wasmtime/runtime/

instance.rs

use crate::linker::{Definition, DefinitionType};
use crate::prelude::*;
use crate::runtime::vm::{
    self, Imports, ModuleRuntimeInfo, UncaughtException, VMFuncRef, VMFunctionImport,
    VMGlobalImport, VMMemoryImport, VMStore, VMTableImport, VMTagImport,
};
use crate::store::{
    AllocateInstanceKind, Asyncness, InstanceId, StoreInstanceId, StoreOpaque, StoreResourceLimiter,
};
use crate::types::matching;
use crate::{
    AsContextMut, Engine, Export, Extern, Func, Global, Memory, Module, ModuleExport, SharedMemory,
    StoreContext, StoreContextMut, Table, Tag, TypedFunc,
};
use alloc::sync::Arc;
use core::ptr::NonNull;
use wasmtime_environ::{
    EntityIndex, EntityType, FuncIndex, GlobalIndex, MemoryIndex, TableIndex, TagIndex, TypeTrace,
};

/// An instantiated WebAssembly module.
///
/// This type represents the instantiation of a [`Module`]. Once instantiated
/// you can access the [`exports`](Instance::exports) which are of type
/// [`Extern`] and provide the ability to call functions, set globals, read
/// memory, etc. When interacting with any wasm code you'll want to make an
/// [`Instance`] to call any code or execute anything.
///
/// Instances are owned by a [`Store`](crate::Store) which is passed in at
/// creation time. It's recommended to create instances with
/// [`Linker::instantiate`](crate::Linker::instantiate) or similar
/// [`Linker`](crate::Linker) methods, but a more low-level constructor is also
/// available as [`Instance::new`].
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
#[repr(C)]
pub struct Instance {
    pub(crate) id: StoreInstanceId,
}

// Double-check that the C representation in `instance.h` matches our in-Rust
// representation here in terms of size/alignment/etc.
const _: () = {
    #[repr(C)]
    struct C(u64, usize);
    assert!(core::mem::size_of::<C>() == core::mem::size_of::<Instance>());
    assert!(core::mem::align_of::<C>() == core::mem::align_of::<Instance>());
    assert!(core::mem::offset_of!(Instance, id) == 0);
};

impl Instance {
    /// Creates a new [`Instance`] from the previously compiled [`Module`] and
    /// list of `imports` specified.
    ///
    /// This method instantiates the `module` provided with the `imports`,
    /// following the procedure in the [core specification][inst] to
    /// instantiate. Instantiation can fail for a number of reasons (many
    /// specified below), but if successful the `start` function will be
    /// automatically run (if specified in the `module`) and then the
    /// [`Instance`] will be returned.
    ///
    /// Per the WebAssembly spec, instantiation includes running the module's
    /// start function, if it has one (not to be confused with the `_start`
    /// function, which is not run).
    ///
    /// Note that this is a low-level function that just performs an
    /// instantiation. See the [`Linker`](crate::Linker) struct for an API which
    /// provides a convenient way to link imports and provides automatic Command
    /// and Reactor behavior.
    ///
    /// ## Providing Imports
    ///
    /// The entries in the list of `imports` are intended to correspond 1:1
    /// with the list of imports returned by [`Module::imports`]. Before
    /// calling [`Instance::new`] you'll want to inspect the return value of
    /// [`Module::imports`] and, for each import type, create an [`Extern`]
    /// which corresponds to that type.  These [`Extern`] values are all then
    /// collected into a list and passed to this function.
    ///
    /// Note that this function is intentionally relatively low level. For an
    /// easier time passing imports by doing name-based resolution it's
    /// recommended to instead use the [`Linker`](crate::Linker) type.
    ///
    /// ## Errors
    ///
    /// This function can fail for a number of reasons, including, but not
    /// limited to:
    ///
    /// * The number of `imports` provided doesn't match the number of imports
    ///   returned by the `module`'s [`Module::imports`] method.
    /// * The type of any [`Extern`] doesn't match the corresponding
    ///   [`ExternType`] entry that it maps to.
    /// * The `start` function in the instance, if present, traps.
    /// * Module/instance resource limits are exceeded.
    /// * The `store` provided requires the use of [`Instance::new_async`]
    ///   instead, such as if epochs or fuel are configured.
    ///
    /// When instantiation fails it's recommended to inspect the return value to
    /// see why it failed, or bubble it upwards. If you'd like to specifically
    /// check for trap errors, you can use `error.downcast::<Trap>()`. For more
    /// about error handling see the [`Trap`] documentation.
    ///
    /// [`Trap`]: crate::Trap
    ///
    /// # Panics
    ///
    /// This function will panic if any [`Extern`] supplied is not owned by
    /// `store`.
    ///
    /// [inst]: https://webassembly.github.io/spec/core/exec/modules.html#exec-instantiation
    /// [`ExternType`]: crate::ExternType
    ///
    /// This function will return an [`OutOfMemory`][crate::OutOfMemory] error when
    /// memory allocation fails. See the `OutOfMemory` type's documentation for
    /// details on Wasmtime's out-of-memory handling.
    pub fn new(
        mut store: impl AsContextMut,
        module: &Module,
        imports: &[Extern],
    ) -> Result<Instance> {
        let mut store = store.as_context_mut();
        store.0.validate_sync_call()?;
        let imports = Instance::typecheck_externs(store.0, module, imports)?;
        // Note that the unsafety here should be satisfied by the call to
        // `typecheck_externs` above which satisfies the condition that all
        // the imports are valid for this module.
        vm::assert_ready(unsafe {
            Instance::new_started(&mut store, module, imports.as_ref(), Asyncness::No)
        })
    }

    /// Same as [`Instance::new`], except for usage in [asynchronous stores].
    ///
    /// For more details about this function see the documentation on
    /// [`Instance::new`]. The only difference between these two methods is that
    /// this one will asynchronously invoke the wasm start function in case it
    /// calls any imported function which is an asynchronous host function (e.g.
    /// created with [`Func::new_async`](crate::Func::new_async).
    ///
    /// # Panics
    ///
    /// This function will panic, like [`Instance::new`], if any [`Extern`]
    /// specified does not belong to `store`.
    ///
    /// # Examples
    ///
    /// An example of using this function:
    ///
    /// ```
    /// use wasmtime::{Result, Store, Engine, Module, Instance};
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<()> {
    ///     let engine = Engine::default();
    ///
    ///     // For this example, a module with no imports is being used hence
    ///     // the empty array to `Instance::new_async`.
    ///     let module = Module::new(&engine, "(module)")?;
    ///     let mut store = Store::new(&engine, ());
    ///     let instance = Instance::new_async(&mut store, &module, &[]).await?;
    ///
    ///     // ... use `instance` and exports and such ...
    ///
    ///     Ok(())
    /// }
    /// ```
    ///
    /// Note, though, that the future returned from this function is only
    /// `Send` if the store's own data is `Send` meaning that this does not
    /// compile for example:
    ///
    /// ```compile_fail
    /// use wasmtime::{Result, Store, Engine, Module, Instance};
    /// use std::rc::Rc;
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<()> {
    ///     let engine = Engine::default();
    ///
    ///     let module = Module::new(&engine, "(module)")?;
    ///
    ///     // Note that `Rc<()>` is NOT `Send`, which is what many future
    ///     // runtimes require and below will cause a failure.
    ///     let mut store = Store::new(&engine, Rc::new(()));
    ///
    ///     // Compile failure because `Store<Rc<()>>` is not `Send`
    ///     assert_send(Instance::new_async(&mut store, &module, &[])).await?;
    ///
    ///     Ok(())
    /// }
    ///
    /// fn assert_send<T: Send>(t: T) -> T { t }
    /// ```
    ///
    /// # Errors
    ///
    /// This function will return an [`OutOfMemory`][crate::OutOfMemory] error when
    /// memory allocation fails. See the `OutOfMemory` type's documentation for
    /// details on Wasmtime's out-of-memory handling.
    #[cfg(feature = "async")]
    pub async fn new_async(
        mut store: impl AsContextMut,
        module: &Module,
        imports: &[Extern],
    ) -> Result<Instance> {
        let mut store = store.as_context_mut();
        let imports = Instance::typecheck_externs(store.0, module, imports)?;
        // See `new` for notes on this unsafety
        unsafe { Instance::new_started(&mut store, module, imports.as_ref(), Asyncness::Yes).await }
    }

    fn typecheck_externs(
        store: &mut StoreOpaque,
        module: &Module,
        imports: &[Extern],
    ) -> Result<OwnedImports> {
        for import in imports {
            if !import.comes_from_same_store(store) {
                bail!("cross-`Store` instantiation is not currently supported");
            }
        }

        typecheck(store.engine(), module, imports, |cx, ty, item| {
            let item = DefinitionType::from(store, item);
            cx.definition(ty, &item)
        })?;

        // When pushing functions into `OwnedImports` it's required that their
        // `wasm_call` fields are all filled out. This `module` is guaranteed
        // to have any trampolines necessary for functions so register the
        // module with the store and then attempt to fill out any outstanding
        // holes.
        //
        // Note that under normal operation this shouldn't do much as the list
        // of funcs-with-holes should generally be empty. As a result the
        // process of filling this out is not super optimized at this point.
        let (modules, engine, breakpoints) = store.modules_and_engine_and_breakpoints_mut();
        modules.register_module(module, engine, breakpoints)?;
        let (funcrefs, modules) = store.func_refs_and_modules();
        funcrefs.fill(modules);

        let mut owned_imports = OwnedImports::new(module)?;
        for import in imports {
            owned_imports.push(import, store)?;
        }
        Ok(owned_imports)
    }

    /// Internal function to create an instance and run the start function.
    ///
    /// This function's unsafety is the same as `Instance::new_raw`.
    pub(crate) async unsafe fn new_started<T>(
        store: &mut StoreContextMut<'_, T>,
        module: &Module,
        imports: Imports<'_>,
        asyncness: Asyncness,
    ) -> Result<Instance> {
        let (instance, needs_startup) = {
            let (mut limiter, store) = store.0.resource_limiter_and_store_opaque();
            // SAFETY: the safety contract of `new_raw` is the same as this
            // function.
            unsafe { Instance::new_raw(store, limiter.as_mut(), module, imports).await? }
        };

        // If this instance requires startup, which is a dynamic decision made
        // at this point in conjunction with analysis at compile time, the
        // instance gets started. Note that this isn't just the wasm start
        // function itself, but it's finalization of initialization of this
        // instance, for example for complicated global initialization
        // expressions.
        if needs_startup {
            if asyncness == Asyncness::No {
                instance.start_raw(store)?;
            } else {
                #[cfg(feature = "async")]
                {
                    store.on_fiber(|store| instance.start_raw(store)).await??;
                }
                #[cfg(not(feature = "async"))]
                unreachable!();
            }
        }
        Ok(instance)
    }

    /// Internal function to create an instance which doesn't have its `start`
    /// function run yet.
    ///
    /// # Unsafety
    ///
    /// This method is unsafe because it does not type-check the `imports`
    /// provided. The `imports` provided must be suitable for the module
    /// provided as well.
    pub(crate) async unsafe fn new_raw(
        store: &mut StoreOpaque,
        mut limiter: Option<&mut StoreResourceLimiter<'_>>,
        module: &Module,
        imports: Imports<'_>,
    ) -> Result<(Instance, bool)> {
        if !Engine::same(store.engine(), module.engine()) {
            bail!("cross-`Engine` instantiation is not currently supported");
        }
        store.bump_resource_counts(module)?;

        // Allocate the GC heap, if necessary.
        if module.env_module().needs_gc_heap {
            store.ensure_gc_store(limiter.as_deref_mut()).await?;
        }

        // Register the module just before instantiation to ensure we keep the module
        // properly referenced while in use by the store.
        let (modules, engine, breakpoints) = store.modules_and_engine_and_breakpoints_mut();
        let module_id = modules.register_module(module, engine, breakpoints)?;

        // The first thing we do is issue an instance allocation request
        // to the instance allocator. This, on success, will give us an
        // instance handle.
        //
        // SAFETY: this module, by construction, was already validated within
        // the store.
        let id = unsafe {
            store
                .allocate_instance(
                    limiter.as_deref_mut(),
                    AllocateInstanceKind::Module(module_id),
                    &ModuleRuntimeInfo::Module(module.clone()),
                    imports,
                )
                .await?
        };

        let instance = Instance::from_wasmtime(id, store);

        let needs_startup = instance.id.get_mut(store).needs_startup();

        // At this point the instance is created and stored within the store,
        // but it's also not quite usable just yet. Initialization hasn't
        // completed (e.g. active data/element segments) and the `start`
        // function additionally has not yet been invoked. That's the
        // responsibility of the caller to handle, however.
        Ok((instance, needs_startup))
    }

    pub(crate) fn from_wasmtime(id: InstanceId, store: &mut StoreOpaque) -> Instance {
        Instance {
            id: StoreInstanceId::new(store.id(), id),
        }
    }

    pub(crate) fn start_raw<T>(&self, store: &mut StoreContextMut<'_, T>) -> Result<()> {
        // If a start function is present, invoke it. Make sure we use all the
        // trap-handling configuration in `store` as well.
        let store_id = store.0.id();
        let (mut instance, registry) = self.id.get_mut_and_module_registry(store.0);
        // SAFETY: the `store_id` is the id of the store that owns this
        // instance and any function stored within the instance.
        let f = unsafe {
            instance
                .as_mut()
                .get_startup_func(registry, store_id)
                .expect("should have a startup function")
        };
        let caller_vmctx = instance.vmctx();
        unsafe {
            let funcref = f.vm_func_ref(store.0);
            super::func::invoke_wasm_and_catch_traps(
                store,
                UncaughtException::Propagate,
                |_default_caller, vm| {
                    VMFuncRef::array_call(funcref, vm, caller_vmctx, NonNull::from(&mut []))
                },
            )?;
        }
        Ok(())
    }

    /// Get this instance's module.
    pub fn module<'a, T: 'static>(&self, store: impl Into<StoreContext<'a, T>>) -> &'a Module {
        self._module(store.into().0)
    }

    pub(crate) fn _module<'a>(&self, store: &'a StoreOpaque) -> &'a Module {
        store.module_for_instance(self.id).unwrap()
    }

    /// Returns the list of exported items from this [`Instance`].
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance, or if memory allocation
    /// fails.
    pub fn exports<'a, T: 'static>(
        &'a self,
        store: impl Into<StoreContextMut<'a, T>>,
    ) -> impl ExactSizeIterator<Item = Export<'a>> + 'a {
        let store = store.into().0;
        let store_id = store.id();
        let engine = store.engine().clone();

        let (instance, registry) = store.instance_and_module_registry_mut(self.id());
        let (module, mut instance) = instance.module_and_self();
        module.exports.iter().map(move |(name, entity)| {
            // SAFETY: the `store_id` owns this instance and all exports
            // contained within.
            let export = unsafe {
                instance
                    .as_mut()
                    .get_export_by_index_mut(registry, store_id, *entity)
            };

            let ext = Extern::from_wasmtime_export(export, &engine);
            Export::new(&module.strings[name], ext)
        })
    }

    /// Looks up an exported [`Extern`] value by name.
    ///
    /// This method will search the module for an export named `name` and return
    /// the value, if found.
    ///
    /// Returns `None` if there was no export named `name`.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    ///
    /// # Why does `get_export` take a mutable context?
    ///
    /// This method requires a mutable context because an instance's exports are
    /// lazily populated, and we cache them as they are accessed. This makes
    /// instantiating a module faster, but also means this method requires a
    /// mutable context.
    pub fn get_export(&self, mut store: impl AsContextMut, name: &str) -> Option<Extern> {
        let store = store.as_context_mut().0;
        let module = store[self.id].env_module();
        let name = module.strings.get_atom(name)?;
        let entity = *module.exports.get(&name)?;
        Some(self._get_export(store, entity))
    }

    /// Looks up an exported [`Extern`] value by a [`ModuleExport`] value.
    ///
    /// This is similar to [`Instance::get_export`] but uses a [`ModuleExport`] value to avoid
    /// string lookups where possible. [`ModuleExport`]s can be obtained by calling
    /// [`Module::get_export_index`] on the [`Module`] that this instance was instantiated with.
    ///
    /// This method will search the module for an export with a matching entity index and return
    /// the value, if found.
    ///
    /// Returns `None` if there was no export with a matching entity index.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_module_export(
        &self,
        mut store: impl AsContextMut,
        export: &ModuleExport,
    ) -> Option<Extern> {
        let store = store.as_context_mut().0;

        // Verify the `ModuleExport` matches the module used in this instance.
        if self._module(store).id() != export.module {
            return None;
        }

        Some(self._get_export(store, export.entity))
    }

    fn _get_export(&self, store: &mut StoreOpaque, entity: EntityIndex) -> Extern {
        let id = store.id();
        // SAFETY: the store `id` owns this instance and all exports contained
        // within.
        let export = unsafe {
            let (instance, registry) = self.id.get_mut_and_module_registry(store);
            instance.get_export_by_index_mut(registry, id, entity)
        };
        Extern::from_wasmtime_export(export, store.engine())
    }

    /// Looks up an exported [`Func`] value by name.
    ///
    /// Returns `None` if there was no export named `name`, or if there was but
    /// it wasn't a function.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_func(&self, store: impl AsContextMut, name: &str) -> Option<Func> {
        self.get_export(store, name)?.into_func()
    }

    /// Looks up an exported [`Func`] value by name and with its type.
    ///
    /// This function is a convenience wrapper over [`Instance::get_func`] and
    /// [`Func::typed`]. For more information see the linked documentation.
    ///
    /// Returns an error if `name` isn't a function export or if the export's
    /// type did not match `Params` or `Results`
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    ///
    /// # Errors
    ///
    /// This function will return an [`OutOfMemory`][crate::OutOfMemory] error when
    /// memory allocation fails. See the `OutOfMemory` type's documentation for
    /// details on Wasmtime's out-of-memory handling.
    pub fn get_typed_func<Params, Results>(
        &self,
        mut store: impl AsContextMut,
        name: &str,
    ) -> Result<TypedFunc<Params, Results>>
    where
        Params: crate::WasmParams,
        Results: crate::WasmResults,
    {
        let f = self
            .get_export(store.as_context_mut(), name)
            .and_then(|f| f.into_func())
            .ok_or_else(|| format_err!("failed to find function export `{name}`"))?;
        Ok(f.typed::<Params, Results>(store)
            .with_context(|| format!("failed to convert function `{name}` to given type"))?)
    }

    /// Looks up an exported [`Table`] value by name.
    ///
    /// Returns `None` if there was no export named `name`, or if there was but
    /// it wasn't a table.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_table(&self, store: impl AsContextMut, name: &str) -> Option<Table> {
        self.get_export(store, name)?.into_table()
    }

    /// Looks up an exported [`Memory`] value by name.
    ///
    /// Returns `None` if there was no export named `name`, or if there was but
    /// it wasn't a memory.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_memory(&self, store: impl AsContextMut, name: &str) -> Option<Memory> {
        self.get_export(store, name)?.into_memory()
    }

    /// Looks up an exported [`SharedMemory`] value by name.
    ///
    /// Returns `None` if there was no export named `name`, or if there was but
    /// it wasn't a shared memory.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_shared_memory(
        &self,
        mut store: impl AsContextMut,
        name: &str,
    ) -> Option<SharedMemory> {
        let mut store = store.as_context_mut();
        self.get_export(&mut store, name)?.into_shared_memory()
    }

    /// Looks up an exported [`Global`] value by name.
    ///
    /// Returns `None` if there was no export named `name`, or if there was but
    /// it wasn't a global.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_global(&self, store: impl AsContextMut, name: &str) -> Option<Global> {
        self.get_export(store, name)?.into_global()
    }

    /// Looks up a tag [`Tag`] by name.
    ///
    /// Returns `None` if there was no export named `name`, or if there was but
    /// it wasn't a tag.
    ///
    /// # Panics
    ///
    /// Panics if `store` does not own this instance.
    pub fn get_tag(&self, store: impl AsContextMut, name: &str) -> Option<Tag> {
        self.get_export(store, name)?.into_tag()
    }

    #[allow(
        dead_code,
        reason = "c-api crate does not yet support exnrefs and causes this method to be dead."
    )]
    pub(crate) fn id(&self) -> InstanceId {
        self.id.instance()
    }

    /// Return a unique-within-Store index for this `Instance`.
    ///
    /// Allows distinguishing instance identities when introspecting
    /// the `Store`, e.g. via debug APIs.
    ///
    /// This index will match the instance's position in the sequence
    /// returned by `Store::debug_all_instances()`.
    #[cfg(feature = "debug")]
    pub fn debug_index_in_store(&self) -> u32 {
        self.id.instance().as_u32()
    }

    /// Get all globals within this instance.
    ///
    /// Returns both import and defined globals.
    ///
    /// Returns both exported and non-exported globals.
    ///
    /// Gives access to the full globals space.
    #[cfg(feature = "coredump")]
    pub(crate) fn all_globals<'a>(
        &'a self,
        store: &'a mut StoreOpaque,
    ) -> impl ExactSizeIterator<Item = (GlobalIndex, Global)> + 'a {
        let store_id = store.id();
        store[self.id].all_globals(store_id)
    }

    /// Get all memories within this instance.
    ///
    /// Returns both import and defined memories.
    ///
    /// Returns both exported and non-exported memories.
    ///
    /// Gives access to the full memories space.
    #[cfg(feature = "coredump")]
    pub(crate) fn all_memories<'a>(
        &'a self,
        store: &'a StoreOpaque,
    ) -> impl ExactSizeIterator<Item = (MemoryIndex, vm::ExportMemory)> + 'a {
        let store_id = store.id();
        store[self.id].all_memories(store_id)
    }
}

pub(crate) struct OwnedImports {
    functions: TryPrimaryMap<FuncIndex, VMFunctionImport>,
    tables: TryPrimaryMap<TableIndex, VMTableImport>,
    memories: TryPrimaryMap<MemoryIndex, VMMemoryImport>,
    globals: TryPrimaryMap<GlobalIndex, VMGlobalImport>,
    tags: TryPrimaryMap<TagIndex, VMTagImport>,
}

impl OwnedImports {
    fn new(module: &Module) -> Result<OwnedImports, OutOfMemory> {
        let mut ret = OwnedImports::empty();
        ret.reserve(module)?;
        Ok(ret)
    }

    pub(crate) fn empty() -> OwnedImports {
        OwnedImports {
            functions: TryPrimaryMap::new(),
            tables: TryPrimaryMap::new(),
            memories: TryPrimaryMap::new(),
            globals: TryPrimaryMap::new(),
            tags: TryPrimaryMap::new(),
        }
    }

    pub(crate) fn reserve(&mut self, module: &Module) -> Result<(), OutOfMemory> {
        let raw = module.compiled_module().module();
        self.functions.reserve(raw.num_imported_funcs)?;
        self.tables.reserve(raw.num_imported_tables)?;
        self.memories.reserve(raw.num_imported_memories)?;
        self.globals.reserve(raw.num_imported_globals)?;
        self.tags.reserve(raw.num_imported_tags)?;
        Ok(())
    }

    #[cfg(feature = "component-model")]
    pub(crate) fn clear(&mut self) {
        self.functions.clear();
        self.tables.clear();
        self.memories.clear();
        self.globals.clear();
        self.tags.clear();
    }

    fn push(&mut self, item: &Extern, store: &mut StoreOpaque) -> Result<(), OutOfMemory> {
        match item {
            Extern::Func(i) => {
                self.functions.push(i.vmimport(store))?;
            }
            Extern::Global(i) => {
                self.globals.push(i.vmimport(store))?;
            }
            Extern::Table(i) => {
                self.tables.push(i.vmimport(store))?;
            }
            Extern::Memory(i) => {
                self.memories.push(i.vmimport(store))?;
            }
            Extern::SharedMemory(i) => {
                self.memories.push(i.vmimport(store))?;
            }
            Extern::Tag(i) => {
                self.tags.push(i.vmimport(store))?;
            }
        }
        Ok(())
    }

    /// Note that this is unsafe as the validity of `item` is not verified and
    /// it contains a bunch of raw pointers.
    #[cfg(feature = "component-model")]
    pub(crate) fn push_export(
        &mut self,
        store: &StoreOpaque,
        item: &crate::runtime::vm::Export,
    ) -> Result<(), OutOfMemory> {
        match item {
            crate::runtime::vm::Export::Function(f) => {
                self.functions.push(f.vmimport(store))?;
            }
            crate::runtime::vm::Export::Global(g) => {
                self.globals.push(g.vmimport(store))?;
            }
            crate::runtime::vm::Export::Table(t) => {
                self.tables.push(t.vmimport(store))?;
            }
            crate::runtime::vm::Export::Memory(m) => {
                self.memories.push(m.vmimport(store))?;
            }
            crate::runtime::vm::Export::SharedMemory(_, vmimport) => {
                self.memories.push(*vmimport)?;
            }
            crate::runtime::vm::Export::Tag(t) => {
                self.tags.push(t.vmimport(store))?;
            }
        }
        Ok(())
    }

    pub(crate) fn as_ref(&self) -> Imports<'_> {
        Imports {
            tables: self.tables.values().as_slice(),
            globals: self.globals.values().as_slice(),
            memories: self.memories.values().as_slice(),
            functions: self.functions.values().as_slice(),
            tags: self.tags.values().as_slice(),
        }
    }
}

/// An instance, pre-instantiation, that is ready to be instantiated.
///
/// This structure represents an instance *just before* it was instantiated,
/// after all type-checking and imports have been resolved. The only thing left
/// to do for this instance is to actually run the process of instantiation.
///
/// Note that an `InstancePre` may not be tied to any particular [`Store`] if
/// none of the imports it closed over are tied to any particular [`Store`].
///
/// This structure is created through the [`Linker::instantiate_pre`] method,
/// which also has some more information and examples.
///
/// [`Store`]: crate::Store
/// [`Linker::instantiate_pre`]: crate::Linker::instantiate_pre
pub struct InstancePre<T> {
    module: Module,

    /// The items which this `InstancePre` use to instantiate the `module`
    /// provided, passed to `Instance::new_started` after inserting them into a
    /// `Store`.
    ///
    /// Note that this is stored as an `Arc` to quickly move a strong reference
    /// to everything internally into a `Store<T>` without having to clone each
    /// individual item.
    items: Arc<TryVec<Definition>>,

    /// A count of `Definition::HostFunc` entries in `items` above to
    /// preallocate space in a `Store` up front for all entries to be inserted.
    host_funcs: usize,

    /// The `VMFuncRef`s for the functions in `items` that do not
    /// have a `wasm_call` trampoline. We pre-allocate and pre-patch these
    /// `VMFuncRef`s so that we don't have to do it at
    /// instantiation time.
    ///
    /// This is an `Arc` for the same reason as `items`.
    func_refs: Arc<TryVec<VMFuncRef>>,

    /// Whether or not any import in `items` is flagged as needing async.
    ///
    /// This is used to update stores during instantiation as to whether they
    /// require async entrypoints.
    asyncness: Asyncness,

    _marker: core::marker::PhantomData<fn() -> T>,
}

/// InstancePre's clone does not require T: Clone
impl<T> Clone for InstancePre<T> {
    fn clone(&self) -> Self {
        Self {
            module: self.module.clone(),
            items: self.items.clone(),
            host_funcs: self.host_funcs,
            func_refs: self.func_refs.clone(),
            asyncness: self.asyncness,
            _marker: self._marker,
        }
    }
}

impl<T: 'static> InstancePre<T> {
    /// Creates a new `InstancePre` which type-checks the `items` provided and
    /// on success is ready to instantiate a new instance.
    ///
    /// `engine` is the engine that `items` belong to, and this returns an error
    /// if that is not also `module`'s engine. This also returns an error if an
    /// individual item within `items` reports an engine of its own that is not
    /// `engine`, which happens when that item was taken from a store belonging
    /// to a different engine than the linker it was defined in.
    ///
    /// # Unsafety
    ///
    /// This method is unsafe as the `T` of the `InstancePre<T>` is not
    /// guaranteed to be the same as the `T` within the `Store`, the caller must
    /// verify that.
    pub(crate) unsafe fn new(
        engine: &Engine,
        module: &Module,
        items: TryVec<Definition>,
    ) -> Result<InstancePre<T>> {
        typecheck(engine, module, &items, |cx, ty, item| {
            cx.definition(ty, &item.ty())
        })?;

        let mut func_refs = TryVec::with_capacity(items.len())?;
        let mut host_funcs = 0;
        let mut asyncness = Asyncness::No;
        for item in &items {
            match item {
                Definition::Extern { .. } => {}
                Definition::HostFunc(f) => {
                    host_funcs += 1;
                    if f.func_ref().wasm_call.is_none() {
                        func_refs.push(VMFuncRef {
                            wasm_call: module
                                .wasm_to_array_trampoline(f.sig_index())
                                .map(|f| f.into()),
                            ..*f.func_ref()
                        })?;
                    }
                    asyncness = asyncness | f.asyncness();
                }
            }
        }

        Ok(InstancePre {
            module: module.clone(),
            items: try_new::<Arc<_>>(items)?,
            host_funcs,
            func_refs: try_new::<Arc<_>>(func_refs)?,
            asyncness,
            _marker: core::marker::PhantomData,
        })
    }

    /// Returns a reference to the module that this [`InstancePre`] will be
    /// instantiating.
    pub fn module(&self) -> &Module {
        &self.module
    }

    /// Instantiates this instance, creating a new instance within the provided
    /// `store`.
    ///
    /// This function will run the actual process of instantiation to
    /// completion. This will use all of the previously-closed-over items as
    /// imports to instantiate the module that this was originally created with.
    ///
    /// For more information about instantiation see [`Instance::new`].
    ///
    /// # Panics
    ///
    /// Panics if any import closed over by this [`InstancePre`] isn't owned by
    /// `store`, or if `store` has async support enabled. Additionally this
    /// function will panic if the `store` provided comes from a different
    /// [`Engine`] than the [`InstancePre`] originally came from.
    ///
    /// # Errors
    ///
    /// This function will return an [`OutOfMemory`][crate::OutOfMemory] error when
    /// memory allocation fails. See the `OutOfMemory` type's documentation for
    /// details on Wasmtime's out-of-memory handling.
    pub fn instantiate(&self, mut store: impl AsContextMut<Data = T>) -> Result<Instance> {
        let mut store = store.as_context_mut();
        let imports = pre_instantiate_raw(
            &mut store.0,
            &self.module,
            &self.items,
            self.host_funcs,
            &self.func_refs,
            self.asyncness,
        )?;

        // Note that this is specifically done after `pre_instantiate_raw` to
        // handle the case that if any imports in this `InstancePre` require
        // async that it's flagged in the store by that point which will reject
        // this instantiation to say "use `instantiate_async` instead".
        store.0.validate_sync_call()?;

        // This unsafety should be handled by the type-checking performed by the
        // constructor of `InstancePre` to assert that all the imports we're passing
        // in match the module we're instantiating.
        vm::assert_ready(unsafe {
            Instance::new_started(&mut store, &self.module, imports.as_ref(), Asyncness::No)
        })
    }

    /// Creates a new instance, running the start function asynchronously
    /// instead of inline.
    ///
    /// For more information about asynchronous instantiation see the
    /// documentation on [`Instance::new_async`].
    ///
    /// # Panics
    ///
    /// Panics if any import closed over by this [`InstancePre`] isn't owned by
    /// `store`, or if `store` does not have async support enabled.
    ///
    /// # Errors
    ///
    /// This function will return an [`OutOfMemory`][crate::OutOfMemory] error when
    /// memory allocation fails. See the `OutOfMemory` type's documentation for
    /// details on Wasmtime's out-of-memory handling.
    #[cfg(feature = "async")]
    pub async fn instantiate_async(
        &self,
        mut store: impl AsContextMut<Data = T>,
    ) -> Result<Instance> {
        let mut store = store.as_context_mut();
        let imports = pre_instantiate_raw(
            &mut store.0,
            &self.module,
            &self.items,
            self.host_funcs,
            &self.func_refs,
            self.asyncness,
        )?;

        // This unsafety should be handled by the type-checking performed by the
        // constructor of `InstancePre` to assert that all the imports we're passing
        // in match the module we're instantiating.
        unsafe {
            Instance::new_started(&mut store, &self.module, imports.as_ref(), Asyncness::Yes).await
        }
    }
}

/// Helper function shared between
/// `InstancePre::{instantiate,instantiate_async}`
///
/// This is an out-of-line function to avoid the generic on `InstancePre` and
/// get this compiled into the `wasmtime` crate to avoid having it monomorphized
/// elsewhere.
fn pre_instantiate_raw(
    store: &mut StoreOpaque,
    module: &Module,
    items: &Arc<TryVec<Definition>>,
    host_funcs: usize,
    func_refs: &Arc<TryVec<VMFuncRef>>,
    asyncness: Asyncness,
) -> Result<OwnedImports> {
    // Register this module and use it to fill out any funcref wasm_call holes
    // we can. For more comments on this see `typecheck_externs`.
    let (modules, engine, breakpoints) = store.modules_and_engine_and_breakpoints_mut();
    modules.register_module(module, engine, breakpoints)?;
    let (funcrefs, modules) = store.func_refs_and_modules();
    funcrefs.fill(modules);

    if host_funcs > 0 {
        // Any linker-defined function of the `Definition::HostFunc` variant
        // will insert a function into the store automatically as part of
        // instantiation, so reserve space here to make insertion more efficient
        // as it won't have to realloc during the instantiation.
        funcrefs.reserve_storage(host_funcs)?;

        // The usage of `to_extern_store_rooted` requires that the items are
        // rooted via another means, which happens here by cloning the list of
        // items into the store once. This avoids cloning each individual item
        // below.
        funcrefs.push_instance_pre_definitions(items.clone())?;
        funcrefs.push_instance_pre_func_refs(func_refs.clone())?;
    }

    store.set_async_required(asyncness);

    let mut func_refs = func_refs.iter().map(|f| NonNull::from(f));
    let mut imports = OwnedImports::new(module)?;
    for import in items.iter() {
        if !import.comes_from_same_store(store) {
            bail!("cross-`Store` instantiation is not currently supported");
        }
        // This unsafety should be encapsulated in the constructor of
        // `InstancePre` where the `T` of the original item should match the
        // `T` of the store. Additionally the rooting necessary has happened
        // above.
        let item = match import {
            Definition::Extern { item, .. } => item.clone(),
            Definition::HostFunc(func) => unsafe {
                func.to_func_store_rooted(
                    store,
                    if func.func_ref().wasm_call.is_none() {
                        Some(func_refs.next().unwrap())
                    } else {
                        None
                    },
                )
                .into()
            },
        };
        imports.push(&item, store)?;
    }

    Ok(imports)
}

/// An item that can be supplied as an import argument during instantiation.
///
/// # Safety
///
/// Implementations must return an associated engine if they own a handle to
/// one. Failure to do so may allow cross-`Engine` type confusion.
///
/// (Items that are just identifiers indexing into a store, for example
/// `Extern::Global(wasmtime::Global)`, do not have their own handle to an
/// engine. Their engine is the engine of the store they belong to, and it is
/// the store, not them, that holds an owning handle to the engine.)
unsafe trait ImportArg {
    fn engine(&self) -> Option<&Engine>;
}

// SAFETY: `Extern::SharedMemory` is the only variant with an `Engine` handle.
unsafe impl ImportArg for Extern {
    fn engine(&self) -> Option<&Engine> {
        match self {
            Extern::SharedMemory(m) => Some(m.engine()),
            Extern::Func(_)
            | Extern::Global(_)
            | Extern::Table(_)
            | Extern::Memory(_)
            | Extern::Tag(_) => None,
        }
    }
}

// SAFETY: `Definition::engine` is complete.
unsafe impl ImportArg for Definition {
    fn engine(&self) -> Option<&Engine> {
        Some(Definition::engine(self))
    }
}

/// Type check the `import_args` against the imports that `module` declares.
///
/// `engine` is the engine that the `import_args` belong to. It must be the same
/// engine as `module`'s: entity types are compared by `VMSharedTypeIndex`, which
/// only means anything within the engine that assigned it, so checking one
/// engine's items against another engine's module would compare unrelated types
/// and consider them equal.
fn typecheck<I>(
    engine: &Engine,
    module: &Module,
    import_args: &[I],
    check: impl Fn(&matching::MatchCx<'_>, &EntityType, &I) -> Result<()>,
) -> Result<()>
where
    I: ImportArg,
{
    ensure!(
        Engine::same(engine, module.engine()),
        "cross-`Engine` instantiation is not currently supported"
    );
    let env_module = module.compiled_module().module();
    let expected_len = env_module.imports().count();
    let actual_len = import_args.len();
    if expected_len != actual_len {
        bail!("expected {expected_len} imports, found {actual_len}");
    }
    let cx = matching::MatchCx::new(module.engine());
    for ((name, field, expected_ty), actual) in env_module.imports().zip(import_args) {
        debug_assert!(expected_ty.is_canonicalized_for_runtime_usage());
        if let Some(actual_engine) = actual.engine() {
            ensure!(
                Engine::same(actual_engine, engine),
                "cross-`Engine` instantiation is not currently supported: \
                 the item provided for `{name}::{field}` belongs to a \
                 different engine than the module being instantiated"
            );
        }
        check(&cx, &expected_ty, actual)
            .with_context(|| format!("incompatible import type for `{name}::{field}`"))?;
    }
    Ok(())
}