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    use core::any::TypeId;
    use value::{FromRuntimeValue, RuntimeValue};
    use {Trap, TrapKind};

    /// Wrapper around slice of [`RuntimeValue`] for using it
    /// as an argument list conveniently.
    ///
    /// [`RuntimeValue`]: enum.RuntimeValue.html
    #[derive(Debug)]
    pub struct RuntimeArgs<'a>(&'a [RuntimeValue]);

    impl<'a> From<&'a [RuntimeValue]> for RuntimeArgs<'a> {
        fn from(inner: &'a [RuntimeValue]) -> Self {
            RuntimeArgs(inner)
        }
    }

    impl<'a> AsRef<[RuntimeValue]> for RuntimeArgs<'a> {
        fn as_ref(&self) -> &[RuntimeValue] {
            self.0
        }
    }

    impl<'a> RuntimeArgs<'a> {
        /// Extract argument by index `idx`.
        ///
        /// # Errors
        ///
        /// Returns `Err` if cast is invalid or not enough arguments.
        pub fn nth_checked<T>(&self, idx: usize) -> Result<T, Trap>
        where
            T: FromRuntimeValue,
        {
            Ok(self
                .nth_value_checked(idx)?
                .try_into()
                .ok_or_else(|| TrapKind::UnexpectedSignature)?)
        }

        /// Extract argument as a [`RuntimeValue`] by index `idx`.
        ///
        /// # Errors
        ///
        /// Returns `Err` if this list has not enough arguments.
        ///
        /// [`RuntimeValue`]: enum.RuntimeValue.html
        pub fn nth_value_checked(&self, idx: usize) -> Result<RuntimeValue, Trap> {
            if self.0.len() <= idx {
                return Err(TrapKind::UnexpectedSignature.into());
            }
            Ok(self.0[idx])
        }

        /// Extract argument by index `idx`.
        ///
        /// # Panics
        ///
        /// Panics if cast is invalid or not enough arguments.
        pub fn nth<T>(&self, idx: usize) -> T
        where
            T: FromRuntimeValue,
        {
            let value = self.nth_value_checked(idx).expect("Invalid argument index");
            value.try_into().expect("Unexpected argument type")
        }

        /// Total number of arguments
        pub fn len(&self) -> usize {
            self.0.len()
        }
    }

    /// Trait that allows the host to return custom error.
    ///
    /// It should be useful for representing custom traps,
    /// troubles at instantiation time or other host specific conditions.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::fmt;
    /// use wasmi::{Error, HostError};
    ///
    /// #[derive(Debug)]
    /// struct MyError {
    ///     code: u32,
    /// }
    ///
    /// impl fmt::Display for MyError {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(f, "MyError, code={}", self.code)
    ///     }
    /// }
    ///
    /// impl HostError for MyError { }
    ///
    /// fn failable_fn() -> Result<(), Error> {
    ///     let my_error = MyError { code: 1312 };
    ///     Err(Error::Host(Box::new(my_error)))
    /// }
    ///
    /// match failable_fn() {
    ///     Err(Error::Host(host_error)) => {
    ///         let my_error = host_error.downcast_ref::<MyError>().unwrap();
    ///         assert_eq!(my_error.code, 1312);
    ///     }
    ///     _ => panic!(),
    /// }
    /// ```
    pub trait HostError: 'static + ::core::fmt::Display + ::core::fmt::Debug + Send + Sync {
        #[doc(hidden)]
        fn __private_get_type_id__(&self) -> TypeId {
            TypeId::of::<Self>()
        }
    }

    impl dyn HostError {
        /// Attempt to downcast this `HostError` to a concrete type by reference.
        pub fn downcast_ref<T: HostError>(&self) -> Option<&T> {
            if self.__private_get_type_id__() == TypeId::of::<T>() {
                unsafe { Some(&*(self as *const dyn HostError as *const T)) }
            } else {
                None
            }
        }

        /// Attempt to downcast this `HostError` to a concrete type by mutable
        /// reference.
        pub fn downcast_mut<T: HostError>(&mut self) -> Option<&mut T> {
            if self.__private_get_type_id__() == TypeId::of::<T>() {
                unsafe { Some(&mut *(self as *mut dyn HostError as *mut T)) }
            } else {
                None
            }
        }
    }

    /// Trait that allows to implement host functions.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use wasmi::{
    ///     Externals, RuntimeValue, RuntimeArgs, Error, ModuleImportResolver,
    ///     FuncRef, ValueType, Signature, FuncInstance, Trap,
    /// };
    ///
    /// struct HostExternals {
    ///     counter: usize,
    /// }
    ///
    /// const ADD_FUNC_INDEX: usize = 0;
    ///
    /// impl Externals for HostExternals {
    ///     fn invoke_index(
    ///         &mut self,
    ///         index: usize,
    ///         args: RuntimeArgs,
    ///     ) -> Result<Option<RuntimeValue>, Trap> {
    ///         match index {
    ///             ADD_FUNC_INDEX => {
    ///                 let a: u32 = args.nth_checked(0)?;
    ///                 let b: u32 = args.nth_checked(1)?;
    ///                 let result = a + b;
    ///
    ///                 Ok(Some(RuntimeValue::I32(result as i32)))
    ///             }
    ///             _ => panic!("Unimplemented function at {}", index),
    ///         }
    ///     }
    /// }
    ///
    /// impl HostExternals {
    ///     fn check_signature(
    ///         &self,
    ///         index: usize,
    ///         signature: &Signature
    ///     ) -> bool {
    ///         let (params, ret_ty): (&[ValueType], Option<ValueType>) = match index {
    ///             ADD_FUNC_INDEX => (&[ValueType::I32, ValueType::I32], Some(ValueType::I32)),
    ///             _ => return false,
    ///         };
    ///         signature.params() == params && signature.return_type() == ret_ty
    ///     }
    /// }
    ///
    /// impl ModuleImportResolver for HostExternals {
    ///     fn resolve_func(
    ///         &self,
    ///         field_name: &str,
    ///         signature: &Signature
    ///     ) -> Result<FuncRef, Error> {
    ///         let index = match field_name {
    ///             "add" => ADD_FUNC_INDEX,
    ///             _ => {
    ///                 return Err(Error::Instantiation(
    ///                     format!("Export {} not found", field_name),
    ///                 ))
    ///             }
    ///         };
    ///
    ///         if !self.check_signature(index, signature) {
    ///             return Err(Error::Instantiation(
    ///                 format!("Export {} has a bad signature", field_name)
    ///             ));
    ///         }
    ///
    ///         Ok(FuncInstance::alloc_host(
    ///             Signature::new(&[ValueType::I32, ValueType::I32][..], Some(ValueType::I32)),
    ///             index,
    ///         ))
    ///     }
    /// }
    /// ```
    pub trait Externals {
        /// Perform invoke of a host function by specified `index`.
        fn invoke_index(
            &mut self,
            index: usize,
            args: RuntimeArgs,
        ) -> Result<Option<RuntimeValue>, Trap>;
    }

    /// Implementation of [`Externals`] that just traps on [`invoke_index`].
    ///
    /// [`Externals`]: trait.Externals.html
    /// [`invoke_index`]: trait.Externals.html#tymethod.invoke_index
    pub struct NopExternals;

    impl Externals for NopExternals {
        fn invoke_index(
            &mut self,
            _index: usize,
            _args: RuntimeArgs,
        ) -> Result<Option<RuntimeValue>, Trap> {
            Err(TrapKind::Unreachable.into())
        }
    }

    #[cfg(test)]
    mod tests {

        use super::{HostError, RuntimeArgs};
        use value::RuntimeValue;

        #[test]
        fn i32_runtime_args() {
            let args: RuntimeArgs = (&[RuntimeValue::I32(0)][..]).into();
            let val: i32 = args.nth_checked(0).unwrap();
            assert_eq!(val, 0);
        }

        #[test]
        fn i64_invalid_arg_cast() {
            let args: RuntimeArgs = (&[RuntimeValue::I64(90534534545322)][..]).into();
            assert!(args.nth_checked::<i32>(0).is_err());
        }

        // Tests that `HostError` trait is object safe.
        fn _host_error_is_object_safe(_: &dyn HostError) {}
    }

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