Add the ability to cache typechecking an instance (#2962)

* Add the ability to cache typechecking an instance

This commit adds the abilty to cache the type-checked imports of an
instance if an instance is going to be instantiated multiple times. This
can also be useful to do a "dry run" of instantiation where no wasm code
is run but it's double-checked that a `Linker` possesses everything
necessary to instantiate the provided module.

This should ideally help cut down repeated instantiation costs slightly
by avoiding type-checking and allocation a `Vec<Extern>` on each
instantiation. It's expected though that the impact on instantiation
time is quite small and likely not super significant. The functionality,
though, of pre-checking can be useful for some embeddings.

* Fix build with async

crates/wasmtime/src/func.rs

@@ -677,7 +677,7 @@ impl Func {
     /// initiates a panic. Also panics if `store` does not own this function.
     pub fn call(&self, mut store: impl AsContextMut, params: &[Val]) -> Result<Box<[Val]>> {
         assert!(
-            !cfg!(feature = "async") || !store.as_context().async_support(),
+            !store.as_context().async_support(),
             "must use `call_async` when async support is enabled on the config",
         );
         let my_ty = self.ty(&store);
@@ -1894,6 +1894,10 @@ impl HostFunc {
         let idx = self.export.anyfunc.as_ref().type_index;
         store.register_host_trampoline(idx, self.trampoline);
     }
+
+    pub(crate) fn sig_index(&self) -> VMSharedSignatureIndex {
+        unsafe { self.export.anyfunc.as_ref().type_index }
+    }
 }
 
 impl Drop for HostFunc {

crates/wasmtime/src/func/typed.rs

@@ -73,7 +73,7 @@ where
     pub fn call(&self, mut store: impl AsContextMut, params: Params) -> Result<Results, Trap> {
         let mut store = store.as_context_mut().opaque();
         assert!(
-            !cfg!(feature = "async") || !store.async_support(),
+            !store.async_support(),
             "must use `call_async` with async stores"
         );
         unsafe { self._call(&mut store, params) }

crates/wasmtime/src/instance.rs

@@ -1,3 +1,4 @@
+use crate::linker::Definition;
 use crate::store::{InstanceId, StoreData, StoreOpaque, StoreOpaqueSend, Stored};
 use crate::types::matching;
 use crate::{
@@ -9,7 +10,8 @@ use std::mem;
 use std::sync::Arc;
 use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::wasm::{
-    EntityIndex, FuncIndex, GlobalIndex, InstanceIndex, MemoryIndex, ModuleIndex, TableIndex,
+    EntityIndex, EntityType, FuncIndex, GlobalIndex, InstanceIndex, MemoryIndex, ModuleIndex,
+    TableIndex,
 };
 use wasmtime_environ::Initializer;
 use wasmtime_runtime::{
@@ -96,32 +98,14 @@ impl Instance {
         module: &Module,
         imports: &[Extern],
     ) -> Result<Instance, Error> {
-        Instance::_new(&mut store.as_context_mut().opaque(), module, imports)
-    }
-
-    fn _new(
-        store: &mut StoreOpaque<'_>,
-        module: &Module,
-        imports: &[Extern],
-    ) -> Result<Instance, Error> {
-        assert!(
-            !store.async_support(),
-            "cannot use `new` when async support is enabled on the config"
-        );
-
-        // NB: this is the same code as `Instance::new_async`. It's intentionally
-        // small but should be kept in sync (modulo the async bits).
-        let mut i = Instantiator::new(store, module, imports)?;
-        loop {
-            if let Some((id, instance)) = i.step(store)? {
-                if let Some(start) = store.instance(id).module().start_func {
-                    Instantiator::start_raw(store, id, start)?;
-                }
-                if let Some(instance) = instance {
-                    break Ok(instance);
-                }
-            }
-        }
+        // This unsafety comes from `Instantiator::new` where we must typecheck
+        // first, which we are sure to do here.
+        let mut i = unsafe {
+            let mut cx = store.as_context_mut().opaque();
+            typecheck_externs(&mut cx, module, imports)?;
+            Instantiator::new(&mut cx, module, ImportSource::Externs(imports))?
+        };
+        i.run(store.as_context_mut().opaque())
     }
 
     /// Same as [`Instance::new`], except for usage in [asynchronous stores].
@@ -151,37 +135,13 @@ impl Instance {
     where
         T: Send,
     {
-        Instance::_new_async(store.as_context_mut().opaque_send(), module, imports).await
-    }
-
-    #[cfg(feature = "async")]
-    async fn _new_async<'a>(
-        mut store: StoreOpaqueSend<'a>,
-        module: &Module,
-        imports: &[Extern],
-    ) -> Result<Instance, Error> {
-        assert!(
-            store.async_support(),
-            "cannot use `new_async` without enabling async support on the config"
-        );
-
-        // NB: this is the same code as `Instance::new`. It's intentionally
-        // small but should be kept in sync (modulo the async bits).
-        let mut i = Instantiator::new(&mut store.opaque(), module, imports)?;
-        loop {
-            let step = i.step(&mut store.opaque())?;
-            if let Some((id, instance)) = step {
-                let start = store.instance(id).module().start_func;
-                if let Some(start) = start {
-                    store
-                        .on_fiber(|store| Instantiator::start_raw(store, id, start))
-                        .await??;
-                }
-                if let Some(instance) = instance {
-                    break Ok(instance);
-                }
-            }
-        }
+        // See `new` for unsafety comments
+        let mut i = unsafe {
+            let mut cx = store.as_context_mut().opaque();
+            typecheck_externs(&mut cx, module, imports)?;
+            Instantiator::new(&mut cx, module, ImportSource::Externs(imports))?
+        };
+        i.run_async(store.as_context_mut().opaque_send()).await
     }
 
     pub(crate) fn from_wasmtime(handle: RuntimeInstance, store: &mut StoreOpaque) -> Instance {
@@ -336,7 +296,8 @@ struct ImportsBuilder<'a> {
 }
 
 enum ImportSource<'a> {
-    Runtime(&'a [Extern]),
+    Externs(&'a [Extern]),
+    Definitions(&'a [Definition]),
     Outer { initializer: usize },
 }
 
@@ -345,33 +306,82 @@ impl<'a> Instantiator<'a> {
     /// directives of a module.
     ///
     /// This doesn't do much work itself beyond setting things up.
-    fn new(
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe for a few reasons:
+    ///
+    /// * This assumes that `imports` has already been typechecked and is of the
+    ///   appropriate length. It is memory unsafe if the types of `imports` are
+    ///   not what `module` expects.
+    ///
+    /// * The `imports` must be safely able to get inserted into `store`. This
+    ///   only applies if `ImportSource::Definitions` is used because this will
+    ///   internally call `Definition::to_extern` which requires that any
+    ///   host functions in the list were created with an original `T` as the
+    ///   store that's being inserted into.
+    ///
+    /// * The `imports` must all come from the `store` specified.
+    unsafe fn new(
         store: &mut StoreOpaque<'_>,
         module: &Module,
-        imports: &'a [Extern],
+        imports: ImportSource<'a>,
     ) -> Result<Instantiator<'a>> {
         if !Engine::same(store.engine(), module.engine()) {
             bail!("cross-`Engine` instantiation is not currently supported");
         }
 
-        // Perform some pre-flight checks before we get into the meat of
-        // instantiation.
-        let expected = module.compiled_module().module().imports().count();
-        if expected != imports.len() {
-            bail!("expected {} imports, found {}", expected, imports.len());
-        }
-        for import in imports {
-            if !import.comes_from_same_store(&store) {
-                bail!("cross-`Store` instantiation is not currently supported");
-            }
-        }
-
         Ok(Instantiator {
             in_progress: Vec::new(),
-            cur: ImportsBuilder::new(module, ImportSource::Runtime(imports)),
+            cur: ImportsBuilder::new(module, imports),
         })
     }
 
+    fn run(&mut self, mut store: StoreOpaque<'_>) -> Result<Instance, Error> {
+        assert!(
+            !store.async_support(),
+            "cannot use `new` when async support is enabled on the config"
+        );
+
+        // NB: this is the same code as `run_async`. It's intentionally
+        // small but should be kept in sync (modulo the async bits).
+        loop {
+            if let Some((id, instance)) = self.step(&mut store)? {
+                if let Some(start) = store.instance(id).module().start_func {
+                    Instantiator::start_raw(&mut store, id, start)?;
+                }
+                if let Some(instance) = instance {
+                    break Ok(instance);
+                }
+            }
+        }
+    }
+
+    #[cfg(feature = "async")]
+    async fn run_async(&mut self, mut store: StoreOpaqueSend<'_>) -> Result<Instance, Error> {
+        assert!(
+            store.async_support(),
+            "cannot use `new_async` without enabling async support on the config"
+        );
+
+        // NB: this is the same code as `run`. It's intentionally
+        // small but should be kept in sync (modulo the async bits).
+        loop {
+            let step = self.step(&mut store.opaque())?;
+            if let Some((id, instance)) = step {
+                let start = store.instance(id).module().start_func;
+                if let Some(start) = start {
+                    store
+                        .on_fiber(|store| Instantiator::start_raw(store, id, start))
+                        .await??;
+                }
+                if let Some(instance) = instance {
+                    break Ok(instance);
+                }
+            }
+        }
+    }
+
     /// Processes the next initializer for the next instance being created
     /// without running any wasm code.
     ///
@@ -410,7 +420,7 @@ impl<'a> Instantiator<'a> {
             .initializers
             .get(self.cur.initializer - 1)
         {
-            Some(Initializer::Import { index, name, field }) => {
+            Some(Initializer::Import { name, field, .. }) => {
                 match &mut self.cur.src {
                     // If imports are coming from the runtime-provided list
                     // (e.g. the root module being instantiated) then we
@@ -418,27 +428,18 @@ impl<'a> Instantiator<'a> {
                     //
                     // Note the `unwrap` here should be ok given the validation
                     // above in `Instantiation::new`.
-                    ImportSource::Runtime(list) => {
+                    ImportSource::Externs(list) => {
                         let (head, remaining) = list.split_first().unwrap();
                         *list = remaining;
-                        let expected_ty =
-                            self.cur.module.compiled_module().module().type_of(*index);
-                        matching::MatchCx {
-                            signatures: self.cur.module.signatures(),
-                            types: self.cur.module.types(),
-                            store_data: store.store_data(),
-                            engine: store.engine(),
-                        }
-                        .extern_(&expected_ty, head)
-                        .with_context(|| {
-                            let extra = match field {
-                                Some(name) => format!("::{}", name),
-                                None => String::new(),
-                            };
-                            format!("incompatible import type for `{}{}`", name, extra)
-                        })?;
                         self.cur.push(head.clone(), store);
                     }
+                    ImportSource::Definitions(list) => {
+                        let (head, remaining) = list.split_first().unwrap();
+                        *list = remaining;
+                        // This unsafety is encapsulated with
+                        // `Instantiator::new`, documented above.
+                        self.cur.push(unsafe { head.to_extern(store) }, store);
+                    }
 
                     // Otherwise if arguments are coming from our outer
                     // instance due to a recursive instantiation then we
@@ -738,25 +739,157 @@ impl<'a> ImportsBuilder<'a> {
 
 pub(crate) type RuntimeInstance = Arc<indexmap::IndexMap<String, Extern>>;
 
-/// An internal structure to this crate to build an `Instance` from a list of
-/// items with names. This is intended to stay private for now, it'll need an
-/// audit of APIs if publicly exported.
-#[derive(Default)]
-pub(crate) struct InstanceBuilder {
-    items: RuntimeInstance,
+/// 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,
+    items: Vec<Definition>,
+    _marker: std::marker::PhantomData<fn() -> T>,
+}
+
+impl<T> InstancePre<T> {
+    pub(crate) unsafe fn new(
+        store: &mut StoreOpaque,
+        module: &Module,
+        items: Vec<Definition>,
+    ) -> Result<InstancePre<T>> {
+        typecheck_defs(store, module, &items)?;
+        Ok(InstancePre {
+            module: module.clone(),
+            items,
+            _marker: std::marker::PhantomData,
+        })
+    }
+
+    /// 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.
+    pub fn instantiate(&self, mut store: impl AsContextMut<Data = T>) -> Result<Instance> {
+        let mut store = store.as_context_mut().opaque();
+        // For the unsafety here the typecheck happened at creation time of this
+        // structure and then othrewise the `T` of `InstancePre<T>` connects any
+        // host functions we have in our definition list to the `store` that was
+        // passed in.
+        unsafe {
+            self.ensure_comes_from_same_store(&store)?;
+            Instantiator::new(
+                &mut store,
+                &self.module,
+                ImportSource::Definitions(&self.items),
+            )?
+            .run(store)
+        }
+    }
+
+    /// 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.
+    #[cfg(feature = "async")]
+    #[cfg_attr(nightlydoc, doc(cfg(feature = "async")))]
+    pub async fn instantiate_async(
+        &self,
+        mut store: impl AsContextMut<Data = T>,
+    ) -> Result<Instance>
+    where
+        T: Send,
+    {
+        // For the unsafety here see above
+        let mut i = unsafe {
+            let mut store = store.as_context_mut().opaque();
+            self.ensure_comes_from_same_store(&store)?;
+            Instantiator::new(
+                &mut store,
+                &self.module,
+                ImportSource::Definitions(&self.items),
+            )?
+        };
+        i.run_async(store.as_context_mut().opaque_send()).await
+    }
+
+    fn ensure_comes_from_same_store(&self, store: &StoreOpaque<'_>) -> Result<()> {
+        for import in self.items.iter() {
+            if !import.comes_from_same_store(store) {
+                bail!("cross-`Store` instantiation is not currently supported");
+            }
+        }
+        Ok(())
+    }
 }
 
-impl InstanceBuilder {
-    pub(crate) fn new() -> InstanceBuilder {
-        InstanceBuilder::default()
+fn typecheck_externs(store: &mut StoreOpaque, module: &Module, imports: &[Extern]) -> Result<()> {
+    for import in imports {
+        if !import.comes_from_same_store(store) {
+            bail!("cross-`Store` instantiation is not currently supported");
+        }
     }
+    typecheck(store, module, imports, |cx, ty, item| cx.extern_(ty, item))
+}
 
-    pub(crate) fn insert(&mut self, name: &str, item: impl Into<Extern>) {
-        let items = Arc::get_mut(&mut self.items).unwrap();
-        items.insert(name.to_string(), item.into());
+fn typecheck_defs(store: &mut StoreOpaque, module: &Module, imports: &[Definition]) -> Result<()> {
+    for import in imports {
+        if !import.comes_from_same_store(store) {
+            bail!("cross-`Store` instantiation is not currently supported");
+        }
     }
+    typecheck(store, module, imports, |cx, ty, item| {
+        cx.definition(ty, item)
+    })
+}
 
-    pub(crate) fn finish(self, store: &mut StoreOpaque) -> Instance {
-        Instance::from_wasmtime(self.items, store)
+fn typecheck<I>(
+    store: &mut StoreOpaque,
+    module: &Module,
+    imports: &[I],
+    check: impl Fn(&matching::MatchCx<'_>, &EntityType, &I) -> Result<()>,
+) -> Result<()> {
+    let env_module = module.compiled_module().module();
+    let expected = env_module.imports().count();
+    if expected != imports.len() {
+        bail!("expected {} imports, found {}", expected, imports.len());
+    }
+    let cx = matching::MatchCx {
+        signatures: module.signatures(),
+        types: module.types(),
+        store_data: store.store_data(),
+        engine: store.engine(),
+    };
+    for ((name, field, expected_ty), actual) in env_module.imports().zip(imports) {
+        check(&cx, &expected_ty, actual).with_context(|| {
+            let extra = match field {
+                Some(name) => format!("::{}", name),
+                None => String::new(),
+            };
+            format!("incompatible import type for `{}{}`", name, extra)
+        })?;
     }
+    Ok(())
 }

crates/wasmtime/src/lib.rs

@@ -387,7 +387,7 @@ pub use crate::config::*;
 pub use crate::engine::*;
 pub use crate::externals::*;
 pub use crate::func::*;
-pub use crate::instance::Instance;
+pub use crate::instance::{Instance, InstancePre};
 pub use crate::limits::*;
 pub use crate::linker::*;
 pub use crate::memory::*;
@@ -428,6 +428,8 @@ fn _assert_send_sync() {
     _assert::<Linker<()>>();
     _assert::<Linker<*mut u8>>();
     _assert::<ExternRef>();
+    _assert::<InstancePre<()>>();
+    _assert::<InstancePre<*mut u8>>();
 
     #[cfg(feature = "async")]
     fn _call_async(s: &mut Store<()>, f: Func) {

crates/wasmtime/src/linker.rs

@@ -1,5 +1,6 @@
 use crate::func::HostFunc;
-use crate::instance::InstanceBuilder;
+use crate::instance::InstancePre;
+use crate::store::StoreOpaque;
 use crate::{
     AsContextMut, Caller, Engine, Extern, ExternType, Func, FuncType, ImportType, Instance,
     IntoFunc, Module, Trap, Val,
@@ -75,7 +76,7 @@ pub struct Linker<T> {
     engine: Engine,
     string2idx: HashMap<Arc<str>, usize>,
     strings: Vec<Arc<str>>,
-    map: HashMap<ImportKey, Definition<T>>,
+    map: HashMap<ImportKey, Definition>,
     allow_shadowing: bool,
     allow_unknown_exports: bool,
     _marker: marker::PhantomData<fn() -> T>,
@@ -87,12 +88,11 @@ struct ImportKey {
     module: usize,
 }
 
-enum Definition<T> {
+#[derive(Clone)]
+pub(crate) enum Definition {
     Extern(Extern),
-    HostFunc {
-        func: Arc<HostFunc>,
-        t: marker::PhantomData<fn() -> T>,
-    },
+    HostFunc(Arc<HostFunc>),
+    Instance(Arc<indexmap::IndexMap<String, Definition>>),
 }
 
 macro_rules! generate_wrap_async_func {
@@ -284,13 +284,7 @@ impl<T> Linker<T> {
     ) -> Result<&mut Self> {
         let func = HostFunc::new(&self.engine, ty, func);
         let key = self.import_key(module, Some(name));
-        self.insert(
-            key,
-            Definition::HostFunc {
-                func: Arc::new(func),
-                t: marker::PhantomData,
-            },
-        )?;
+        self.insert(key, Definition::HostFunc(Arc::new(func)))?;
         Ok(self)
     }
 
@@ -394,13 +388,7 @@ impl<T> Linker<T> {
     ) -> Result<&mut Self> {
         let func = HostFunc::wrap(&self.engine, func);
         let key = self.import_key(module, Some(name));
-        self.insert(
-            key,
-            Definition::HostFunc {
-                func: Arc::new(func),
-                t: marker::PhantomData,
-            },
-        )?;
+        self.insert(key, Definition::HostFunc(Arc::new(func)))?;
         Ok(self)
     }
 
@@ -590,7 +578,10 @@ impl<T> Linker<T> {
         mut store: impl AsContextMut<Data = T>,
         module_name: &str,
         module: &Module,
-    ) -> Result<&mut Self> {
+    ) -> Result<&mut Self>
+    where
+        T: 'static,
+    {
         match ModuleKind::categorize(module)? {
             ModuleKind::Command => self.command(store, module_name, module),
             ModuleKind::Reactor => {
@@ -614,18 +605,20 @@ impl<T> Linker<T> {
         mut store: impl AsContextMut<Data = T>,
         module_name: &str,
         module: &Module,
-    ) -> Result<&mut Self> {
+    ) -> Result<&mut Self>
+    where
+        T: 'static,
+    {
         for export in module.exports() {
             if let Some(func_ty) = export.ty().func() {
-                let imports = self.compute_imports(&mut store, module)?;
-                let module = module.clone();
+                let instance_pre = self.instantiate_pre(&mut store, module)?;
                 let export_name = export.name().to_owned();
                 let func = Func::new(
                     &mut store,
                     func_ty.clone(),
                     move |mut caller, params, results| {
                         // Create a new instance for this command execution.
-                        let instance = Instance::new(&mut caller, &module, &imports)?;
+                        let instance = instance_pre.instantiate(&mut caller)?;
 
                         // `unwrap()` everything here because we know the instance contains a
                         // function export with the given name and signature because we're
@@ -739,7 +732,7 @@ impl<T> Linker<T> {
         Ok(())
     }
 
-    fn insert(&mut self, key: ImportKey, item: Definition<T>) -> Result<()> {
+    fn insert(&mut self, key: ImportKey, item: Definition) -> Result<()> {
         match self.map.entry(key) {
             Entry::Occupied(_) if !self.allow_shadowing => {
                 let module = &self.strings[key.module];
@@ -831,8 +824,7 @@ impl<T> Linker<T> {
         mut store: impl AsContextMut<Data = T>,
         module: &Module,
     ) -> Result<Instance> {
-        let imports = self.compute_imports(&mut store, module)?;
-        Instance::new(store, module, &imports)
+        self.instantiate_pre(&mut store, module)?.instantiate(store)
     }
 
     /// Attempts to instantiate the `module` provided. This is the same as
@@ -847,22 +839,72 @@ impl<T> Linker<T> {
     where
         T: Send,
     {
-        let imports = self.compute_imports(&mut store, module)?;
-        Instance::new_async(store, module, &imports).await
+        self.instantiate_pre(&mut store, module)?
+            .instantiate_async(store)
+            .await
     }
 
-    fn compute_imports(
+    /// Performs all checks necessary for instantiating `module` with this
+    /// linker within `store`, except that instantiation doesn't actually
+    /// finish.
+    ///
+    /// This method is used for front-loading type-checking information as well
+    /// as collecting the imports to use to instantiate a module with. The
+    /// returned [`InstancePre`] represents a ready-to-be-instantiated module,
+    /// which can also be instantiated multiple times if desired.
+    ///
+    /// # Panics
+    ///
+    /// This method will panic if any item defined in this linker used by
+    /// `module` is not owned by `store`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let engine = Engine::default();
+    /// # let mut store = Store::new(&engine, ());
+    /// let mut linker = Linker::new(&engine);
+    /// linker.func_wrap("host", "double", |x: i32| x * 2)?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "double" (func (param i32) (result i32)))
+    ///     )
+    /// "#;
+    /// let module = Module::new(&engine, wat)?;
+    /// let instance_pre = linker.instantiate_pre(&mut store, &module)?;
+    ///
+    /// // Finish instantiation after the type-checking has all completed...
+    /// let instance = instance_pre.instantiate(&mut store)?;
+    ///
+    /// // ... and we can even continue to keep instantiating if desired!
+    /// instance_pre.instantiate(&mut store)?;
+    /// instance_pre.instantiate(&mut store)?;
+    ///
+    /// // Note that functions defined in a linker with `func_wrap` and similar
+    /// // constructors are not owned by any particular `Store`, so we can also
+    /// // instantiate our `instance_pre` in other stores because no imports
+    /// // belong to the original store.
+    /// let mut new_store = Store::new(&engine, ());
+    /// instance_pre.instantiate(&mut new_store)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn instantiate_pre(
         &self,
         mut store: impl AsContextMut<Data = T>,
         module: &Module,
-    ) -> Result<Vec<Extern>> {
-        module
+    ) -> Result<InstancePre<T>> {
+        let imports = module
             .imports()
             .map(|import| {
-                self.get_by_import(&mut store, &import)
+                self._get_by_import(&import)
                     .ok_or_else(|| self.link_error(&import))
             })
-            .collect()
+            .collect::<Result<_>>()?;
+        unsafe { InstancePre::new(&mut store.as_context_mut().opaque(), module, imports) }
     }
 
     fn link_error(&self, import: &ImportType) -> Error {
@@ -887,10 +929,12 @@ impl<T> Linker<T> {
         mut store: impl AsContextMut<Data = T> + 'p,
     ) -> impl Iterator<Item = (&str, &str, Extern)> + 'p {
         self.map.iter().map(move |(key, item)| {
+            let mut store = store.as_context_mut().opaque();
             (
                 &*self.strings[key.module],
                 &*self.strings[key.name],
-                item.to_extern(&mut store),
+                // Should be safe since `T` is connecting the linker and store
+                unsafe { item.to_extern(&mut store) },
             )
         })
     }
@@ -902,10 +946,16 @@ impl<T> Linker<T> {
     /// [`Linker`].
     pub fn get(
         &self,
-        store: impl AsContextMut<Data = T>,
+        mut store: impl AsContextMut<Data = T>,
         module: &str,
         name: Option<&str>,
     ) -> Option<Extern> {
+        let mut store = store.as_context_mut().opaque();
+        // Should be safe since `T` is connecting the linker and store
+        Some(unsafe { self._get(module, name)?.to_extern(&mut store) })
+    }
+
+    fn _get(&self, module: &str, name: Option<&str>) -> Option<&Definition> {
         let key = ImportKey {
             module: *self.string2idx.get(module)?,
             name: match name {
@@ -913,7 +963,7 @@ impl<T> Linker<T> {
                 None => usize::max_value(),
             },
         };
-        Some(self.map.get(&key)?.to_extern(store))
+        self.map.get(&key)
     }
 
     /// Looks up a value in this `Linker` which matches the `import` type
@@ -925,8 +975,14 @@ impl<T> Linker<T> {
         mut store: impl AsContextMut<Data = T>,
         import: &ImportType,
     ) -> Option<Extern> {
-        if let Some(item) = self.get(&mut store, import.module(), import.name()) {
-            return Some(item);
+        let mut store = store.as_context_mut().opaque();
+        // Should be safe since `T` is connecting the linker and store
+        Some(unsafe { self._get_by_import(import)?.to_extern(&mut store) })
+    }
+
+    fn _get_by_import(&self, import: &ImportType) -> Option<Definition> {
+        if let Some(item) = self._get(import.module(), import.name()) {
+            return Some(item.clone());
         }
 
         if import.name().is_some() {
@@ -949,12 +1005,12 @@ impl<T> Linker<T> {
             // it each time a module is instantiated. For now though while the
             // module linking proposal is under development this should hopefully
             // suffice.
-            let mut builder = InstanceBuilder::new();
+            let mut map = indexmap::IndexMap::new();
             for export in t.exports() {
-                let item = self.get(&mut store, import.module(), Some(export.name()))?;
-                builder.insert(export.name(), item);
+                let item = self._get(import.module(), Some(export.name()))?;
+                map.insert(export.name().to_string(), item.clone());
             }
-            return Some(builder.finish(&mut store.as_context_mut().opaque()).into());
+            return Some(Definition::Instance(Arc::new(map)));
         }
 
         None
@@ -999,31 +1055,30 @@ impl<T> Default for Linker<T> {
     }
 }
 
-impl<T> Definition<T> {
-    fn to_extern(&self, mut store: impl AsContextMut<Data = T>) -> Extern {
+impl Definition {
+    /// Note the unsafety here is due to calling `HostFunc::to_func`. The
+    /// requirement here is that the `T` that was originally used to create the
+    /// `HostFunc` matches the `T` on the store.
+    pub(crate) unsafe fn to_extern(&self, store: &mut StoreOpaque) -> Extern {
         match self {
             Definition::Extern(e) => e.clone(),
-
-            // Note the unsafety here is due to calling `to_func`. The
-            // requirement here is that the `T` that was originally used to
-            // create the `HostFunc` matches the `T` on the store. This is done
-            // with the `T` on `Definition` (and `Linker`) as well as the
-            // `Data=T` bound above.
-            Definition::HostFunc { func, .. } => unsafe {
-                func.to_func(&mut store.as_context_mut().opaque()).into()
-            },
+            Definition::HostFunc(func) => func.to_func(store).into(),
+            Definition::Instance(i) => {
+                let items = Arc::new(
+                    i.iter()
+                        .map(|(name, item)| (name.clone(), item.to_extern(store)))
+                        .collect(),
+                );
+                Instance::from_wasmtime(items, store).into()
+            }
         }
     }
-}
 
-impl<T> Clone for Definition<T> {
-    fn clone(&self) -> Definition<T> {
+    pub(crate) fn comes_from_same_store(&self, store: &StoreOpaque) -> bool {
         match self {
-            Definition::Extern(e) => Definition::Extern(e.clone()),
-            Definition::HostFunc { func, t } => Definition::HostFunc {
-                func: func.clone(),
-                t: *t,
-            },
+            Definition::Extern(e) => e.comes_from_same_store(store),
+            Definition::HostFunc(_func) => true,
+            Definition::Instance(i) => i.values().all(|e| e.comes_from_same_store(store)),
         }
     }
 }

crates/wasmtime/src/store.rs

@@ -575,7 +575,7 @@ impl<T: ?Sized> StoreInner<T> {
     }
 
     pub fn async_support(&self) -> bool {
-        self.engine().config().async_support
+        cfg!(feature = "async") && self.engine().config().async_support
     }
 
     pub fn engine(&self) -> &Engine {

crates/wasmtime/src/types/matching.rs

@@ -1,3 +1,4 @@
+use crate::linker::Definition;
 use crate::store::StoreData;
 use crate::{signatures::SignatureCollection, Engine, Extern};
 use anyhow::{bail, Context, Result};
@@ -5,6 +6,7 @@ use wasmtime_environ::wasm::{
     EntityType, Global, InstanceTypeIndex, Memory, ModuleTypeIndex, SignatureIndex, Table,
 };
 use wasmtime_jit::TypeTables;
+use wasmtime_runtime::VMSharedSignatureIndex;
 
 pub struct MatchCx<'a> {
     pub signatures: &'a SignatureCollection,
@@ -73,8 +75,24 @@ impl MatchCx<'_> {
     }
 
     pub fn func(&self, expected: SignatureIndex, actual: &crate::Func) -> Result<()> {
+        self.vmshared_signature_index(expected, actual.sig_index(self.store_data))
+    }
+
+    pub(crate) fn host_func(
+        &self,
+        expected: SignatureIndex,
+        actual: &crate::func::HostFunc,
+    ) -> Result<()> {
+        self.vmshared_signature_index(expected, actual.sig_index())
+    }
+
+    pub fn vmshared_signature_index(
+        &self,
+        expected: SignatureIndex,
+        actual: VMSharedSignatureIndex,
+    ) -> Result<()> {
         let matches = match self.signatures.shared_signature(expected) {
-            Some(idx) => actual.sig_index(self.store_data) == idx,
+            Some(idx) => actual == idx,
             // If our expected signature isn't registered, then there's no way
             // that `actual` can match it.
             None => false,
@@ -295,4 +313,44 @@ impl MatchCx<'_> {
             EntityType::Event(_) => unimplemented!(),
         }
     }
+
+    /// Validates that the `expected` type matches the type of `actual`
+    pub(crate) fn definition(&self, expected: &EntityType, actual: &Definition) -> Result<()> {
+        match actual {
+            Definition::Extern(e) => self.extern_(expected, e),
+            Definition::HostFunc(f) => match expected {
+                EntityType::Function(expected) => self.host_func(*expected, f),
+                _ => bail!("expected {}, but found func", entity_desc(expected)),
+            },
+            Definition::Instance(items) => match expected {
+                EntityType::Instance(expected) => {
+                    for (name, expected) in self.types.instance_signatures[*expected].exports.iter()
+                    {
+                        match items.get(name) {
+                            Some(item) => {
+                                self.definition(expected, item).with_context(|| {
+                                    format!("instance export {:?} incompatible", name)
+                                })?;
+                            }
+                            None => bail!("instance type missing export {:?}", name),
+                        }
+                    }
+                    Ok(())
+                }
+                _ => bail!("expected {}, but found instance", entity_desc(expected)),
+            },
+        }
+    }
+}
+
+fn entity_desc(ty: &EntityType) -> &'static str {
+    match ty {
+        EntityType::Global(_) => "global",
+        EntityType::Table(_) => "table",
+        EntityType::Memory(_) => "memory",
+        EntityType::Function(_) => "func",
+        EntityType::Instance(_) => "instance",
+        EntityType::Module(_) => "module",
+        EntityType::Event(_) => "event",
+    }
 }

tests/all/linker.rs

@@ -308,3 +308,35 @@ fn alias_one() -> Result<()> {
     assert!(linker.get(&mut store, "c", Some("d")).is_some());
     Ok(())
 }
+
+#[test]
+fn instance_pre() -> Result<()> {
+    let engine = Engine::default();
+    let mut linker = Linker::new(&engine);
+    linker.func_wrap("", "", || {})?;
+
+    let module = Module::new(&engine, r#"(module (import "" "" (func)))"#)?;
+    let instance_pre = linker.instantiate_pre(&mut Store::new(&engine, ()), &module)?;
+    instance_pre.instantiate(&mut Store::new(&engine, ()))?;
+    instance_pre.instantiate(&mut Store::new(&engine, ()))?;
+
+    let mut store = Store::new(&engine, ());
+    let global = Global::new(
+        &mut store,
+        GlobalType::new(ValType::I32, Mutability::Const),
+        1.into(),
+    )?;
+    linker.define("", "g", global)?;
+
+    let module = Module::new(
+        &engine,
+        r#"(module
+            (import "" "" (func))
+            (import "" "g" (global i32))
+        )"#,
+    )?;
+    let instance_pre = linker.instantiate_pre(&mut store, &module)?;
+    instance_pre.instantiate(&mut store)?;
+    instance_pre.instantiate(&mut store)?;
+    Ok(())
+}