  /// Moves the type information from another type into this one.
  MemoryType(MemoryType &&other) = default;
  /// Moves the type information from another type into this one.
  MemoryType &operator=(MemoryType &&other) = default;

  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator->() { return &ref; }
  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator*() { return &ref; }
};

/**
 * \brief Type information about a WebAssembly table.
 */
class TableType {
  friend class Table;

  struct deleter {
    void operator()(wasm_tabletype_t *p) const { wasm_tabletype_delete(p); }
  };

  std::unique_ptr<wasm_tabletype_t, deleter> ptr;

public:
  /// Non-owning reference to a `TableType`, must not be used after the original
  /// owner is deleted.
  class Ref {
    friend class TableType;

    const wasm_tabletype_t *ptr;

  public:
    /// Creates a reference from the raw underlying C API representation.
    Ref(const wasm_tabletype_t *ptr) : ptr(ptr) {}
    /// Creates a reference to the provided `TableType`.
    Ref(const TableType &ty) : Ref(ty.ptr.get()) {}

    /// Returns the minimum size of this table type, in elements.
    uint32_t min() const { return wasm_tabletype_limits(ptr)->min; }

    /// Returns the maximum size of this table type, in elements, if present.
    std::optional<uint32_t> max() const {
      const auto *limits = wasm_tabletype_limits(ptr);
      if (limits->max == wasm_limits_max_default) {
        return std::nullopt;
      }
      return limits->max;
    }

    /// Returns the type of value that is stored in this table.
    ValType::Ref element() const { return wasm_tabletype_element(ptr); }
  };

private:
  Ref ref;
  TableType(wasm_tabletype_t *ptr) : ptr(ptr), ref(ptr) {}

public:
  /// Creates a new table type from the specified value type and minimum size.
  /// The returned table will have no maximum size.
  TableType(ValType ty, uint32_t min) : ptr(nullptr), ref(nullptr) {
    wasm_limits_t limits;
    limits.min = min;
    limits.max = wasm_limits_max_default;
    ptr.reset(wasm_tabletype_new(ty.ptr.release(), &limits));
    ref = ptr.get();
  }
  /// Creates a new table type from the specified value type, minimum size, and
  /// maximum size.
  TableType(ValType ty, uint32_t min, uint32_t max) // NOLINT
      : ptr(nullptr), ref(nullptr) {
    wasm_limits_t limits;
    limits.min = min;
    limits.max = max;
    ptr.reset(wasm_tabletype_new(ty.ptr.release(), &limits));
    ref = ptr.get();
  }
  /// Clones the given reference into a new table type.
  TableType(Ref other) : TableType(wasm_tabletype_copy(other.ptr)) {}
  /// Copies another table type into this one.
  TableType(const TableType &other)
      : TableType(wasm_tabletype_copy(other.ptr.get())) {}
  /// Copies another table type into this one.
  TableType &operator=(const TableType &other) {
    ptr.reset(wasm_tabletype_copy(other.ptr.get()));
    return *this;
  }
  ~TableType() = default;
  /// Moves the table type resources from another type to this one.
  TableType(TableType &&other) = default;
  /// Moves the table type resources from another type to this one.
  TableType &operator=(TableType &&other) = default;

  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator->() { return &ref; }
  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator*() { return &ref; }
};

/**
 * \brief Type information about a WebAssembly global
 */
class GlobalType {
  friend class Global;

  struct deleter {
    void operator()(wasm_globaltype_t *p) const { wasm_globaltype_delete(p); }
  };

  std::unique_ptr<wasm_globaltype_t, deleter> ptr;

public:
  /// Non-owning reference to a `Global`, must not be used after the original
  /// owner is deleted.
  class Ref {
    friend class GlobalType;
    const wasm_globaltype_t *ptr;

  public:
    /// Creates a new reference from the raw underlying C API representation.
    Ref(const wasm_globaltype_t *ptr) : ptr(ptr) {}
    /// Creates a new reference to the specified type.
    Ref(const GlobalType &ty) : Ref(ty.ptr.get()) {}

    /// Returns whether or not this global type is mutable.
    bool is_mutable() const {
      return wasm_globaltype_mutability(ptr) == WASM_VAR;
    }

    /// Returns the type of value stored within this global type.
    ValType::Ref content() const { return wasm_globaltype_content(ptr); }
  };

private:
  Ref ref;
  GlobalType(wasm_globaltype_t *ptr) : ptr(ptr), ref(ptr) {}

public:
  /// Creates a new global type from the specified value type and mutability.
  GlobalType(ValType ty, bool mut)
      : GlobalType(wasm_globaltype_new(
            ty.ptr.release(),
            (wasm_mutability_t)(mut ? WASM_VAR : WASM_CONST))) {}
  /// Clones a reference into a uniquely owned global type.
  GlobalType(Ref other) : GlobalType(wasm_globaltype_copy(other.ptr)) {}
  /// Copies other type information into this one.
  GlobalType(const GlobalType &other)
      : GlobalType(wasm_globaltype_copy(other.ptr.get())) {}
  /// Copies other type information into this one.
  GlobalType &operator=(const GlobalType &other) {
    ptr.reset(wasm_globaltype_copy(other.ptr.get()));
    return *this;
  }
  ~GlobalType() = default;
  /// Moves the underlying type information from another global into this one.
  GlobalType(GlobalType &&other) = default;
  /// Moves the underlying type information from another global into this one.
  GlobalType &operator=(GlobalType &&other) = default;

  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator->() { return &ref; }
  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator*() { return &ref; }
};

/**
 * \brief Type information for a WebAssembly function.
 */
class FuncType {
  friend class Func;
  friend class Linker;

  struct deleter {
    void operator()(wasm_functype_t *p) const { wasm_functype_delete(p); }
  };

  std::unique_ptr<wasm_functype_t, deleter> ptr;

public:
  /// Non-owning reference to a `FuncType`, must not be used after the original
  /// owner has been deleted.
  class Ref {
    friend class FuncType;
    const wasm_functype_t *ptr;

  public:
    /// Creates a new reference from the underlying C API representation.
    Ref(const wasm_functype_t *ptr) : ptr(ptr) {}
    /// Creates a new reference to the given type.
    Ref(const FuncType &ty) : Ref(ty.ptr.get()) {}

    /// Returns the list of types this function type takes as parameters.
    ValType::ListRef params() const { return wasm_functype_params(ptr); }

    /// Returns the list of types this function type returns.
    ValType::ListRef results() const { return wasm_functype_results(ptr); }
  };

private:
  Ref ref;
  FuncType(wasm_functype_t *ptr) : ptr(ptr), ref(ptr) {}

public:
  /// Creates a new function type from the given list of parameters and results.
  FuncType(std::initializer_list<ValType> params,
           std::initializer_list<ValType> results)
      : ref(nullptr) {
    *this = FuncType::from_iters(params, results);
  }

  /// Copies a reference into a uniquely owned function type.
  FuncType(Ref other) : FuncType(wasm_functype_copy(other.ptr)) {}
  /// Copies another type's information into this one.
  FuncType(const FuncType &other)
      : FuncType(wasm_functype_copy(other.ptr.get())) {}
  /// Copies another type's information into this one.
  FuncType &operator=(const FuncType &other) {
    ptr.reset(wasm_functype_copy(other.ptr.get()));
    return *this;
  }
  ~FuncType() = default;
  /// Moves type information from another type into this one.
  FuncType(FuncType &&other) = default;
  /// Moves type information from another type into this one.
  FuncType &operator=(FuncType &&other) = default;

  /// Creates a new function type from the given list of parameters and results.
  template <typename P, typename R>
  static FuncType from_iters(P params, R results) {
    wasm_valtype_vec_t param_vec;
    wasm_valtype_vec_t result_vec;
    wasm_valtype_vec_new_uninitialized(&param_vec, params.size());
    wasm_valtype_vec_new_uninitialized(&result_vec, results.size());
    size_t i = 0;

    for (auto val : params) {
      param_vec.data[i++] = val.ptr.release(); // NOLINT
    }
    i = 0;
    for (auto val : results) {
      result_vec.data[i++] = val.ptr.release(); // NOLINT
    }

    return wasm_functype_new(&param_vec, &result_vec);
  }

  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator->() { return &ref; }
  /// \brief Returns the underlying `Ref`, a non-owning reference pointing to
  /// this instance.
  Ref *operator*() { return &ref; }
};

/**
 * \brief Type information about a WebAssembly import.
 */
class ImportType {
public:
  /// Non-owning reference to an `ImportType`, must not be used after the
  /// original owner is deleted.
  class Ref {
    friend class ExternType;

    const wasm_importtype_t *ptr;

    // TODO: can this circle be broken another way?
    const wasm_externtype_t *raw_type() { return wasm_importtype_type(ptr); }

  public:
    /// Creates a new reference from the raw underlying C API representation.
    Ref(const wasm_importtype_t *ptr) : ptr(ptr) {}

    /// Returns the module name associated with this import.
    std::string_view module() {
      const auto *name = wasm_importtype_module(ptr);
      return std::string_view(name->data, name->size);
    }

    /// Returns the field name associated with this import.
    std::string_view name() {
      const auto *name = wasm_importtype_name(ptr);
      return std::string_view(name->data, name->size);
    }
  };

  /// An owned list of `ImportType` instances.
  class List {
    friend class Module;
    wasm_importtype_vec_t list;

  public:
    /// Creates an empty list
    List() : list{} {
      list.size = 0;
      list.data = nullptr;
    }
    List(const List &other) = delete;
    /// Moves another list into this one.
    List(List &&other) noexcept : list(other.list) { other.list.size = 0; }
    ~List() {
      if (list.size > 0) {
        wasm_importtype_vec_delete(&list);
      }
    }

    List &operator=(const List &other) = delete;
    /// Moves another list into this one.
    List &operator=(List &&other) noexcept {
      std::swap(list, other.list);
      return *this;
    }

    /// Iterator type, which is a list of non-owning `ImportType::Ref`
    /// instances.
    typedef const Ref *iterator;
    /// Returns the start of iteration.
    iterator begin() const {
      return reinterpret_cast<iterator>(&list.data[0]); // NOLINT
    }
    /// Returns the end of iteration.
    iterator end() const {
      return reinterpret_cast<iterator>(&list.data[list.size]); // NOLINT
    }
    /// Returns the size of this list.
    size_t size() const { return list.size; }
  };
};

/**
 * \brief Type information about a WebAssembly export
 */
class ExportType {

public:
  /// \brief Non-owning reference to an `ExportType`.
  ///
  /// Note to get type information you can use `ExternType::from_export`.
  class Ref {
    friend class ExternType;

    const wasm_exporttype_t *ptr;

    const wasm_externtype_t *raw_type() { return wasm_exporttype_type(ptr); }

  public:
    /// Creates a new reference from the raw underlying C API representation.
    Ref(const wasm_exporttype_t *ptr) : ptr(ptr) {}

    /// Returns the name of this export.
    std::string_view name() {
      const auto *name = wasm_exporttype_name(ptr);
      return std::string_view(name->data, name->size);
    }
  };

  /// An owned list of `ExportType` instances.
  class List {
    friend class Module;
    wasm_exporttype_vec_t list;

  public:
    /// Creates an empty list
    List() : list{} {
      list.size = 0;
      list.data = nullptr;
    }
    List(const List &other) = delete;
    /// Moves another list into this one.
    List(List &&other) noexcept : list(other.list) { other.list.size = 0; }
    ~List() {
      if (list.size > 0) {
        wasm_exporttype_vec_delete(&list);
      }
    }

    List &operator=(const List &other) = delete;
    /// Moves another list into this one.
    List &operator=(List &&other) noexcept {
      std::swap(list, other.list);
      return *this;
    }

    /// Iterator type, which is a list of non-owning `ExportType::Ref`
    /// instances.
    typedef const Ref *iterator;
    /// Returns the start of iteration.
    iterator begin() const {
      return reinterpret_cast<iterator>(&list.data[0]); // NOLINT
    }
    /// Returns the end of iteration.
    iterator end() const {
      return reinterpret_cast<iterator>(&list.data[list.size]); // NOLINT
    }
    /// Returns the size of this list.
    size_t size() const { return list.size; }
  };
};

/**
 * \brief Generic type of a WebAssembly item.
 */
class ExternType {
  friend class ExportType;
  friend class ImportType;

public:
  /// \typedef Ref
  /// \brief Non-owning reference to an item's type
  ///
  /// This cannot be used after the original owner has been deleted, and
  /// otherwise this is used to determine what the actual type of the outer item
  /// is.
  typedef std::variant<FuncType::Ref, GlobalType::Ref, TableType::Ref,
                       MemoryType::Ref>
      Ref;

  /// Extract the type of the item imported by the provided type.
  static Ref from_import(ImportType::Ref ty) {
    // TODO: this would ideally be some sort of implicit constructor, unsure how
    // to do that though...
    return ref_from_c(ty.raw_type());
  }

  /// Extract the type of the item exported by the provided type.
  static Ref from_export(ExportType::Ref ty) {
    // TODO: this would ideally be some sort of implicit constructor, unsure how
    // to do that though...
    return ref_from_c(ty.raw_type());
  }

private:
  static Ref ref_from_c(const wasm_externtype_t *ptr) {
    switch (wasm_externtype_kind(ptr)) {
    case WASM_EXTERN_FUNC:
      return wasm_externtype_as_functype_const(ptr);
    case WASM_EXTERN_GLOBAL:
      return wasm_externtype_as_globaltype_const(ptr);
    case WASM_EXTERN_TABLE:
      return wasm_externtype_as_tabletype_const(ptr);
    case WASM_EXTERN_MEMORY:
      return wasm_externtype_as_memorytype_const(ptr);
    }
    std::abort();
  }
};

/**
 * \brief Non-owning reference to a WebAssembly function frame as part of a
 * `Trace`
 *
 * A `FrameRef` represents a WebAssembly function frame on the stack which was
 * collected as part of a trap.
 */
class FrameRef {
  wasm_frame_t *frame;

public:
  /// Returns the WebAssembly function index of this function, in the original
  /// module.
  uint32_t func_index() const { return wasm_frame_func_index(frame); }
  /// Returns the offset, in bytes from the start of the function in the
  /// original module, to this frame's program counter.
  size_t func_offset() const { return wasm_frame_func_offset(frame); }
  /// Returns the offset, in bytes from the start of the original module,
  /// to this frame's program counter.
  size_t module_offset() const { return wasm_frame_module_offset(frame); }

  /// Returns the name, if present, associated with this function.
  ///
  /// Note that this requires that the `name` section is present in the original
  /// WebAssembly binary.
  std::optional<std::string_view> func_name() const {
    const auto *name = wasmtime_frame_func_name(frame);
    if (name != nullptr) {
      return std::string_view(name->data, name->size);
    }
    return std::nullopt;
  }

  /// Returns the name, if present, associated with this function's module.
  ///
  /// Note that this requires that the `name` section is present in the original
  /// WebAssembly binary.
  std::optional<std::string_view> module_name() const {
    const auto *name = wasmtime_frame_module_name(frame);
    if (name != nullptr) {
      return std::string_view(name->data, name->size);
    }
    return std::nullopt;
  }
};

/**
 * \brief An owned vector of `FrameRef` instances representing the WebAssembly
 * call-stack on a trap.
 *
 * This can be used to iterate over the frames of a trap and determine what was
 * running when a trap happened.
 */
class Trace {
  friend class Trap;
  friend class Error;

  wasm_frame_vec_t vec;

  Trace(wasm_frame_vec_t vec) : vec(vec) {}

public:
  ~Trace() { wasm_frame_vec_delete(&vec); }

  Trace(const Trace &other) = delete;
  Trace(Trace &&other) = delete;
  Trace &operator=(const Trace &other) = delete;
  Trace &operator=(Trace &&other) = delete;

  /// Iterator used to iterate over this trace.
  typedef const FrameRef *iterator;

  /// Returns the start of iteration
  iterator begin() const {
    return reinterpret_cast<FrameRef *>(&vec.data[0]); // NOLINT
  }
  /// Returns the end of iteration
  iterator end() const {
    return reinterpret_cast<FrameRef *>(&vec.data[vec.size]); // NOLINT
  }
  /// Returns the size of this trace, or how many frames it contains.
  size_t size() const { return vec.size; }
};

inline Trace Error::trace() const {
  wasm_frame_vec_t frames;
  wasmtime_error_wasm_trace(ptr.get(), &frames);
  return Trace(frames);
}

/**
 * \brief Information about a WebAssembly trap.
 *
 * Traps can happen during normal wasm execution (such as the `unreachable`
 * instruction) but they can also happen in host-provided functions to a host
 * function can simulate raising a trap.
 *
 * Traps have a message associated with them as well as a trace of WebAssembly
 * frames on the stack.
 */
class Trap {
  friend class Linker;
  friend class Instance;
  friend class Func;
  template <typename Params, typename Results> friend class TypedFunc;

  struct deleter {
    void operator()(wasm_trap_t *p) const { wasm_trap_delete(p); }
  };

  std::unique_ptr<wasm_trap_t, deleter> ptr;

  Trap(wasm_trap_t *ptr) : ptr(ptr) {}

public:
  /// Creates a new host-defined trap with the specified message.
  explicit Trap(std::string_view msg)
      : Trap(wasmtime_trap_new(msg.data(), msg.size())) {}

  /// Returns the descriptive message associated with this trap
  std::string message() const {
    wasm_byte_vec_t msg;
    wasm_trap_message(ptr.get(), &msg);
    std::string ret(msg.data, msg.size - 1);
    wasm_byte_vec_delete(&msg);
    return ret;
  }

  /// Returns the trace of WebAssembly frames associated with this trap.
  ///
  /// Note that the `trace` cannot outlive this error object.
  Trace trace() const {
    wasm_frame_vec_t frames;
    wasm_trap_trace(ptr.get(), &frames);
    return Trace(frames);
  }
};

/// Structure used to represent either a `Trap` or an `Error`.
struct TrapError {
  /// Storage for what this trap represents.
  std::variant<Trap, Error> data;

  /// Creates a new `TrapError` from a `Trap`
  TrapError(Trap t) : data(std::move(t)) {}
  /// Creates a new `TrapError` from an `Error`
  TrapError(Error e) : data(std::move(e)) {}

  /// Dispatches internally to return the message associated with this error.
  std::string message() const {
    if (const auto *trap = std::get_if<Trap>(&data)) {
      return trap->message();
    }
    if (const auto *error = std::get_if<Error>(&data)) {
      return std::string(error->message());
    }
    std::abort();
  }
};

/// Result used by functions which can fail because of invariants being violated
/// (such as a type error) as well as because of a WebAssembly trap.
template <typename T> using TrapResult = Result<T, TrapError>;

/**
 * \brief Representation of a compiled WebAssembly module.
 *
 * This type contains JIT code of a compiled WebAssembly module. A `Module` is
 * connected to an `Engine` and can only be instantiated within that `Engine`.
 * You can inspect a `Module` for its type information. This is passed as an
 * argument to other APIs to instantiate it.
 */
class Module {
  friend class Store;
  friend class Instance;
  friend class Linker;

  struct deleter {
    void operator()(wasmtime_module_t *p) const { wasmtime_module_delete(p); }
  };

  std::unique_ptr<wasmtime_module_t, deleter> ptr;

  Module(wasmtime_module_t *raw) : ptr(raw) {}

public:
  /// Copies another module into this one.
  Module(const Module &other) : ptr(wasmtime_module_clone(other.ptr.get())) {}
  /// Copies another module into this one.
  Module &operator=(const Module &other) {
    ptr.reset(wasmtime_module_clone(other.ptr.get()));
    return *this;
  }
  ~Module() = default;
  /// Moves resources from another module into this one.
  Module(Module &&other) = default;
  /// Moves resources from another module into this one.
  Module &operator=(Module &&other) = default;

  /**
   * \brief Compiles a module from the WebAssembly text format.
   *
   * This function will automatically use `wat2wasm` on the input and then
   * delegate to the #compile function.
   */
  static Result<Module> compile(Engine &engine, std::string_view wat) {
    auto wasm = wat2wasm(wat);
    if (!wasm) {
      return wasm.err();
    }
    auto bytes = wasm.ok();
    return compile(engine, bytes);
  }

  /**
   * \brief Compiles a module from the WebAssembly binary format.
   *
   * This function compiles the provided WebAssembly binary specified by `wasm`
   * within the compilation settings configured by `engine`. This method is
   * synchronous and will not return until the module has finished compiling.
   *
   * This function can fail if the WebAssembly binary is invalid or doesn't
   * validate (or similar).
   */
  static Result<Module> compile(Engine &engine, Span<uint8_t> wasm) {
    wasmtime_module_t *ret = nullptr;
    auto *error =
        wasmtime_module_new(engine.ptr.get(), wasm.data(), wasm.size(), &ret);
    if (error != nullptr) {
      return Error(error);
    }
    return Module(ret);
  }

  /**
   * \brief Validates the provided WebAssembly binary without compiling it.
   *
   * This function will validate whether the provided binary is indeed valid
   * within the compilation settings of the `engine` provided.
   */
  static Result<std::monostate> validate(Engine &engine, Span<uint8_t> wasm) {
    auto *error =
        wasmtime_module_validate(engine.ptr.get(), wasm.data(), wasm.size());
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /**
   * \brief Deserializes a previous list of bytes created with `serialize`.
   *
   * This function is intended to be much faster than `compile` where it uses
   * the artifacts of a previous compilation to quickly create an in-memory
   * module ready for instantiation.
   *
   * It is not safe to pass arbitrary input to this function, it is only safe to
   * pass in output from previous calls to `serialize`. For more information see
   * the Rust documentation -
   * https://docs.wasmtime.dev/api/wasmtime/struct.Module.html#method.deserialize
   */
  static Result<Module> deserialize(Engine &engine, Span<uint8_t> wasm) {
    wasmtime_module_t *ret = nullptr;
    auto *error = wasmtime_module_deserialize(engine.ptr.get(), wasm.data(),
                                              wasm.size(), &ret);
    if (error != nullptr) {
      return Error(error);
    }
    return Module(ret);
  }

  /**
   * \brief Deserializes a module from an on-disk file.
   *
   * This function is the same as `deserialize` except that it reads the data
   * for the serialized module from the path on disk. This can be faster than
   * the alternative which may require copying the data around.
   *
   * It is not safe to pass arbitrary input to this function, it is only safe to
   * pass in output from previous calls to `serialize`. For more information see
   * the Rust documentation -
   * https://docs.wasmtime.dev/api/wasmtime/struct.Module.html#method.deserialize
   */
  static Result<Module> deserialize_file(Engine &engine,
                                         const std::string &path) {
    wasmtime_module_t *ret = nullptr;
    auto *error =
        wasmtime_module_deserialize_file(engine.ptr.get(), path.c_str(), &ret);
    if (error != nullptr) {
      return Error(error);
    }
    return Module(ret);
  }

  /// Returns the list of types imported by this module.
  ImportType::List imports() const {
    ImportType::List list;
    wasmtime_module_imports(ptr.get(), &list.list);
    return list;
  }

  /// Returns the list of types exported by this module.
  ExportType::List exports() const {
    ExportType::List list;
    wasmtime_module_exports(ptr.get(), &list.list);
    return list;
  }

  /**
   * \brief Serializes this module to a list of bytes.
   *
   * The returned bytes can then be used to later pass to `deserialize` to
   * quickly recreate this module in a different process perhaps.
   */
  Result<std::vector<uint8_t>> serialize() const {
    wasm_byte_vec_t bytes;
    auto *error = wasmtime_module_serialize(ptr.get(), &bytes);
    if (error != nullptr) {
      return Error(error);
    }
    std::vector<uint8_t> ret;
    // NOLINTNEXTLINE TODO can this be done without triggering lints?
    Span<uint8_t> raw(reinterpret_cast<uint8_t *>(bytes.data), bytes.size);
    ret.assign(raw.begin(), raw.end());
    wasm_byte_vec_delete(&bytes);
    return ret;
  }
};

/**
 * \brief Configuration for an instance of WASI.
 *
 * This is inserted into a store with `Store::Context::set_wasi`.
 */
class WasiConfig {
  friend class Store;

  struct deleter {
    void operator()(wasi_config_t *p) const { wasi_config_delete(p); }
  };

  std::unique_ptr<wasi_config_t, deleter> ptr;

public:
  /// Creates a new configuration object with default settings.
  WasiConfig() : ptr(wasi_config_new()) {}

  /// Configures the argv explicitly with the given string array.
  void argv(const std::vector<std::string> &args) {
    std::vector<const char *> ptrs;
    ptrs.reserve(args.size());
    for (const auto &arg : args) {
      ptrs.push_back(arg.c_str());
    }

    wasi_config_set_argv(ptr.get(), (int)args.size(), ptrs.data());
  }

  /// Configures the argv for wasm to be inherited from this process itself.
  void inherit_argv() { wasi_config_inherit_argv(ptr.get()); }

  /// Configures the environment variables available to wasm, specified here as
  /// a list of pairs where the first element of the pair is the key and the
  /// second element is the value.
  void env(const std::vector<std::pair<std::string, std::string>> &env) {
    std::vector<const char *> names;
    std::vector<const char *> values;
    for (const auto &[name, value] : env) {
      names.push_back(name.c_str());
      values.push_back(value.c_str());
    }
    wasi_config_set_env(ptr.get(), (int)env.size(), names.data(),
                        values.data());
  }

  /// Indicates that the entire environment of this process should be inherited
  /// by the wasi configuration.
  void inherit_env() { wasi_config_inherit_env(ptr.get()); }

  /// Configures the provided file to be used for the stdin of this WASI
  /// configuration.
  [[nodiscard]] bool stdin_file(const std::string &path) {
    return wasi_config_set_stdin_file(ptr.get(), path.c_str());
  }

  /// Configures this WASI configuration to inherit its stdin from the host
  /// process.
  void inherit_stdin() { return wasi_config_inherit_stdin(ptr.get()); }

  /// Configures the provided file to be created and all stdout output will be
  /// written there.
  [[nodiscard]] bool stdout_file(const std::string &path) {
    return wasi_config_set_stdout_file(ptr.get(), path.c_str());
  }

  /// Configures this WASI configuration to inherit its stdout from the host
  /// process.
  void inherit_stdout() { return wasi_config_inherit_stdout(ptr.get()); }

  /// Configures the provided file to be created and all stderr output will be
  /// written there.
  [[nodiscard]] bool stderr_file(const std::string &path) {
    return wasi_config_set_stderr_file(ptr.get(), path.c_str());
  }

  /// Configures this WASI configuration to inherit its stdout from the host
  /// process.
  void inherit_stderr() { return wasi_config_inherit_stderr(ptr.get()); }

  /// Opens `path` to be opened as `guest_path` in the WASI pseudo-filesystem.
  [[nodiscard]] bool preopen_dir(const std::string &path,
                                 const std::string &guest_path,
                                 size_t dir_perms,
                                 size_t file_perms) {
    return wasi_config_preopen_dir(ptr.get(), path.c_str(), guest_path.c_str(), dir_perms, file_perms);
  }
};

class Caller;
template <typename Params, typename Results> class TypedFunc;

/**
 * \brief Owner of all WebAssembly objects
 *
 * A `Store` owns all WebAssembly objects such as instances, globals, functions,
 * memories, etc. A `Store` is one of the main central points about working with
 * WebAssembly since it's an argument to almost all APIs. The `Store` serves as
 * a form of "context" to give meaning to the pointers of `Func` and friends.
 *
 * A `Store` can be sent between threads but it cannot generally be shared
 * concurrently between threads. Memory associated with WebAssembly instances
 * will be deallocated when the `Store` is deallocated.
 */
class Store {
  struct deleter {
    void operator()(wasmtime_store_t *p) const { wasmtime_store_delete(p); }
  };

  std::unique_ptr<wasmtime_store_t, deleter> ptr;

  static void finalizer(void *ptr) {
    std::unique_ptr<std::any> _ptr(static_cast<std::any *>(ptr));
  }

public:
  /// Creates a new `Store` within the provided `Engine`.
  explicit Store(Engine &engine)
      : ptr(wasmtime_store_new(engine.ptr.get(), nullptr, finalizer)) {}

  /**
   * \brief An interior pointer into a `Store`.
   *
   * A `Context` object is created from either a `Store` or a `Caller`. It is an
   * interior pointer into a `Store` and cannot be used outside the lifetime of
   * the original object it was created from.
   *
   * This object is an argument to most APIs in Wasmtime but typically doesn't
   * need to be constructed explicitly since it can be created from a `Store&`
   * or a `Caller&`.
   */
  class Context {
    friend class Global;
    friend class Table;
    friend class Memory;
    friend class Func;
    friend class Instance;
    friend class Linker;
    friend class ExternRef;
    friend class Val;
    wasmtime_context_t *ptr;

    Context(wasmtime_context_t *ptr) : ptr(ptr) {}

  public:
    /// Creates a context referencing the provided `Store`.
    Context(Store &store) : Context(wasmtime_store_context(store.ptr.get())) {}
    /// Creates a context referencing the provided `Store`.
    Context(Store *store) : Context(*store) {}
    /// Creates a context referencing the provided `Caller`.
    Context(Caller &caller);
    /// Creates a context referencing the provided `Caller`.
    Context(Caller *caller);

    /// Runs a garbage collection pass in the referenced store to collect loose
    /// `externref` values, if any are available.
    void gc() { wasmtime_context_gc(ptr); }

    /// Injects fuel to be consumed within this store.
    ///
    /// Stores start with 0 fuel and if `Config::consume_fuel` is enabled then
    /// this is required if you want to let WebAssembly actually execute.
    ///
    /// Returns an error if fuel consumption isn't enabled.
    Result<std::monostate> set_fuel(uint64_t fuel) {
      auto *error = wasmtime_context_set_fuel(ptr, fuel);
      if (error != nullptr) {
        return Error(error);
      }
      return std::monostate();
    }

    /// Returns the amount of fuel consumed so far by executing WebAssembly.
    ///
    /// Returns `std::nullopt` if fuel consumption is not enabled.
    Result<uint64_t> get_fuel() const {
      uint64_t fuel = 0;
      auto *error = wasmtime_context_get_fuel(ptr, &fuel);
      if (error != nullptr) {
        return Error(error);
      }
      return fuel;
    }

    /// Set user specified data associated with this store.
    void set_data(std::any data) const {
      finalizer(static_cast<std::any *>(wasmtime_context_get_data(ptr)));
      wasmtime_context_set_data(
          ptr, std::make_unique<std::any>(std::move(data)).release());
    }

    /// Get user specified data associated with this store.
    std::any &get_data() const {
      return *static_cast<std::any *>(wasmtime_context_get_data(ptr));
    }

    /// Configures the WASI state used by this store.
    ///
    /// This will only have an effect if used in conjunction with
    /// `Linker::define_wasi` because otherwise no host functions will use the
    /// WASI state.
    Result<std::monostate> set_wasi(WasiConfig config) {
      auto *error = wasmtime_context_set_wasi(ptr, config.ptr.release());
      if (error != nullptr) {
        return Error(error);
      }
      return std::monostate();
    }

    /// Configures this store's epoch deadline to be the specified number of
    /// ticks beyond the engine's current epoch.
    ///
    /// By default the deadline is the current engine's epoch, immediately
    /// interrupting code if epoch interruption is enabled. This must be called
    /// to extend the deadline to allow interruption.
    void set_epoch_deadline(uint64_t ticks_beyond_current) {
      wasmtime_context_set_epoch_deadline(ptr, ticks_beyond_current);
    }

    /// Returns the raw context pointer for the C API.
    wasmtime_context_t *raw_context() { return ptr; }
  };

  /// \brief Provides limits for a store. Used by hosts to limit resource
  /// consumption of instances. Use negative value to keep the default value
  /// for the limit.
  ///
  /// \param memory_size the maximum number of bytes a linear memory can grow
  /// to. Growing a linear memory beyond this limit will fail. By default,
  /// linear memory will not be limited.
  ///
  /// \param table_elements the maximum number of elements in a table.
  /// Growing a table beyond this limit will fail. By default, table elements
  /// will not be limited.
  ///
  /// \param instances the maximum number of instances that can be created
  /// for a Store. Module instantiation will fail if this limit is exceeded.
  /// This value defaults to 10,000.
  ///
  /// \param tables the maximum number of tables that can be created for a
  /// Store. Module instantiation will fail if this limit is exceeded. This
  /// value defaults to 10,000.
  ///
  /// \param memories the maximum number of linear
  /// memories that can be created for a Store. Instantiation will fail with an
  /// error if this limit is exceeded. This value defaults to 10,000.
  ///
  /// Use any negative value for the parameters that should be kept on
  /// the default values.
  ///
  /// Note that the limits are only used to limit the creation/growth of
  /// resources in the future, this does not retroactively attempt to apply
  /// limits to the store.
  void limiter(int64_t memory_size, int64_t table_elements, int64_t instances,
               int64_t tables, int64_t memories) {
    wasmtime_store_limiter(ptr.get(), memory_size, table_elements, instances,
                           tables, memories);
  }

  /// Explicit function to acquire a `Context` from this store.
  Context context() { return this; }
};

/**
 * \brief Representation of a WebAssembly `externref` value.
 *
 * This class represents an value that cannot be forged by WebAssembly itself.
 * All `ExternRef` values are guaranteed to be created by the host and its
 * embedding. It's suitable to place private data structures in here which
 * WebAssembly will not have access to, only other host functions will have
 * access to them.
 *
 * Note that `ExternRef` values are rooted within a `Store` and must be manually
 * unrooted via the `unroot` function. If this is not used then values will
 * never be candidates for garbage collection.
 */
class ExternRef {
  friend class Val;

  wasmtime_externref_t val;

  static void finalizer(void *ptr) {
    std::unique_ptr<std::any> _ptr(static_cast<std::any *>(ptr));
  }

public:
  /// Creates a new `ExternRef` directly from its C-API representation.
  explicit ExternRef(wasmtime_externref_t val) : val(val) {}

  /// Creates a new `externref` value from the provided argument.
  ///
  /// Note that `val` should be safe to send across threads and should own any
  /// memory that it points to. Also note that `ExternRef` is similar to a
  /// `std::shared_ptr` in that there can be many references to the same value.
  template <typename T> explicit ExternRef(Store::Context cx, T val) {
    void *ptr = std::make_unique<std::any>(std::move(val)).release();
    bool ok = wasmtime_externref_new(cx.ptr, ptr, finalizer, &this->val);
    if (!ok)  {
      fprintf(stderr, "failed to allocate a new externref");
      abort();
    }
  }

  /// Creates a new `ExternRef` which is separately rooted from this one.
  ExternRef clone(Store::Context cx) {
    wasmtime_externref_t other;
    wasmtime_externref_clone(cx.ptr, &val, &other);
    return ExternRef(other);
  }

  /// Returns the underlying host data associated with this `ExternRef`.
  std::any &data(Store::Context cx) {
    return *static_cast<std::any *>(wasmtime_externref_data(cx.ptr, &val));
  }

  /// Unroots this value from the context provided, enabling a future GC to
  /// collect the internal object if there are no more references.
  void unroot(Store::Context cx) {
    wasmtime_externref_unroot(cx.ptr, &val);
  }

  /// Returns the raw underlying C API value.
  ///
  /// This class still retains ownership of the pointer.
  const wasmtime_externref_t *raw() const { return &val; }
};

class Func;
class Global;
class Instance;
class Memory;
class Table;

/// \typedef Extern
/// \brief Representation of an external WebAssembly item
typedef std::variant<Func, Global, Memory, Table> Extern;

/// \brief Container for the `v128` WebAssembly type.
struct V128 {
  /// \brief The little-endian bytes of the `v128` value.
  wasmtime_v128 v128;

  /// \brief Creates a new zero-value `v128`.
  V128() : v128{} { memset(&v128[0], 0, sizeof(wasmtime_v128)); }

  /// \brief Creates a new `V128` from its C API representation.
  V128(const wasmtime_v128 &v) : v128{} {
    memcpy(&v128[0], &v[0], sizeof(wasmtime_v128));
  }
};

/**
 * \brief Representation of a generic WebAssembly value.
 *
 * This is roughly equivalent to a tagged union of all possible WebAssembly
 * values. This is later used as an argument with functions, globals, tables,
 * etc.
 *
 * Note that a `Val` can represent owned GC pointers. In this case the `unroot`
 * method must be used to ensure that they can later be garbage-collected.
 */
class Val {
  friend class Global;
  friend class Table;
  friend class Func;

  wasmtime_val_t val;

  Val() : val{} {
    val.kind = WASMTIME_I32;
    val.of.i32 = 0;
  }
  Val(wasmtime_val_t val) : val(val) {}

public:
  /// Creates a new `i32` WebAssembly value.
  Val(int32_t i32) : val{} {
    val.kind = WASMTIME_I32;
    val.of.i32 = i32;
  }
  /// Creates a new `i64` WebAssembly value.
  Val(int64_t i64) : val{} {
    val.kind = WASMTIME_I64;
    val.of.i64 = i64;
  }
  /// Creates a new `f32` WebAssembly value.
  Val(float f32) : val{} {
    val.kind = WASMTIME_F32;
    val.of.f32 = f32;
  }
  /// Creates a new `f64` WebAssembly value.
  Val(double f64) : val{} {
    val.kind = WASMTIME_F64;
    val.of.f64 = f64;
  }
  /// Creates a new `v128` WebAssembly value.
  Val(const V128 &v128) : val{} {
    val.kind = WASMTIME_V128;
    memcpy(&val.of.v128[0], &v128.v128[0], sizeof(wasmtime_v128));
  }
  /// Creates a new `funcref` WebAssembly value.
  Val(std::optional<Func> func);
  /// Creates a new `funcref` WebAssembly value which is not `ref.null func`.
  Val(Func func);
  /// Creates a new `externref` value.
  Val(std::optional<ExternRef> ptr) : val{} {
    val.kind = WASMTIME_EXTERNREF;
    if (ptr) {
      val.of.externref = ptr->val;
    } else {
      wasmtime_externref_set_null(&val.of.externref);
    }
  }
  /// Creates a new `externref` WebAssembly value which is not `ref.null
  /// extern`.
  Val(ExternRef ptr);

  /// Returns the kind of value that this value has.
  ValKind kind() const {
    switch (val.kind) {
    case WASMTIME_I32:
      return ValKind::I32;
    case WASMTIME_I64:
      return ValKind::I64;
    case WASMTIME_F32:
      return ValKind::F32;
    case WASMTIME_F64:
      return ValKind::F64;
    case WASMTIME_FUNCREF:
      return ValKind::FuncRef;
    case WASMTIME_EXTERNREF:
      return ValKind::ExternRef;
    case WASMTIME_V128:
      return ValKind::V128;
    }
    std::abort();
  }

  /// Returns the underlying `i32`, requires `kind() == KindI32` or aborts the
  /// process.
  int32_t i32() const {
    if (val.kind != WASMTIME_I32) {
      std::abort();
    }
    return val.of.i32;
  }

  /// Returns the underlying `i64`, requires `kind() == KindI64` or aborts the
  /// process.
  int64_t i64() const {
    if (val.kind != WASMTIME_I64) {
      std::abort();
    }
    return val.of.i64;
  }

  /// Returns the underlying `f32`, requires `kind() == KindF32` or aborts the
  /// process.
  float f32() const {
    if (val.kind != WASMTIME_F32) {
      std::abort();
    }
    return val.of.f32;
  }

  /// Returns the underlying `f64`, requires `kind() == KindF64` or aborts the
  /// process.
  double f64() const {
    if (val.kind != WASMTIME_F64) {
      std::abort();
    }
    return val.of.f64;
  }

  /// Returns the underlying `v128`, requires `kind() == KindV128` or aborts
  /// the process.
  V128 v128() const {
    if (val.kind != WASMTIME_V128) {
      std::abort();
    }
    return val.of.v128;
  }

  /// Returns the underlying `externref`, requires `kind() == KindExternRef` or
  /// aborts the process.
  ///
  /// Note that `externref` is a nullable reference, hence the `optional` return
  /// value.
  std::optional<ExternRef> externref(Store::Context cx) const {
    if (val.kind != WASMTIME_EXTERNREF) {
      std::abort();
    }
    if (val.of.externref.store_id == 0) {
      return std::nullopt;
    }
    wasmtime_externref_t other;
    wasmtime_externref_clone(cx.ptr, &val.of.externref, &other);
    return ExternRef(other);
  }

  /// Returns the underlying `funcref`, requires `kind() == KindFuncRef` or
  /// aborts the process.
  ///
  /// Note that `funcref` is a nullable reference, hence the `optional` return
  /// value.
  std::optional<Func> funcref() const;

  /// Unroots any GC references this `Val` points to within the `cx` provided.
  void unroot(Store::Context cx) {
    wasmtime_val_unroot(cx.ptr, &val);
  }
};

/**
 * \brief Structure provided to host functions to lookup caller information or
 * acquire a `Store::Context`.
 *
 * This structure is passed to all host functions created with `Func`. It can be
 * used to create a `Store::Context`.
 */
class Caller {
  friend class Func;
  friend class Store;
  wasmtime_caller_t *ptr;
  Caller(wasmtime_caller_t *ptr) : ptr(ptr) {}

public:
  /// Attempts to load an exported item from the calling instance.
  ///
  /// For more information see the Rust documentation -
  /// https://docs.wasmtime.dev/api/wasmtime/struct.Caller.html#method.get_export
  std::optional<Extern> get_export(std::string_view name);

  /// Explicitly acquire a `Store::Context` from this `Caller`.
  Store::Context context() { return this; }
};

inline Store::Context::Context(Caller &caller)
    : Context(wasmtime_caller_context(caller.ptr)) {}
inline Store::Context::Context(Caller *caller) : Context(*caller) {}

namespace detail {

/// A "trait" for native types that correspond to WebAssembly types for use with
/// `Func::wrap` and `TypedFunc::call`
template <typename T> struct WasmType { static const bool valid = false; };

/// Helper macro to define `WasmType` definitions for primitive types like
/// int32_t and such.
// NOLINTNEXTLINE
#define NATIVE_WASM_TYPE(native, valkind, field)                               \
  template <> struct WasmType<native> {                                        \
    static const bool valid = true;                                            \
    static const ValKind kind = ValKind::valkind;                              \
    static void store(Store::Context cx, wasmtime_val_raw_t *p,                \
                      const native &t) {                                       \
      p->field = t;                                                            \
    }                                                                          \
    static native load(Store::Context cx, wasmtime_val_raw_t *p) {             \
      return p->field;                                                         \
    }                                                                          \
  };

NATIVE_WASM_TYPE(int32_t, I32, i32)
NATIVE_WASM_TYPE(uint32_t, I32, i32)
NATIVE_WASM_TYPE(int64_t, I64, i64)
NATIVE_WASM_TYPE(uint64_t, I64, i64)
NATIVE_WASM_TYPE(float, F32, f32)
NATIVE_WASM_TYPE(double, F64, f64)

#undef NATIVE_WASM_TYPE

/// Type information for `externref`, represented on the host as an optional
/// `ExternRef`.
template <> struct WasmType<std::optional<ExternRef>> {
  static const bool valid = true;
  static const ValKind kind = ValKind::ExternRef;
  static void store(Store::Context cx, wasmtime_val_raw_t *p,
                    const std::optional<ExternRef> &ref) {
    if (ref) {
      p->externref = wasmtime_externref_to_raw(cx.raw_context(), ref->raw());
    } else {
      p->externref = 0;
    }
  }
  static std::optional<ExternRef> load(Store::Context cx,
                                       wasmtime_val_raw_t *p) {
    if (p->externref == 0) {
      return std::nullopt;
    }
    wasmtime_externref_t val;
    wasmtime_externref_from_raw(cx.raw_context(), p->externref, &val);
    return ExternRef(val);
  }
};

/// Type information for the `V128` host value used as a wasm value.
template <> struct WasmType<V128> {
  static const bool valid = true;
  static const ValKind kind = ValKind::V128;
  static void store(Store::Context cx, wasmtime_val_raw_t *p, const V128 &t) {
    memcpy(&p->v128[0], &t.v128[0], sizeof(wasmtime_v128));
  }
  static V128 load(Store::Context cx, wasmtime_val_raw_t *p) { return p->v128; }
};

/// A "trait" for a list of types and operations on them, used for `Func::wrap`
/// and `TypedFunc::call`
///
/// The base case is a single type which is a list of one element.
template <typename T> struct WasmTypeList {
  static const bool valid = WasmType<T>::valid;
  static const size_t size = 1;
  static bool matches(ValType::ListRef types) {
    return WasmTypeList<std::tuple<T>>::matches(types);
  }
  static void store(Store::Context cx, wasmtime_val_raw_t *storage,
                    const T &t) {
    WasmType<T>::store(cx, storage, t);
  }
  static T load(Store::Context cx, wasmtime_val_raw_t *storage) {
    return WasmType<T>::load(cx, storage);
  }
  static std::vector<ValType> types() { return {WasmType<T>::kind}; }
};

/// std::monostate translates to an empty list of types.
template <> struct WasmTypeList<std::monostate> {
  static const bool valid = true;
  static const size_t size = 0;
  static bool matches(ValType::ListRef types) { return types.size() == 0; }
  static void store(Store::Context cx, wasmtime_val_raw_t *storage,
                    const std::monostate &t) {}
  static std::monostate load(Store::Context cx, wasmtime_val_raw_t *storage) {
    return std::monostate();
  }
  static std::vector<ValType> types() { return {}; }
};

/// std::tuple<> translates to the corresponding list of types
template <typename... T> struct WasmTypeList<std::tuple<T...>> {
  static const bool valid = (WasmType<T>::valid && ...);
  static const size_t size = sizeof...(T);
  static bool matches(ValType::ListRef types) {
    if (types.size() != size) {
      return false;
    }
    size_t n = 0;
    return ((WasmType<T>::kind == types.begin()[n++].kind()) && ...);
  }
  static void store(Store::Context cx, wasmtime_val_raw_t *storage,
                    const std::tuple<T...> &t) {
    size_t n = 0;
    std::apply(
        [&](const auto &...val) {
          (WasmType<T>::store(cx, &storage[n++], val), ...); // NOLINT
        },
        t);
  }
  static std::tuple<T...> load(Store::Context cx, wasmtime_val_raw_t *storage) {
    size_t n = 0;
    return std::tuple<T...>{WasmType<T>::load(cx, &storage[n++])...}; // NOLINT
  }
  static std::vector<ValType> types() { return {WasmType<T>::kind...}; }
};

/// A "trait" for what can be returned from closures specified to `Func::wrap`.
///
/// The base case here is a bare return value like `int32_t`.
template <typename R> struct WasmHostRet {
  using Results = WasmTypeList<R>;

  template <typename F, typename... A>
  static std::optional<Trap> invoke(F f, Caller cx, wasmtime_val_raw_t *raw,
                                    A... args) {
    auto ret = f(args...);
    Results::store(cx, raw, ret);
    return std::nullopt;
  }
};

/// Host functions can return nothing
template <> struct WasmHostRet<void> {
  using Results = WasmTypeList<std::tuple<>>;

  template <typename F, typename... A>
  static std::optional<Trap> invoke(F f, Caller cx, wasmtime_val_raw_t *raw,
                                    A... args) {
    f(args...);
    return std::nullopt;
  }
};

// Alternative method of returning "nothing" (also enables `std::monostate` in
// the `R` type of `Result` below)
template <> struct WasmHostRet<std::monostate> : public WasmHostRet<void> {};

/// Host functions can return a result which allows them to also possibly return
/// a trap.
template <typename R> struct WasmHostRet<Result<R, Trap>> {
  using Results = WasmTypeList<R>;

  template <typename F, typename... A>
  static std::optional<Trap> invoke(F f, Caller cx, wasmtime_val_raw_t *raw,
                                    A... args) {
    Result<R, Trap> ret = f(args...);
    if (!ret) {
      return ret.err();
    }
    Results::store(cx, raw, ret.ok());
    return std::nullopt;
  }
};

template <typename F, typename = void> struct WasmHostFunc;

/// Base type information for host free-function pointers being used as wasm
/// functions
template <typename R, typename... A> struct WasmHostFunc<R (*)(A...)> {
  using Params = WasmTypeList<std::tuple<A...>>;
  using Results = typename WasmHostRet<R>::Results;

  template <typename F>
  static std::optional<Trap> invoke(F &f, Caller cx, wasmtime_val_raw_t *raw) {
    auto params = Params::load(cx, raw);
    return std::apply(
        [&](const auto &...val) {
          return WasmHostRet<R>::invoke(f, cx, raw, val...);
        },
        params);
  }
};

/// Function type information, but with a `Caller` first parameter
template <typename R, typename... A>
struct WasmHostFunc<R (*)(Caller, A...)> : public WasmHostFunc<R (*)(A...)> {
  // Override `invoke` here to pass the `cx` as the first parameter
  template <typename F>
  static std::optional<Trap> invoke(F &f, Caller cx, wasmtime_val_raw_t *raw) {
    auto params = WasmTypeList<std::tuple<A...>>::load(cx, raw);
    return std::apply(
        [&](const auto &...val) {
          return WasmHostRet<R>::invoke(f, cx, raw, cx, val...);
        },
        params);
  }
};

/// Function type information, but with a class method.
template <typename R, typename C, typename... A>
struct WasmHostFunc<R (C::*)(A...)> : public WasmHostFunc<R (*)(A...)> {};

/// Function type information, but with a const class method.
template <typename R, typename C, typename... A>
struct WasmHostFunc<R (C::*)(A...) const> : public WasmHostFunc<R (*)(A...)> {};

/// Function type information, but as a host method with a `Caller` first
/// parameter.
template <typename R, typename C, typename... A>
struct WasmHostFunc<R (C::*)(Caller, A...)>
    : public WasmHostFunc<R (*)(Caller, A...)> {};

/// Function type information, but as a host const method with a `Caller`
/// first parameter.
template <typename R, typename C, typename... A>
struct WasmHostFunc<R (C::*)(Caller, A...) const>
    : public WasmHostFunc<R (*)(Caller, A...)> {};

/// Base type information for host callables being used as wasm
/// functions
template <typename T>
struct WasmHostFunc<T, std::void_t<decltype(&T::operator())>>
    : public WasmHostFunc<decltype(&T::operator())> {};

} // namespace detail

using namespace detail;

/**
 * \brief Representation of a WebAssembly function.
 *
 * This class represents a WebAssembly function, either created through
 * instantiating a module or a host function.
 *
 * Note that this type does not itself own any resources. It points to resources
 * owned within a `Store` and the `Store` must be passed in as the first
 * argument to the functions defined on `Func`. Note that if the wrong `Store`
 * is passed in then the process will be aborted.
 */
class Func {
  friend class Val;
  friend class Instance;
  friend class Linker;
  template <typename Params, typename Results> friend class TypedFunc;

  wasmtime_func_t func;

  template <typename F>
  static wasm_trap_t *raw_callback(void *env, wasmtime_caller_t *caller,
                                   const wasmtime_val_t *args, size_t nargs,
                                   wasmtime_val_t *results, size_t nresults) {
    static_assert(alignof(Val) == alignof(wasmtime_val_t));
    static_assert(sizeof(Val) == sizeof(wasmtime_val_t));
    F *func = reinterpret_cast<F *>(env);                          // NOLINT
    Span<const Val> args_span(reinterpret_cast<const Val *>(args), // NOLINT
                              nargs);
    Span<Val> results_span(reinterpret_cast<Val *>(results), // NOLINT
                           nresults);
    Result<std::monostate, Trap> result =
        (*func)(Caller(caller), args_span, results_span);
    if (!result) {
      return result.err().ptr.release();
    }
    return nullptr;
  }

  template <typename F>
  static wasm_trap_t *
  raw_callback_unchecked(void *env, wasmtime_caller_t *caller,
                         wasmtime_val_raw_t *args_and_results,
                         size_t nargs_and_results) {
    using HostFunc = WasmHostFunc<F>;
    Caller cx(caller);
    F *func = reinterpret_cast<F *>(env); // NOLINT
    auto trap = HostFunc::invoke(*func, cx, args_and_results);
    if (trap) {
      return trap->ptr.release();
    }
    return nullptr;
  }

  template <typename F> static void raw_finalize(void *env) {
    std::unique_ptr<F> ptr(reinterpret_cast<F *>(env)); // NOLINT
  }

public:
  /// Creates a new function from the raw underlying C API representation.
  Func(wasmtime_func_t func) : func(func) {}

  /**
   * \brief Creates a new host-defined function.
   *
   * This constructor is used to create a host function within the store
   * provided. This is how WebAssembly can call into the host and make use of
   * external functionality.
   *
   * > **Note**: host functions created this way are more flexible but not
   * > as fast to call as those created by `Func::wrap`.
   *
   * \param cx the store to create the function within
   * \param ty the type of the function that will be created
   * \param f the host callback to be executed when this function is called.
   *
   * The parameter `f` is expected to be a lambda (or a lambda lookalike) which
   * takes three parameters:
   *
   * * The first parameter is a `Caller` to get recursive access to the store
   *   and other caller state.
   * * The second parameter is a `Span<const Val>` which is the list of
   *   parameters to the function. These parameters are guaranteed to be of the
   *   types specified by `ty` when constructing this function.
   * * The last argument is `Span<Val>` which is where to write the return
   *   values of the function. The function must produce the types of values
   *   specified by `ty` or otherwise a trap will be raised.
   *
   * The parameter `f` is expected to return `Result<std::monostate, Trap>`.
   * This allows `f` to raise a trap if desired, or otherwise return no trap and
   * finish successfully. If a trap is raised then the results pointer does not
   * need to be written to.
   */
  template <typename F,
            std::enable_if_t<
                std::is_invocable_r_v<Result<std::monostate, Trap>, F, Caller,
                                      Span<const Val>, Span<Val>>,
                bool> = true>
  Func(Store::Context cx, const FuncType &ty, F f) : func({}) {
    wasmtime_func_new(cx.ptr, ty.ptr.get(), raw_callback<F>,
                      std::make_unique<F>(f).release(), raw_finalize<F>, &func);
  }

  /**
   * \brief Creates a new host function from the provided callback `f`,
   * inferring the WebAssembly function type from the host signature.
   *
   * This function is akin to the `Func` constructor except that the WebAssembly
   * type does not need to be specified and additionally the signature of `f`
   * is different. The main goal of this function is to enable WebAssembly to
   * call the function `f` as-fast-as-possible without having to validate any
   * types or such.
   *
   * The function `f` can optionally take a `Caller` as its first parameter,
   * but otherwise its arguments are translated to WebAssembly types:
   *
   * * `int32_t`, `uint32_t` - `i32`
   * * `int64_t`, `uint64_t` - `i64`
   * * `float` - `f32`
   * * `double` - `f64`
   * * `std::optional<Func>` - `funcref`
   * * `std::optional<ExternRef>` - `externref`
   * * `wasmtime::V128` - `v128`
   *
   * The function may only take these arguments and if it takes any other kinds
   * of arguments then it will fail to compile.
   *
   * The function may return a few different flavors of return values:
   *
   * * `void` - interpreted as returning nothing
   * * Any type above - interpreted as a singular return value.
   * * `std::tuple<T...>` where `T` is one of the valid argument types -
   *   interpreted as returning multiple values.
   * * `Result<T, Trap>` where `T` is another valid return type - interpreted as
   *   a function that returns `T` to wasm but is optionally allowed to also
   *   raise a trap.
   *
   * It's recommended, if possible, to use this function over the `Func`
   * constructor since this is generally easier to work with and also enables
   * a faster path for WebAssembly to call this function.
   */
  template <typename F,
            std::enable_if_t<WasmHostFunc<F>::Params::valid, bool> = true,
            std::enable_if_t<WasmHostFunc<F>::Results::valid, bool> = true>
  static Func wrap(Store::Context cx, F f) {
    using HostFunc = WasmHostFunc<F>;
    auto params = HostFunc::Params::types();
    auto results = HostFunc::Results::types();
    auto ty = FuncType::from_iters(params, results);
    wasmtime_func_t func;
    wasmtime_func_new_unchecked(cx.ptr, ty.ptr.get(), raw_callback_unchecked<F>,
                                std::make_unique<F>(f).release(),
                                raw_finalize<F>, &func);
    return func;
  }

  /**
   * \brief Invoke a WebAssembly function.
   *
   * This function will execute this WebAssembly function. This function muts be
   * defined within the `cx`'s store provided. The `params` argument is the list
   * of parameters that are passed to the wasm function, and the types of the
   * values within `params` must match the type signature of this function.
   *
   * This may return one of three values:
   *
   * * First the function could succeed, returning a vector of values
   *   representing the results of the function.
   * * Otherwise a `Trap` might be generated by the WebAssembly function.
   * * Finally an `Error` could be returned indicating that `params` were not of
   *   the right type.
   *
   * > **Note**: for optimized calls into WebAssembly where the function
   * > signature is statically known it's recommended to use `Func::typed` and
   * > `TypedFunc::call`.
   */
  template <typename I>
  TrapResult<std::vector<Val>> call(Store::Context cx, const I &begin,
                                    const I &end) const {
    std::vector<wasmtime_val_t> raw_params;
    raw_params.reserve(end - begin);
    for (auto i = begin; i != end; i++) {
      raw_params.push_back(i->val);
    }
    size_t nresults = this->type(cx)->results().size();
    std::vector<wasmtime_val_t> raw_results(nresults);

    wasm_trap_t *trap = nullptr;
    auto *error =
        wasmtime_func_call(cx.ptr, &func, raw_params.data(), raw_params.size(),
                           raw_results.data(), raw_results.capacity(), &trap);
    if (error != nullptr) {
      return TrapError(Error(error));
    }
    if (trap != nullptr) {
      return TrapError(Trap(trap));
    }

    std::vector<Val> results;
    results.reserve(nresults);
    for (size_t i = 0; i < nresults; i++) {
      results.push_back(raw_results[i]);
    }
    return results;
  }

  TrapResult<std::vector<Val>> call(Store::Context cx,
                                    const std::vector<Val> &params) const {
    return this->call(cx, params.begin(), params.end());
  }

  TrapResult<std::vector<Val>>
  call(Store::Context cx, const std::initializer_list<Val> &params) const {
    return this->call(cx, params.begin(), params.end());
  }

  /// Returns the type of this function.
  FuncType type(Store::Context cx) const {
    return wasmtime_func_type(cx.ptr, &func);
  }

  /**
   * \brief Statically checks this function against the provided types.
   *
   * This function will check whether it takes the statically known `Params`
   * and returns the statically known `Results`. If the type check succeeds then
   * a `TypedFunc` is returned which enables a faster method of invoking
   * WebAssembly functions.
   *
   * The `Params` and `Results` specified as template parameters here are the
   * parameters and results of the wasm function. They can either be a bare
   * type which means that the wasm takes/returns one value, or they can be a
   * `std::tuple<T...>` of types to represent multiple arguments or multiple
   * returns.
   *
   * The valid types for this function are those mentioned as the arguments
   * for `Func::wrap`.
   */
  template <typename Params, typename Results,
            std::enable_if_t<WasmTypeList<Params>::valid, bool> = true,
            std::enable_if_t<WasmTypeList<Results>::valid, bool> = true>
  Result<TypedFunc<Params, Results>, Trap> typed(Store::Context cx) const {
    auto ty = this->type(cx);
    if (!WasmTypeList<Params>::matches(ty->params()) ||
        !WasmTypeList<Results>::matches(ty->results())) {
      return Trap("static type for this function does not match actual type");
    }
    TypedFunc<Params, Results> ret(*this);
    return ret;
  }

  /// Returns the raw underlying C API function this is using.
  const wasmtime_func_t &raw_func() const { return func; }
};

/**
 * \brief A version of a WebAssembly `Func` where the type signature of the
 * function is statically known.
 */
template <typename Params, typename Results> class TypedFunc {
  friend class Func;
  Func f;
  TypedFunc(Func func) : f(func) {}

public:
  /**
   * \brief Calls this function with the provided parameters.
   *
   * This function is akin to `Func::call` except that since static type
   * information is available it statically takes its parameters and statically
   * returns its results.
   *
   * Note that this function still may return a `Trap` indicating that calling
   * the WebAssembly function failed.
   */
  TrapResult<Results> call(Store::Context cx, Params params) const {
    std::array<wasmtime_val_raw_t, std::max(WasmTypeList<Params>::size,
                                            WasmTypeList<Results>::size)>
        storage;
    wasmtime_val_raw_t *ptr = storage.data();
    if (ptr == nullptr)
      ptr = reinterpret_cast<wasmtime_val_raw_t*>(alignof(wasmtime_val_raw_t));
    WasmTypeList<Params>::store(cx, ptr, params);
    wasm_trap_t *trap = nullptr;
    auto *error = wasmtime_func_call_unchecked(
        cx.raw_context(), &f.func, ptr, storage.size(), &trap);
    if (error != nullptr) {
      return TrapError(Error(error));
    }
    if (trap != nullptr) {
      return TrapError(Trap(trap));
    }
    return WasmTypeList<Results>::load(cx, ptr);
  }

  /// Returns the underlying un-typed `Func` for this function.
  const Func &func() const { return f; }
};

inline Val::Val(std::optional<Func> func) : val{} {
  val.kind = WASMTIME_FUNCREF;
  if (func) {
    val.of.funcref = (*func).func;
  } else {
    wasmtime_funcref_set_null(&val.of.funcref);
  }
}

inline Val::Val(Func func) : Val(std::optional(func)) {}
inline Val::Val(ExternRef ptr) : Val(std::optional(ptr)) {}

inline std::optional<Func> Val::funcref() const {
  if (val.kind != WASMTIME_FUNCREF) {
    std::abort();
  }
  if (val.of.funcref.store_id == 0) {
    return std::nullopt;
  }
  return Func(val.of.funcref);
}

/// Definition for the `funcref` native wasm type
template <> struct detail::WasmType<std::optional<Func>> {
  /// @private
  static const bool valid = true;
  /// @private
  static const ValKind kind = ValKind::FuncRef;
  /// @private
  static void store(Store::Context cx, wasmtime_val_raw_t *p,
                    const std::optional<Func> func) {
    if (func) {
      p->funcref = wasmtime_func_to_raw(cx.raw_context(), &func->raw_func());
    } else {
      p->funcref = 0;
    }
  }
  /// @private
  static std::optional<Func> load(Store::Context cx, wasmtime_val_raw_t *p) {
    if (p->funcref == 0) {
      return std::nullopt;
    }
    wasmtime_func_t ret;
    wasmtime_func_from_raw(cx.raw_context(), p->funcref, &ret);
    return ret;
  }
};

/**
 * \brief A WebAssembly global.
 *
 * This class represents a WebAssembly global, either created through
 * instantiating a module or a host global. Globals contain a WebAssembly value
 * and can be read and optionally written to.
 *
 * Note that this type does not itself own any resources. It points to resources
 * owned within a `Store` and the `Store` must be passed in as the first
 * argument to the functions defined on `Global`. Note that if the wrong `Store`
 * is passed in then the process will be aborted.
 */
class Global {
  friend class Instance;
  wasmtime_global_t global;

public:
  /// Creates as global from the raw underlying C API representation.
  Global(wasmtime_global_t global) : global(global) {}

  /**
   * \brief Create a new WebAssembly global.
   *
   * \param cx the store in which to create the global
   * \param ty the type that this global will have
   * \param init the initial value of the global
   *
   * This function can fail if `init` does not have a value that matches `ty`.
   */
  static Result<Global> create(Store::Context cx, const GlobalType &ty,
                               const Val &init) {
    wasmtime_global_t global;
    auto *error = wasmtime_global_new(cx.ptr, ty.ptr.get(), &init.val, &global);
    if (error != nullptr) {
      return Error(error);
    }
    return Global(global);
  }

  /// Returns the type of this global.
  GlobalType type(Store::Context cx) const {
    return wasmtime_global_type(cx.ptr, &global);
  }

  /// Returns the current value of this global.
  Val get(Store::Context cx) const;

  /// Sets this global to a new value.
  ///
  /// This can fail if `val` has the wrong type or if this global isn't mutable.
  Result<std::monostate> set(Store::Context cx, const Val &val) const {
    auto *error = wasmtime_global_set(cx.ptr, &global, &val.val);
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }
};

/**
 * \brief A WebAssembly table.
 *
 * This class represents a WebAssembly table, either created through
 * instantiating a module or a host table. Tables are contiguous vectors of
 * WebAssembly reference types, currently either `externref` or `funcref`.
 *
 * Note that this type does not itself own any resources. It points to resources
 * owned within a `Store` and the `Store` must be passed in as the first
 * argument to the functions defined on `Table`. Note that if the wrong `Store`
 * is passed in then the process will be aborted.
 */
class Table {
  friend class Instance;
  wasmtime_table_t table;

public:
  /// Creates a new table from the raw underlying C API representation.
  Table(wasmtime_table_t table) : table(table) {}

  /**
   * \brief Creates a new host-defined table.
   *
   * \param cx the store in which to create the table.
   * \param ty the type of the table to be created
   * \param init the initial value for all table slots.
   *
   * Returns an error if `init` has the wrong value for the `ty` specified.
   */
  static Result<Table> create(Store::Context cx, const TableType &ty,
                              const Val &init) {
    wasmtime_table_t table;
    auto *error = wasmtime_table_new(cx.ptr, ty.ptr.get(), &init.val, &table);
    if (error != nullptr) {
      return Error(error);
    }
    return Table(table);
  }

  /// Returns the type of this table.
  TableType type(Store::Context cx) const {
    return wasmtime_table_type(cx.ptr, &table);
  }

  /// Returns the size, in elements, that the table currently has.
  uint64_t size(Store::Context cx) const {
    return wasmtime_table_size(cx.ptr, &table);
  }

  /// Loads a value from the specified index in this table.
  ///
  /// Returns `std::nullopt` if `idx` is out of bounds.
  std::optional<Val> get(Store::Context cx, uint64_t idx) const {
    Val val;
    if (wasmtime_table_get(cx.ptr, &table, idx, &val.val)) {
      return val;
    }
    return std::nullopt;
  }

  /// Stores a value into the specified index in this table.
  ///
  /// Returns an error if `idx` is out of bounds or if `val` has the wrong type.
  Result<std::monostate> set(Store::Context cx, uint64_t idx,
                             const Val &val) const {
    auto *error = wasmtime_table_set(cx.ptr, &table, idx, &val.val);
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /// Grow this table.
  ///
  /// \param cx the store that owns this table.
  /// \param delta the number of new elements to be added to this table.
  /// \param init the initial value of all new elements in this table.
  ///
  /// Returns an error if `init` has the wrong type for this table. Otherwise
  /// returns the previous size of the table before growth.
  Result<uint64_t> grow(Store::Context cx, uint64_t delta,
                        const Val &init) const {
    uint64_t prev = 0;
    auto *error = wasmtime_table_grow(cx.ptr, &table, delta, &init.val, &prev);
    if (error != nullptr) {
      return Error(error);
    }
    return prev;
  }
};

// gcc 8.3.0 seems to require that this comes after the definition of `Table`. I
// don't know why...
inline Val Global::get(Store::Context cx) const {
  Val val;
  wasmtime_global_get(cx.ptr, &global, &val.val);
  return val;
}

/**
 * \brief A WebAssembly linear memory.
 *
 * This class represents a WebAssembly memory, either created through
 * instantiating a module or a host memory.
 *
 * Note that this type does not itself own any resources. It points to resources
 * owned within a `Store` and the `Store` must be passed in as the first
 * argument to the functions defined on `Table`. Note that if the wrong `Store`
 * is passed in then the process will be aborted.
 */
class Memory {
  friend class Instance;
  wasmtime_memory_t memory;

public:
  /// Creates a new memory from the raw underlying C API representation.
  Memory(wasmtime_memory_t memory) : memory(memory) {}

  /// Creates a new host-defined memory with the type specified.
  static Result<Memory> create(Store::Context cx, const MemoryType &ty) {
    wasmtime_memory_t memory;
    auto *error = wasmtime_memory_new(cx.ptr, ty.ptr.get(), &memory);
    if (error != nullptr) {
      return Error(error);
    }
    return Memory(memory);
  }

  /// Returns the type of this memory.
  MemoryType type(Store::Context cx) const {
    return wasmtime_memory_type(cx.ptr, &memory);
  }

  /// Returns the size, in WebAssembly pages, of this memory.
  uint64_t size(Store::Context cx) const {
    return wasmtime_memory_size(cx.ptr, &memory);
  }

  /// Returns a `span` of where this memory is located in the host.
  ///
  /// Note that embedders need to be very careful in their usage of the returned
  /// `span`. It can be invalidated with calls to `grow` and/or calls into
  /// WebAssembly.
  Span<uint8_t> data(Store::Context cx) const {
    auto *base = wasmtime_memory_data(cx.ptr, &memory);
    auto size = wasmtime_memory_data_size(cx.ptr, &memory);
    return {base, size};
  }

  /// Grows the memory by `delta` WebAssembly pages.
  ///
  /// On success returns the previous size of this memory in units of
  /// WebAssembly pages.
  Result<uint64_t> grow(Store::Context cx, uint64_t delta) const {
    uint64_t prev = 0;
    auto *error = wasmtime_memory_grow(cx.ptr, &memory, delta, &prev);
    if (error != nullptr) {
      return Error(error);
    }
    return prev;
  }
};

/**
 * \brief A WebAssembly instance.
 *
 * This class represents a WebAssembly instance, created by instantiating a
 * module. An instance is the collection of items exported by the module, which
 * can be accessed through the `Store` that owns the instance.
 *
 * Note that this type does not itself own any resources. It points to resources
 * owned within a `Store` and the `Store` must be passed in as the first
 * argument to the functions defined on `Instance`. Note that if the wrong
 * `Store` is passed in then the process will be aborted.
 */
class Instance {
  friend class Linker;
  friend class Caller;

  wasmtime_instance_t instance;

  static Extern cvt(wasmtime_extern_t &e) {
    switch (e.kind) {
    case WASMTIME_EXTERN_FUNC:
      return Func(e.of.func);
    case WASMTIME_EXTERN_GLOBAL:
      return Global(e.of.global);
    case WASMTIME_EXTERN_MEMORY:
      return Memory(e.of.memory);
    case WASMTIME_EXTERN_TABLE:
      return Table(e.of.table);
    }
    std::abort();
  }

  static void cvt(const Extern &e, wasmtime_extern_t &raw) {
    if (const auto *func = std::get_if<Func>(&e)) {
      raw.kind = WASMTIME_EXTERN_FUNC;
      raw.of.func = func->func;
    } else if (const auto *global = std::get_if<Global>(&e)) {
      raw.kind = WASMTIME_EXTERN_GLOBAL;
      raw.of.global = global->global;
    } else if (const auto *table = std::get_if<Table>(&e)) {
      raw.kind = WASMTIME_EXTERN_TABLE;
      raw.of.table = table->table;
    } else if (const auto *memory = std::get_if<Memory>(&e)) {
      raw.kind = WASMTIME_EXTERN_MEMORY;
      raw.of.memory = memory->memory;
    } else {
      std::abort();
    }
  }

public:
  /// Creates a new instance from the raw underlying C API representation.
  Instance(wasmtime_instance_t instance) : instance(instance) {}

  /**
   * \brief Instantiates the module `m` with the provided `imports`
   *
   * \param cx the store in which to instantiate the provided module
   * \param m the module to instantiate
   * \param imports the list of imports to use to instantiate the module
   *
   * This `imports` parameter is expected to line up 1:1 with the imports
   * required by the `m`. The type of `m` can be inspected to determine in which
   * order to provide the imports. Note that this is a relatively low-level API
   * and it's generally recommended to use `Linker` instead for name-based
   * instantiation.
   *
   * This function can return an error if any of the `imports` have the wrong
   * type, or if the wrong number of `imports` is provided.
   */
  static TrapResult<Instance> create(Store::Context cx, const Module &m,
                                     const std::vector<Extern> &imports) {
    std::vector<wasmtime_extern_t> raw_imports;
    for (const auto &item : imports) {
      raw_imports.push_back(wasmtime_extern_t{});
      auto &last = raw_imports.back();
      Instance::cvt(item, last);
    }
    wasmtime_instance_t instance;
    wasm_trap_t *trap = nullptr;
    auto *error = wasmtime_instance_new(cx.ptr, m.ptr.get(), raw_imports.data(),
                                        raw_imports.size(), &instance, &trap);
    if (error != nullptr) {
      return TrapError(Error(error));
    }
    if (trap != nullptr) {
      return TrapError(Trap(trap));
    }
    return Instance(instance);
  }

  /**
   * \brief Load an instance's export by name.
   *
   * This function will look for an export named `name` on this instance and, if
   * found, return it as an `Extern`.
   */
  std::optional<Extern> get(Store::Context cx, std::string_view name) {
    wasmtime_extern_t e;
    if (!wasmtime_instance_export_get(cx.ptr, &instance, name.data(),
                                      name.size(), &e)) {
      return std::nullopt;
    }
    return Instance::cvt(e);
  }

  /**
   * \brief Load an instance's export by index.
   *
   * This function will look for the `idx`th export of this instance. This will
   * return both the name of the export as well as the exported item itself.
   */
  std::optional<std::pair<std::string_view, Extern>> get(Store::Context cx,
                                                         size_t idx) {
    wasmtime_extern_t e;
    // I'm not sure why clang-tidy thinks this is using va_list or anything
    // related to that...
    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg)
    char *name = nullptr;
    size_t len = 0;
    if (!wasmtime_instance_export_nth(cx.ptr, &instance, idx, &name, &len,
                                      &e)) {
      return std::nullopt;
    }
    std::string_view n(name, len);
    return std::pair(n, Instance::cvt(e));
  }
};

inline std::optional<Extern> Caller::get_export(std::string_view name) {
  wasmtime_extern_t item;
  if (wasmtime_caller_export_get(ptr, name.data(), name.size(), &item)) {
    return Instance::cvt(item);
  }
  return std::nullopt;
}

/**
 * \brief Helper class for linking modules together with name-based resolution.
 *
 * This class is used for easily instantiating `Module`s by defining names into
 * the linker and performing name-based resolution during instantiation. A
 * `Linker` can also be used to link in WASI functions to instantiate a module.
 */
class Linker {
  struct deleter {
    void operator()(wasmtime_linker_t *p) const { wasmtime_linker_delete(p); }
  };

  std::unique_ptr<wasmtime_linker_t, deleter> ptr;

public:
  /// Creates a new linker which will instantiate in the given engine.
  explicit Linker(Engine &engine)
      : ptr(wasmtime_linker_new(engine.ptr.get())) {}

  /// Configures whether shadowing previous names is allowed or not.
  ///
  /// By default shadowing is not allowed.
  void allow_shadowing(bool allow) {
    wasmtime_linker_allow_shadowing(ptr.get(), allow);
  }

  /// Defines the provided item into this linker with the given name.
  Result<std::monostate> define(Store::Context cx, std::string_view module,
                                std::string_view name, const Extern &item) {
    wasmtime_extern_t raw;
    Instance::cvt(item, raw);
    auto *error =
        wasmtime_linker_define(ptr.get(), cx.ptr, module.data(), module.size(),
                               name.data(), name.size(), &raw);
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /// Defines WASI functions within this linker.
  ///
  /// Note that `Store::Context::set_wasi` must also be used for instantiated
  /// modules to have access to configured WASI state.
  Result<std::monostate> define_wasi() {
    auto *error = wasmtime_linker_define_wasi(ptr.get());
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /// Defines all exports of the `instance` provided in this linker with the
  /// given module name of `name`.
  Result<std::monostate>
  define_instance(Store::Context cx, std::string_view name, Instance instance) {
    auto *error = wasmtime_linker_define_instance(
        ptr.get(), cx.ptr, name.data(), name.size(), &instance.instance);
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /// Instantiates the module `m` provided within the store `cx` using the items
  /// defined within this linker.
  TrapResult<Instance> instantiate(Store::Context cx, const Module &m) {
    wasmtime_instance_t instance;
    wasm_trap_t *trap = nullptr;
    auto *error = wasmtime_linker_instantiate(ptr.get(), cx.ptr, m.ptr.get(),
                                              &instance, &trap);
    if (error != nullptr) {
      return TrapError(Error(error));
    }
    if (trap != nullptr) {
      return TrapError(Trap(trap));
    }
    return Instance(instance);
  }

  /// Defines instantiations of the module `m` within this linker under the
  /// given `name`.
  Result<std::monostate> module(Store::Context cx, std::string_view name,
                                const Module &m) {
    auto *error = wasmtime_linker_module(ptr.get(), cx.ptr, name.data(),
                                         name.size(), m.ptr.get());
    if (error != nullptr) {
      return Error(error);
    }
    return std::monostate();
  }

  /// Attempts to load the specified named item from this linker, returning
  /// `std::nullopt` if it was not defined.
  [[nodiscard]] std::optional<Extern>
  get(Store::Context cx, std::string_view module, std::string_view name) {
    wasmtime_extern_t item;
    if (wasmtime_linker_get(ptr.get(), cx.ptr, module.data(), module.size(),
                            name.data(), name.size(), &item)) {
      return Instance::cvt(item);
    }
    return std::nullopt;
  }

  /// Defines a new function in this linker in the style of the `Func`
  /// constructor.
  template <typename F,
            std::enable_if_t<
                std::is_invocable_r_v<Result<std::monostate, Trap>, F, Caller,
                                      Span<const Val>, Span<Val>>,
                bool> = true>
  Result<std::monostate> func_new(std::string_view module,
                                  std::string_view name, const FuncType &ty,
                                  F&& f) {

    auto *error = wasmtime_linker_define_func(
        ptr.get(), module.data(), module.length(), name.data(), name.length(),
        ty.ptr.get(), Func::raw_callback<std::remove_reference_t<F>>, std::make_unique<std::remove_reference_t<F>>(std::forward<F>(f)).release(),
        Func::raw_finalize<std::remove_reference_t<F>>);

    if (error != nullptr) {
      return Error(error);
    }

    return std::monostate();
  }

  /// Defines a new function in this linker in the style of the `Func::wrap`
  /// constructor.
  template <typename F,
            std::enable_if_t<WasmHostFunc<F>::Params::valid, bool> = true,
            std::enable_if_t<WasmHostFunc<F>::Results::valid, bool> = true>
  Result<std::monostate> func_wrap(std::string_view module,
                                   std::string_view name, F&& f) {
    using HostFunc = WasmHostFunc<F>;
    auto params = HostFunc::Params::types();
    auto results = HostFunc::Results::types();
    auto ty = FuncType::from_iters(params, results);
    auto *error = wasmtime_linker_define_func_unchecked(
        ptr.get(), module.data(), module.length(), name.data(), name.length(),
        ty.ptr.get(), Func::raw_callback_unchecked<std::remove_reference_t<F>>,
        std::make_unique<std::remove_reference_t<F>>(std::forward<F>(f)).release(), Func::raw_finalize<std::remove_reference_t<F>>);

    if (error != nullptr) {
      return Error(error);
    }

    return std::monostate();
  }

  /// Loads the "default" function, according to WASI commands and reactors, of
  /// the module named `name` in this linker.
  Result<Func> get_default(Store::Context cx, std::string_view name) {
    wasmtime_func_t item;
    auto *error = wasmtime_linker_get_default(ptr.get(), cx.ptr, name.data(),
                                              name.size(), &item);
    if (error != nullptr) {
      return Error(error);
    }
    return Func(item);
  }
};

} // namespace wasmtime

#endif // WASMTIME_HH