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    use alloc::{
        borrow::ToOwned,
        rc::Rc,
        string::{String, ToString},
        vec::Vec,
    };
    use core::cell::RefCell;
    use core::fmt;
    use Trap;

    use alloc::collections::BTreeMap;

    use core::cell::Ref;
    use func::{FuncBody, FuncInstance, FuncRef};
    use global::{GlobalInstance, GlobalRef};
    use host::Externals;
    use imports::ImportResolver;
    use memory::MemoryRef;
    use memory_units::Pages;
    use parity_wasm::elements::{External, InitExpr, Instruction, Internal, ResizableLimits, Type};
    use runner::StackRecycler;
    use table::TableRef;
    use types::{GlobalDescriptor, MemoryDescriptor, TableDescriptor};
    use validation::{DEFAULT_MEMORY_INDEX, DEFAULT_TABLE_INDEX};
    use {Error, MemoryInstance, Module, RuntimeValue, Signature, TableInstance};

    /// Reference to a [`ModuleInstance`].
    ///
    /// This reference has a reference-counting semantics.
    ///
    /// All [`ModuleInstance`] have strong references to it's components (i.e.
    /// globals, memories, funcs, tables), however, this components have
    /// weak references to it's containing module. This might be a problem
    /// at execution time.
    ///
    /// So if have to make sure that all modules which might be needed at execution time
    /// should be retained.
    ///
    /// [`ModuleInstance`]: struct.ModuleInstance.html
    #[derive(Clone, Debug)]
    pub struct ModuleRef(pub(crate) Rc<ModuleInstance>);

    impl ::core::ops::Deref for ModuleRef {
        type Target = ModuleInstance;
        fn deref(&self) -> &ModuleInstance {
            &self.0
        }
    }

    /// An external value is the runtime representation of an entity
    /// that can be imported or exported.
    pub enum ExternVal {
        /// [Function][`FuncInstance`].
        ///
        /// [`FuncInstance`]: struct.FuncInstance.html
        Func(FuncRef),
        /// [Table][`TableInstance`].
        ///
        /// [`TableInstance`]: struct.TableInstance.html
        Table(TableRef),
        /// [Memory][`MemoryInstance`].
        ///
        /// [`MemoryInstance`]: struct.MemoryInstance.html
        Memory(MemoryRef),
        /// [Global][`GlobalInstance`].
        ///
        /// Should be immutable.
        ///
        /// [`GlobalInstance`]: struct.GlobalInstance.html
        Global(GlobalRef),
    }

    impl Clone for ExternVal {
        fn clone(&self) -> Self {
            match *self {
                ExternVal::Func(ref func) => ExternVal::Func(func.clone()),
                ExternVal::Table(ref table) => ExternVal::Table(table.clone()),
                ExternVal::Memory(ref memory) => ExternVal::Memory(memory.clone()),
                ExternVal::Global(ref global) => ExternVal::Global(global.clone()),
            }
        }
    }

    impl fmt::Debug for ExternVal {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(
                f,
                "ExternVal {{ {} }}",
                match *self {
                    ExternVal::Func(_) => "Func",
                    ExternVal::Table(_) => "Table",
                    ExternVal::Memory(_) => "Memory",
                    ExternVal::Global(_) => "Global",
                }
            )
        }
    }

    impl ExternVal {
        /// Get underlying function reference if this `ExternVal` contains
        /// a function, or `None` if it is some other kind.
        pub fn as_func(&self) -> Option<&FuncRef> {
            match *self {
                ExternVal::Func(ref func) => Some(func),
                _ => None,
            }
        }

        /// Get underlying table reference if this `ExternVal` contains
        /// a table, or `None` if it is some other kind.
        pub fn as_table(&self) -> Option<&TableRef> {
            match *self {
                ExternVal::Table(ref table) => Some(table),
                _ => None,
            }
        }

        /// Get underlying memory reference if this `ExternVal` contains
        /// a memory, or `None` if it is some other kind.
        pub fn as_memory(&self) -> Option<&MemoryRef> {
            match *self {
                ExternVal::Memory(ref memory) => Some(memory),
                _ => None,
            }
        }

        /// Get underlying global variable reference if this `ExternVal` contains
        /// a global, or `None` if it is some other kind.
        pub fn as_global(&self) -> Option<&GlobalRef> {
            match *self {
                ExternVal::Global(ref global) => Some(global),
                _ => None,
            }
        }
    }

    /// A module instance is the runtime representation of a [module][`Module`].
    ///
    /// It is created by instantiating a [module][`Module`], and collects runtime representations
    /// of all entities that are imported or defined by the module, namely:
    ///
    /// - [functions][`FuncInstance`],
    /// - [memories][`MemoryInstance`],
    /// - [tables][`TableInstance`],
    /// - [globals][`GlobalInstance`],
    ///
    /// In order to instantiate a module you need to provide entities to satisfy
    /// every module's imports (i.e. wasm modules don't have optional imports).
    ///
    /// After module is instantiated you can start invoking it's exported functions with [`invoke_export`].
    ///
    /// [`Module`]: struct.Module.html
    /// [`FuncInstance`]: struct.FuncInstance.html
    /// [`MemoryInstance`]: struct.MemoryInstance.html
    /// [`TableInstance`]: struct.TableInstance.html
    /// [`GlobalInstance`]: struct.GlobalInstance.html
    /// [`invoke_export`]: #method.invoke_export
    #[derive(Debug)]
    pub struct ModuleInstance {
        signatures: RefCell<Vec<Rc<Signature>>>,
        tables: RefCell<Vec<TableRef>>,
        funcs: RefCell<Vec<FuncRef>>,
        memories: RefCell<Vec<MemoryRef>>,
        globals: RefCell<Vec<GlobalRef>>,
        exports: RefCell<BTreeMap<String, ExternVal>>,
    }

    impl ModuleInstance {
        fn default() -> Self {
            ModuleInstance {
                funcs: RefCell::new(Vec::new()),
                signatures: RefCell::new(Vec::new()),
                tables: RefCell::new(Vec::new()),
                memories: RefCell::new(Vec::new()),
                globals: RefCell::new(Vec::new()),
                exports: RefCell::new(BTreeMap::new()),
            }
        }

        pub(crate) fn memory_by_index(&self, idx: u32) -> Option<MemoryRef> {
            self.memories.borrow_mut().get(idx as usize).cloned()
        }

        pub(crate) fn table_by_index(&self, idx: u32) -> Option<TableRef> {
            self.tables.borrow_mut().get(idx as usize).cloned()
        }

        pub(crate) fn global_by_index(&self, idx: u32) -> Option<GlobalRef> {
            self.globals.borrow_mut().get(idx as usize).cloned()
        }

        pub(crate) fn func_by_index(&self, idx: u32) -> Option<FuncRef> {
            self.funcs.borrow().get(idx as usize).cloned()
        }

        pub(crate) fn signature_by_index(&self, idx: u32) -> Option<Rc<Signature>> {
            self.signatures.borrow().get(idx as usize).cloned()
        }

        fn push_func(&self, func: FuncRef) {
            self.funcs.borrow_mut().push(func);
        }

        fn push_signature(&self, signature: Rc<Signature>) {
            self.signatures.borrow_mut().push(signature)
        }

        fn push_memory(&self, memory: MemoryRef) {
            self.memories.borrow_mut().push(memory)
        }

        fn push_table(&self, table: TableRef) {
            self.tables.borrow_mut().push(table)
        }

        fn push_global(&self, global: GlobalRef) {
            self.globals.borrow_mut().push(global)
        }

        /// Access all globals. This is a non-standard API so it's unlikely to be
        /// portable to other engines.
        pub fn globals<'a>(&self) -> Ref<Vec<GlobalRef>> {
            self.globals.borrow()
        }

        fn insert_export<N: Into<String>>(&self, name: N, extern_val: ExternVal) {
            self.exports.borrow_mut().insert(name.into(), extern_val);
        }

        fn alloc_module<'i, I: Iterator<Item = &'i ExternVal>>(
            loaded_module: &Module,
            extern_vals: I,
        ) -> Result<ModuleRef, Error> {
            let module = loaded_module.module();
            let instance = ModuleRef(Rc::new(ModuleInstance::default()));

            for &Type::Function(ref ty) in module.type_section().map(|ts| ts.types()).unwrap_or(&[]) {
                let signature = Rc::new(Signature::from_elements(ty));
                instance.push_signature(signature);
            }

            {
                let mut imports = module
                    .import_section()
                    .map(|is| is.entries())
                    .unwrap_or(&[])
                    .into_iter();
                let mut extern_vals = extern_vals;
                loop {
                    // Iterate on imports and extern_vals in lockstep, a-la `Iterator:zip`.
                    // We can't use `Iterator::zip` since we want to check if lengths of both iterators are same and
                    // `Iterator::zip` just returns `None` if either of iterators return `None`.
                    let (import, extern_val) = match (imports.next(), extern_vals.next()) {
                        (Some(import), Some(extern_val)) => (import, extern_val),
                        (None, None) => break,
                        (Some(_), None) | (None, Some(_)) => {
                            return Err(Error::Instantiation(
                                "extern_vals length is not equal to import section entries".to_owned(),
                            ));
                        }
                    };

                    match (import.external(), extern_val) {
                        (&External::Function(fn_type_idx), &ExternVal::Func(ref func)) => {
                            let expected_fn_type = instance
                                .signature_by_index(fn_type_idx)
                                .expect("Due to validation function type should exists");
                            let actual_fn_type = func.signature();
                            if &*expected_fn_type != actual_fn_type {
                                return Err(Error::Instantiation(format!(
    								"Expected function with type {:?}, but actual type is {:?} for entry {}",
    								expected_fn_type,
    								actual_fn_type,
    								import.field(),
    							)));
                            }
                            instance.push_func(func.clone())
                        }
                        (&External::Table(ref tt), &ExternVal::Table(ref table)) => {
                            match_limits(table.limits(), tt.limits())?;
                            instance.push_table(table.clone());
                        }
                        (&External::Memory(ref mt), &ExternVal::Memory(ref memory)) => {
                            match_limits(memory.limits(), mt.limits())?;
                            instance.push_memory(memory.clone());
                        }
                        (&External::Global(ref gl), &ExternVal::Global(ref global)) => {
                            if gl.content_type() != global.elements_value_type() {
                                return Err(Error::Instantiation(format!(
                                    "Expect global with {:?} type, but provided global with {:?} type",
                                    gl.content_type(),
                                    global.value_type(),
                                )));
                            }
                            instance.push_global(global.clone());
                        }
                        (expected_import, actual_extern_val) => {
                            return Err(Error::Instantiation(format!(
                                "Expected {:?} type, but provided {:?} extern_val",
                                expected_import, actual_extern_val
                            )));
                        }
                    }
                }
            }

            let code = loaded_module.code();
            {
                let funcs = module
                    .function_section()
                    .map(|fs| fs.entries())
                    .unwrap_or(&[]);
                let bodies = module.code_section().map(|cs| cs.bodies()).unwrap_or(&[]);
                debug_assert!(
                    funcs.len() == bodies.len(),
                    "Due to validation func and body counts must match"
                );

                for (index, (ty, body)) in
                    Iterator::zip(funcs.into_iter(), bodies.into_iter()).enumerate()
                {
                    let signature = instance
                        .signature_by_index(ty.type_ref())
                        .expect("Due to validation type should exists");
                    let code = code.get(index).expect(
    					"At func validation time labels are collected; Collected labels are added by index; qed",
    				).clone();
                    let func_body = FuncBody {
                        locals: body.locals().to_vec(),
                        code: code,
                    };
                    let func_instance =
                        FuncInstance::alloc_internal(Rc::downgrade(&instance.0), signature, func_body);
                    instance.push_func(func_instance);
                }
            }

            for table_type in module.table_section().map(|ts| ts.entries()).unwrap_or(&[]) {
                let table =
                    TableInstance::alloc(table_type.limits().initial(), table_type.limits().maximum())?;
                instance.push_table(table);
            }

            for memory_type in module
                .memory_section()
                .map(|ms| ms.entries())
                .unwrap_or(&[])
            {
                let initial: Pages = Pages(memory_type.limits().initial() as usize);
                let maximum: Option<Pages> = memory_type.limits().maximum().map(|m| Pages(m as usize));

                let memory = MemoryInstance::alloc(initial, maximum)
                    .expect("Due to validation `initial` and `maximum` should be valid");
                instance.push_memory(memory);
            }

            for global_entry in module
                .global_section()
                .map(|gs| gs.entries())
                .unwrap_or(&[])
            {
                let init_val = eval_init_expr(global_entry.init_expr(), &*instance);
                let global = GlobalInstance::alloc(init_val, global_entry.global_type().is_mutable());
                instance.push_global(global);
            }

            for export in module
                .export_section()
                .map(|es| es.entries())
                .unwrap_or(&[])
            {
                let field = export.field();
                let extern_val: ExternVal = match *export.internal() {
                    Internal::Function(idx) => {
                        let func = instance
                            .func_by_index(idx)
                            .expect("Due to validation func should exists");
                        ExternVal::Func(func)
                    }
                    Internal::Global(idx) => {
                        let global = instance
                            .global_by_index(idx)
                            .expect("Due to validation global should exists");
                        ExternVal::Global(global)
                    }
                    Internal::Memory(idx) => {
                        let memory = instance
                            .memory_by_index(idx)
                            .expect("Due to validation memory should exists");
                        ExternVal::Memory(memory)
                    }
                    Internal::Table(idx) => {
                        let table = instance
                            .table_by_index(idx)
                            .expect("Due to validation table should exists");
                        ExternVal::Table(table)
                    }
                };
                instance.insert_export(field, extern_val);
            }

            Ok(instance)
        }

        /// Instantiate a module with given [external values][ExternVal] as imports.
        ///
        /// See [new] for details.
        ///
        /// [new]: #method.new
        /// [ExternVal]: https://webassembly.github.io/spec/core/exec/runtime.html#syntax-externval
        pub fn with_externvals<'a, 'i, I: Iterator<Item = &'i ExternVal>>(
            loaded_module: &'a Module,
            extern_vals: I,
        ) -> Result<NotStartedModuleRef<'a>, Error> {
            let module = loaded_module.module();

            let module_ref = ModuleInstance::alloc_module(loaded_module, extern_vals)?;

            for element_segment in module
                .elements_section()
                .map(|es| es.entries())
                .unwrap_or(&[])
            {
                let offset = element_segment
                    .offset()
                    .as_ref()
                    .expect("passive segments are rejected due to validation");
                let offset_val = match eval_init_expr(offset, &module_ref) {
                    RuntimeValue::I32(v) => v as u32,
                    _ => panic!("Due to validation elem segment offset should evaluate to i32"),
                };

                let table_inst = module_ref
                    .table_by_index(DEFAULT_TABLE_INDEX)
                    .expect("Due to validation default table should exists");

                // This check is not only for bailing out early, but also to check the case when
                // segment consist of 0 members.
                if offset_val as u64 + element_segment.members().len() as u64
                    > table_inst.current_size() as u64
                {
                    return Err(Error::Instantiation(
                        "elements segment does not fit".to_string(),
                    ));
                }

                for (j, func_idx) in element_segment.members().into_iter().enumerate() {
                    let func = module_ref
                        .func_by_index(*func_idx)
                        .expect("Due to validation funcs from element segments should exists");

                    table_inst.set(offset_val + j as u32, Some(func))?;
                }
            }

            for data_segment in module.data_section().map(|ds| ds.entries()).unwrap_or(&[]) {
                let offset = data_segment
                    .offset()
                    .as_ref()
                    .expect("passive segments are rejected due to validation");
                let offset_val = match eval_init_expr(offset, &module_ref) {
                    RuntimeValue::I32(v) => v as u32,
                    _ => panic!("Due to validation data segment offset should evaluate to i32"),
                };

                let memory_inst = module_ref
                    .memory_by_index(DEFAULT_MEMORY_INDEX)
                    .expect("Due to validation default memory should exists");
                memory_inst.set(offset_val, data_segment.value())?;
            }

            Ok(NotStartedModuleRef {
                loaded_module,
                instance: module_ref,
            })
        }

        /// Instantiate a [module][`Module`].
        ///
        /// Note that in case of successful instantiation this function returns a reference to
        /// a module which `start` function is not called.
        /// In order to complete instantiatiation `start` function must be called. However, there are
        /// situations where you might need to do additional setup before calling `start` function.
        /// For such sitations this separation might be useful.
        ///
        /// See [`NotStartedModuleRef`] for details.
        ///
        /// # Errors
        ///
        /// Returns `Err` if the module cannot be instantiated.
        ///
        /// This can happen if one of the imports can't
        /// be satisfied (e.g module isn't registered in `imports` [resolver][`ImportResolver`]) or
        /// there is a mismatch between requested import and provided (e.g. module requested memory with no
        /// maximum size limit, however, was provided memory with the maximum size limit).
        ///
        /// # Examples
        ///
        /// ```rust
        /// use wasmi::{ModuleInstance, ImportsBuilder, NopExternals};
        /// # fn func() -> Result<(), ::wasmi::Error> {
        /// # let module = wasmi::Module::from_buffer(&[0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00]).unwrap();
        ///
        /// // ModuleInstance::new returns instance which `start` function isn't called.
        /// let not_started = ModuleInstance::new(
        ///     &module,
        ///     &ImportsBuilder::default()
        /// )?;
        /// // Call `start` function if any.
        /// let instance = not_started.run_start(&mut NopExternals)?;
        ///
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// If you sure that the module doesn't have `start` function you can use [`assert_no_start`] to get
        /// instantiated module without calling `start` function.
        ///
        /// ```rust
        /// use wasmi::{ModuleInstance, ImportsBuilder, NopExternals};
        /// # fn func() -> Result<(), ::wasmi::Error> {
        /// # let module = wasmi::Module::from_buffer(&[0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00]).unwrap();
        ///
        /// // This will panic if the module actually contain `start` function.
        /// let not_started = ModuleInstance::new(
        ///     &module,
        ///     &ImportsBuilder::default()
        /// )?.assert_no_start();
        ///
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// [`Module`]: struct.Module.html
        /// [`NotStartedModuleRef`]: struct.NotStartedModuleRef.html
        /// [`ImportResolver`]: trait.ImportResolver.html
        /// [`assert_no_start`]: struct.NotStartedModuleRef.html#method.assert_no_start
        pub fn new<'m, I: ImportResolver>(
            loaded_module: &'m Module,
            imports: &I,
        ) -> Result<NotStartedModuleRef<'m>, Error> {
            let module = loaded_module.module();

            let mut extern_vals = Vec::new();
            for import_entry in module.import_section().map(|s| s.entries()).unwrap_or(&[]) {
                let module_name = import_entry.module();
                let field_name = import_entry.field();
                let extern_val = match *import_entry.external() {
                    External::Function(fn_ty_idx) => {
                        let types = module.type_section().map(|s| s.types()).unwrap_or(&[]);
                        let &Type::Function(ref func_type) = types
                            .get(fn_ty_idx as usize)
                            .expect("Due to validation functions should have valid types");
                        let signature = Signature::from_elements(func_type);
                        let func = imports.resolve_func(module_name, field_name, &signature)?;
                        ExternVal::Func(func)
                    }
                    External::Table(ref table_type) => {
                        let table_descriptor = TableDescriptor::from_elements(table_type);
                        let table =
                            imports.resolve_table(module_name, field_name, &table_descriptor)?;
                        ExternVal::Table(table)
                    }
                    External::Memory(ref memory_type) => {
                        let memory_descriptor = MemoryDescriptor::from_elements(memory_type);
                        let memory =
                            imports.resolve_memory(module_name, field_name, &memory_descriptor)?;
                        ExternVal::Memory(memory)
                    }
                    External::Global(ref global_type) => {
                        let global_descriptor = GlobalDescriptor::from_elements(global_type);
                        let global =
                            imports.resolve_global(module_name, field_name, &global_descriptor)?;
                        ExternVal::Global(global)
                    }
                };
                extern_vals.push(extern_val);
            }

            Self::with_externvals(loaded_module, extern_vals.iter())
        }

        /// Invoke exported function by a name.
        ///
        /// This function finds exported function by a name, and calls it with provided arguments and
        /// external state.
        ///
        /// # Errors
        ///
        /// Returns `Err` if:
        ///
        /// - there are no export with a given name or this export is not a function,
        /// - given arguments doesn't match to function signature,
        /// - trap occurred at the execution time,
        ///
        /// # Examples
        ///
        /// Invoke a function that takes two numbers and returns sum of them.
        ///
        /// ```rust
        /// # extern crate wasmi;
        /// # extern crate wabt;
        /// # use wasmi::{ModuleInstance, ImportsBuilder, NopExternals, RuntimeValue};
        /// # fn main() {
        /// # let wasm_binary: Vec<u8> = wabt::wat2wasm(
        /// #   r#"
        /// #   (module
        /// #       (func (export "add") (param i32 i32) (result i32)
        /// #           get_local 0
        /// #           get_local 1
        /// #           i32.add
        /// #       )
        /// #   )
        /// #   "#,
        /// # ).expect("failed to parse wat");
        /// # let module = wasmi::Module::from_buffer(&wasm_binary).expect("failed to load wasm");
        /// # let instance = ModuleInstance::new(
        /// # &module,
        /// # &ImportsBuilder::default()
        /// # ).expect("failed to instantiate wasm module").assert_no_start();
        /// assert_eq!(
        ///     instance.invoke_export(
        ///         "add",
        ///         &[RuntimeValue::I32(5), RuntimeValue::I32(3)],
        ///         &mut NopExternals,
        ///     ).expect("failed to execute export"),
        ///     Some(RuntimeValue::I32(8)),
        /// );
        /// # }
        /// ```
        pub fn invoke_export<E: Externals>(
            &self,
            func_name: &str,
            args: &[RuntimeValue],
            externals: &mut E,
        ) -> Result<Option<RuntimeValue>, Error> {
            let func_instance = self.func_by_name(func_name)?;

            FuncInstance::invoke(&func_instance, args, externals).map_err(|t| Error::Trap(t))
        }

        /// Invoke exported function by a name using recycled stacks.
        ///
        /// # Errors
        ///
        /// Same as [`invoke_export`].
        ///
        /// [`invoke_export`]: #method.invoke_export
        pub fn invoke_export_with_stack<E: Externals>(
            &self,
            func_name: &str,
            args: &[RuntimeValue],
            externals: &mut E,
            stack_recycler: &mut StackRecycler,
        ) -> Result<Option<RuntimeValue>, Error> {
            let func_instance = self.func_by_name(func_name)?;

            FuncInstance::invoke_with_stack(&func_instance, args, externals, stack_recycler)
                .map_err(|t| Error::Trap(t))
        }

        fn func_by_name(&self, func_name: &str) -> Result<FuncRef, Error> {
            let extern_val = self
                .export_by_name(func_name)
                .ok_or_else(|| Error::Function(format!("Module doesn't have export {}", func_name)))?;

            match extern_val {
                ExternVal::Func(func_instance) => Ok(func_instance),
                unexpected => Err(Error::Function(format!(
                    "Export {} is not a function, but {:?}",
                    func_name, unexpected
                ))),
            }
        }

        /// Find export by a name.
        ///
        /// Returns `None` if there is no export with such name.
        pub fn export_by_name(&self, name: &str) -> Option<ExternVal> {
            self.exports.borrow().get(name).cloned()
        }
    }

    /// Mostly instantiated [`ModuleRef`].
    ///
    /// At this point memory segments and tables are copied. However, `start` function (if any) is not called.
    /// To get [fully instantiated module instance][`ModuleRef`], [running `start` function][`run_start`] is required.
    ///
    /// You can still access not fully initialized instance by calling [`not_started_instance`],
    /// but keep in mind, that this is sort of escape hatch: module really might depend on initialization
    /// done in `start` function. It's definitely not recommended to call any exports on [`ModuleRef`]
    /// returned by this function.
    ///
    /// If you sure, that there is no `start` function (e.g. because you created it without one), you can
    /// call [`assert_no_start`] which returns [`ModuleRef`] without calling `start` function. However,
    /// it will panic if module contains `start` function.
    ///
    /// [`ModuleRef`]: struct.ModuleRef.html
    /// [`run_start`]: #method.run_start
    /// [`assert_no_start`]: #method.assert_no_start
    /// [`not_started_instance`]: #method.not_started_instance
    pub struct NotStartedModuleRef<'a> {
        loaded_module: &'a Module,
        instance: ModuleRef,
    }

    impl<'a> NotStartedModuleRef<'a> {
        /// Returns not fully initialized instance.
        ///
        /// To fully initialize the instance you need to call either [`run_start`] or
        /// [`assert_no_start`]. See struct documentation for details.
        ///
        /// [`NotStartedModuleRef`]: struct.NotStartedModuleRef.html
        /// [`ModuleRef`]: struct.ModuleRef.html
        /// [`run_start`]: #method.run_start
        /// [`assert_no_start`]: #method.assert_no_start
        pub fn not_started_instance(&self) -> &ModuleRef {
            &self.instance
        }

        /// Executes `start` function (if any) and returns fully instantiated module.
        ///
        /// # Errors
        ///
        /// Returns `Err` if start function traps.
        pub fn run_start<E: Externals>(self, state: &mut E) -> Result<ModuleRef, Trap> {
            if let Some(start_fn_idx) = self.loaded_module.module().start_section() {
                let start_func = self
                    .instance
                    .func_by_index(start_fn_idx)
                    .expect("Due to validation start function should exists");
                FuncInstance::invoke(&start_func, &[], state)?;
            }
            Ok(self.instance)
        }

        /// Returns fully instantiated module without running `start` function.
        ///
        /// # Panics
        ///
        /// This function panics if original module contains `start` function.
        pub fn assert_no_start(self) -> ModuleRef {
            if self.loaded_module.module().start_section().is_some() {
                panic!("assert_no_start called on module with `start` function");
            }
            self.instance
        }

        /// Whether or not the module has a `start` function.
        ///
        /// Returns `true` if it has a `start` function.
        pub fn has_start(&self) -> bool {
            self.loaded_module.module().start_section().is_some()
        }
    }

    fn eval_init_expr(init_expr: &InitExpr, module: &ModuleInstance) -> RuntimeValue {
        let code = init_expr.code();
        debug_assert!(
            code.len() == 2,
            "Due to validation `code`.len() should be 2"
        );
        match code[0] {
            Instruction::I32Const(v) => v.into(),
            Instruction::I64Const(v) => v.into(),
            Instruction::F32Const(v) => RuntimeValue::decode_f32(v),
            Instruction::F64Const(v) => RuntimeValue::decode_f64(v),
            Instruction::GetGlobal(idx) => {
                let global = module
                    .global_by_index(idx)
                    .expect("Due to validation global should exists in module");
                global.get()
            }
            _ => panic!("Due to validation init should be a const expr"),
        }
    }

    fn match_limits(l1: &ResizableLimits, l2: &ResizableLimits) -> Result<(), Error> {
        if l1.initial() < l2.initial() {
            return Err(Error::Instantiation(format!(
                "trying to import with limits l1.initial={} and l2.initial={}",
                l1.initial(),
                l2.initial()
            )));
        }

        match (l1.maximum(), l2.maximum()) {
            (_, None) => (),
            (Some(m1), Some(m2)) if m1 <= m2 => (),
            _ => {
                return Err(Error::Instantiation(format!(
                    "trying to import with limits l1.max={:?} and l2.max={:?}",
                    l1.maximum(),
                    l2.maximum()
                )));
            }
        }

        Ok(())
    }

    pub fn check_limits(limits: &ResizableLimits) -> Result<(), Error> {
        if let Some(maximum) = limits.maximum() {
            if maximum < limits.initial() {
                return Err(Error::Instantiation(format!(
                    "maximum limit {} is less than minimum {}",
                    maximum,
                    limits.initial()
                )));
            }
        }

        Ok(())
    }

    #[cfg(test)]
    mod tests {
        use super::{ExternVal, ModuleInstance};
        use func::FuncInstance;
        use imports::ImportsBuilder;
        use tests::parse_wat;
        use types::{Signature, ValueType};

        #[should_panic]
        #[test]
        fn assert_no_start_panics_on_module_with_start() {
            let module_with_start = parse_wat(
                r#"
    			(module
    				(func $f)
    				(start $f))
    			"#,
            );
            let module = ModuleInstance::new(&module_with_start, &ImportsBuilder::default()).unwrap();
            assert!(!module.has_start());
            module.assert_no_start();
        }

        #[test]
        fn imports_provided_by_externvals() {
            let module_with_single_import = parse_wat(
                r#"
    			(module
    				(import "foo" "bar" (func))
    				)
    			"#,
            );

            assert!(ModuleInstance::with_externvals(
                &module_with_single_import,
                [ExternVal::Func(FuncInstance::alloc_host(
                    Signature::new(&[][..], None),
                    0
                ),)]
                .iter(),
            )
            .is_ok());

            // externval vector is longer than import count.
            assert!(ModuleInstance::with_externvals(
                &module_with_single_import,
                [
                    ExternVal::Func(FuncInstance::alloc_host(Signature::new(&[][..], None), 0)),
                    ExternVal::Func(FuncInstance::alloc_host(Signature::new(&[][..], None), 1)),
                ]
                .iter(),
            )
            .is_err());

            // externval vector is shorter than import count.
            assert!(ModuleInstance::with_externvals(&module_with_single_import, [].iter(),).is_err());

            // externval vector has an unexpected type.
            assert!(ModuleInstance::with_externvals(
                &module_with_single_import,
                [ExternVal::Func(FuncInstance::alloc_host(
                    Signature::new(&[][..], Some(ValueType::I32)),
                    0
                ),)]
                .iter(),
            )
            .is_err());
        }
    }

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