Add a wasmtime::Linker type (#1384)

* Add a `wasmtime::Linker` type

This commit adds a new type to the `wasmtime` crate, a `Linker`. This
linker is intended to vastly simplify calling `Instance::new` by easily
performing name resolution and incrementally defining state over time.
The goal here is to start down a path of making linking wasm modules in
`wasmtime` a first-class and ergonomic operation. This is highly likely
to evolve over time and get tweaked through releases as we iterate
towards a design well-suited for `wasmtime`, but this is intended to at
least be the initial foundation for such functionality.

This commit additionally also adds a C API for the linker and switches
the existing linking examples to using this linker in both Rust and C.

One piece of future work I'd like to tackle next is to integrate WASI
into the `wasmtime` crate in a more first-class manner. This [`Linker`]
type provides a great location to hook into the instantiation process to
easily instantiate modules with WASI imports. That's a relatively large
refactoring for now though and I figured it'd be best left for a
different time.

Closes #727

Cargo.lock

@@ -2395,6 +2395,7 @@ dependencies = [
 name = "wasmtime-c-api"
 version = "0.12.0"
 dependencies = [
+ "anyhow",
  "wasi-common",
  "wasmtime",
  "wasmtime-wasi",

crates/api/src/lib.rs

@@ -12,6 +12,7 @@ mod externals;
 mod frame_info;
 mod func;
 mod instance;
+mod linker;
 mod module;
 mod r#ref;
 mod runtime;
@@ -24,6 +25,7 @@ pub use crate::externals::*;
 pub use crate::frame_info::FrameInfo;
 pub use crate::func::*;
 pub use crate::instance::Instance;
+pub use crate::linker::*;
 pub use crate::module::Module;
 pub use crate::r#ref::{AnyRef, HostInfo, HostRef};
 pub use crate::runtime::*;

crates/api/src/linker.rs

@@ -0,0 +1,368 @@
+use crate::{
+    Extern, ExternType, Func, FuncType, GlobalType, ImportType, Instance, IntoFunc, Module, Store,
+};
+use anyhow::{bail, Result};
+use std::collections::hash_map::{Entry, HashMap};
+use std::rc::Rc;
+
+/// Structure used to link wasm modules/instances together.
+///
+/// This structure is used to assist in instantiating a [`Module`]. A `Linker`
+/// is a way of performing name resolution to make instantiating a module easier
+/// (as opposed to calling [`Instance::new`]). `Linker` is a name-based resolver
+/// where names are dynamically defined and then used to instantiate a
+/// [`Module`]. The goal of a `Linker` is to have a one-argument method,
+/// [`Linker::instantiate`], which takes a [`Module`] and produces an
+/// [`Instance`].  This method will automatically select all the right imports
+/// for the [`Module`] to be instantiated, and will otherwise return an error
+/// if an import isn't satisfied.
+///
+/// ## Name Resolution
+///
+/// As mentioned previously, `Linker` is a form of name resolver. It will be
+/// using the string-based names of imports on a module to attempt to select a
+/// matching item to hook up to it. This name resolution has two-levels of
+/// namespaces, a module level and a name level. Each item is defined within a
+/// module and then has its own name. This basically follows the wasm standard
+/// for modularization.
+///
+/// Names in a `Linker` can be defined twice, but only for different signatures
+/// of items. This means that every item defined in a `Linker` has a unique
+/// name/type pair. For example you can define two functions with the module
+/// name `foo` and item name `bar`, so long as they have different function
+/// signatures. Currently duplicate memories and tables are not allowed, only
+/// one-per-name is allowed.
+pub struct Linker {
+    store: Store,
+    string2idx: HashMap<Rc<str>, usize>,
+    strings: Vec<Rc<str>>,
+    map: HashMap<ImportKey, Extern>,
+}
+
+#[derive(Hash, PartialEq, Eq)]
+struct ImportKey {
+    name: usize,
+    module: usize,
+    kind: ImportKind,
+}
+
+#[derive(Hash, PartialEq, Eq, Debug)]
+enum ImportKind {
+    Func(FuncType),
+    Global(GlobalType),
+    Memory,
+    Table,
+}
+
+impl Linker {
+    /// Creates a new [`Linker`].
+    ///
+    /// This function will create a new [`Linker`] which is ready to start
+    /// linking modules. All items defined in this linker and produced by this
+    /// linker will be connected with `store` and must come from the same
+    /// `store`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use wasmtime::{Linker, Store};
+    ///
+    /// let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// // ...
+    /// ```
+    pub fn new(store: &Store) -> Linker {
+        Linker {
+            store: store.clone(),
+            map: HashMap::new(),
+            string2idx: HashMap::new(),
+            strings: Vec::new(),
+        }
+    }
+
+    /// Defines a new item in this [`Linker`].
+    ///
+    /// This method will add a new definition, by name, to this instance of
+    /// [`Linker`]. The `module` and `name` provided are what to name the
+    /// `item`.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the `module` and `name` already identify an item
+    /// of the same type as the `item` provided. For more information see the
+    /// documentation on [`Linker`].
+    ///
+    /// Also returns an error if `item` comes from a different store than this
+    /// [`Linker`] was created with.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// let ty = GlobalType::new(ValType::I32, Mutability::Const);
+    /// let global = Global::new(&store, ty, Val::I32(0x1234))?;
+    /// linker.define("host", "offset", global);
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "offset" (global i32))
+    ///         (memory 1)
+    ///         (data (global.get 0) "foo")
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn define(
+        &mut self,
+        module: &str,
+        name: &str,
+        item: impl Into<Extern>,
+    ) -> Result<&mut Self> {
+        self._define(module, name, item.into())
+    }
+
+    fn _define(&mut self, module: &str, name: &str, item: Extern) -> Result<&mut Self> {
+        if !item.comes_from_same_store(&self.store) {
+            bail!("all linker items must be from the same store");
+        }
+        self.insert(module, name, &item.ty(), item)?;
+        Ok(self)
+    }
+
+    /// Convenience wrapper to define a function import.
+    ///
+    /// This method is a convenience wrapper around [`Linker::define`] which
+    /// internally delegates to [`Func::wrap`].
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the `module` and `name` already identify a function
+    /// of the same signature as `func`. For more information see the
+    /// documentation on [`Linker`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// linker.func("host", "double", |x: i32| x * 2)?;
+    /// linker.func("host", "log_i32", |x: i32| println!("{}", x))?;
+    /// linker.func("host", "log_str", |caller: Caller, ptr: i32, len: i32| {
+    ///     // ...
+    /// })?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "double" (func (param i32) (result i32)))
+    ///         (import "host" "log_i32" (func (param i32)))
+    ///         (import "host" "log_str" (func (param i32 i32)))
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn func<Params, Args>(
+        &mut self,
+        module: &str,
+        name: &str,
+        func: impl IntoFunc<Params, Args>,
+    ) -> Result<&mut Self> {
+        self._define(module, name, Func::wrap(&self.store, func).into())
+    }
+
+    /// Convenience wrapper to define an entire [`Instance`] in this linker.
+    ///
+    /// This function is a convenience wrapper around [`Linker::define`] which
+    /// will define all exports on `instance` into this linker. The module name
+    /// for each export is `module_name`, and the name for each export is the
+    /// name in the instance itself.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the any item is redefined twice in this linker (for
+    /// example the same `module_name` was already defined), or if `instance`
+    /// comes from a different [`Store`] than this [`Linker`] originally was
+    /// created with.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    ///
+    /// // Instantiate a small instance...
+    /// let wat = r#"(module (func (export "run") ))"#;
+    /// let module = Module::new(&store, wat)?;
+    /// let instance = linker.instantiate(&module)?;
+    ///
+    /// // ... and inform the linker that the name of this instance is
+    /// // `instance1`. This defines the `instance1::run` name for our next
+    /// // module to use.
+    /// linker.instance("instance1", &instance)?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "instance1" "run" (func $instance1_run))
+    ///         (func (export "run")
+    ///             call $instance1_run
+    ///         )
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// let instance = linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn instance(&mut self, module_name: &str, instance: &Instance) -> Result<&mut Self> {
+        if !Store::same(&self.store, instance.store()) {
+            bail!("all linker items must be from the same store");
+        }
+        for (export, item) in instance.module().exports().iter().zip(instance.exports()) {
+            self.insert(module_name, export.name(), export.ty(), item.clone())?;
+        }
+        Ok(self)
+    }
+
+    fn insert(&mut self, module: &str, name: &str, ty: &ExternType, item: Extern) -> Result<()> {
+        let key = self.import_key(module, name, ty);
+        match self.map.entry(key) {
+            Entry::Occupied(o) => bail!(
+                "import of `{}::{}` with as {:?} defined twice",
+                module,
+                name,
+                o.key().kind,
+            ),
+            Entry::Vacant(v) => {
+                v.insert(item);
+            }
+        }
+        Ok(())
+    }
+
+    fn import_key(&mut self, module: &str, name: &str, ty: &ExternType) -> ImportKey {
+        ImportKey {
+            module: self.intern_str(module),
+            name: self.intern_str(name),
+            kind: self.import_kind(ty),
+        }
+    }
+
+    fn import_kind(&self, ty: &ExternType) -> ImportKind {
+        match ty {
+            ExternType::Func(f) => ImportKind::Func(f.clone()),
+            ExternType::Global(f) => ImportKind::Global(f.clone()),
+            ExternType::Memory(_) => ImportKind::Memory,
+            ExternType::Table(_) => ImportKind::Table,
+        }
+    }
+
+    fn intern_str(&mut self, string: &str) -> usize {
+        if let Some(idx) = self.string2idx.get(string) {
+            return *idx;
+        }
+        let string: Rc<str> = string.into();
+        let idx = self.strings.len();
+        self.strings.push(string.clone());
+        self.string2idx.insert(string, idx);
+        idx
+    }
+
+    /// Attempts to instantiate the `module` provided.
+    ///
+    /// This method will attempt to assemble a list of imports that correspond
+    /// to the imports required by the [`Module`] provided. This list
+    /// of imports is then passed to [`Instance::new`] to continue the
+    /// instantiation process.
+    ///
+    /// Each import of `module` will be looked up in this [`Linker`] and must
+    /// have previously been defined. If it was previously defined with an
+    /// incorrect signature or if it was not prevoiusly defined then an error
+    /// will be returned because the import can not be satisfied.
+    ///
+    /// # Errors
+    ///
+    /// This method can fail because an import may not be found, or because
+    /// instantiation itself may fail. For information on instantiation
+    /// failures see [`Instance::new`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use wasmtime::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// # let store = Store::default();
+    /// let mut linker = Linker::new(&store);
+    /// linker.func("host", "double", |x: i32| x * 2)?;
+    ///
+    /// let wat = r#"
+    ///     (module
+    ///         (import "host" "double" (func (param i32) (result i32)))
+    ///     )
+    /// "#;
+    /// let module = Module::new(&store, wat)?;
+    /// linker.instantiate(&module)?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn instantiate(&self, module: &Module) -> Result<Instance> {
+        let mut imports = Vec::new();
+        for import in module.imports() {
+            if let Some(item) = self.import_get(import) {
+                imports.push(item.clone());
+                continue;
+            }
+
+            let mut options = String::new();
+            for i in self.map.keys() {
+                if &*self.strings[i.module] != import.module()
+                    || &*self.strings[i.name] != import.name()
+                {
+                    continue;
+                }
+                options.push_str("  * ");
+                options.push_str(&format!("{:?}", i.kind));
+                options.push_str("\n");
+            }
+            if options.len() == 0 {
+                bail!(
+                    "import of `{}::{}` has not been defined",
+                    import.module(),
+                    import.name()
+                )
+            }
+
+            bail!(
+                "failed to find import of `{}::{}` with matching signature\n\
+                 desired signature was: {:?}\n\
+                 signatures available:\n\n{}",
+                import.module(),
+                import.name(),
+                import.ty(),
+                options,
+            )
+        }
+
+        Instance::new(module, &imports)
+    }
+
+    fn import_get(&self, import: &ImportType) -> Option<&Extern> {
+        let key = ImportKey {
+            module: *self.string2idx.get(import.module())?,
+            name: *self.string2idx.get(import.name())?,
+            kind: self.import_kind(import.ty()),
+        };
+        self.map.get(&key)
+    }
+}

crates/api/src/types.rs

@@ -5,7 +5,7 @@ use wasmtime_environ::{ir, wasm};
 // Type attributes
 
 /// Indicator of whether a global is mutable or not
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
 pub enum Mutability {
     /// The global is constant and its value does not change
     Const,
@@ -17,7 +17,7 @@ pub enum Mutability {
 /// table/memory types.
 ///
 /// A minimum is always available but the maximum may not be present.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct Limits {
     min: u32,
     max: Option<u32>,
@@ -48,7 +48,7 @@ impl Limits {
 // Value Types
 
 /// A list of all possible value types in WebAssembly.
-#[derive(Debug, Clone, Eq, PartialEq)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub enum ValType {
     /// Signed 32 bit integer.
     I32,
@@ -114,7 +114,7 @@ impl ValType {
 ///
 /// This list can be found in [`ImportType`] or [`ExportType`], so these types
 /// can either be imported or exported.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub enum ExternType {
     /// This external type is the type of a WebAssembly function.
     Func(FuncType),
@@ -168,7 +168,7 @@ fn from_wasmtime_abiparam(param: &ir::AbiParam) -> Option<ValType> {
 /// A descriptor for a function in a WebAssembly module.
 ///
 /// WebAssembly functions can have 0 or more parameters and results.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct FuncType {
     params: Box<[ValType]>,
     results: Box<[ValType]>,
@@ -249,7 +249,7 @@ impl FuncType {
 /// This type describes an instance of a global in a WebAssembly module. Globals
 /// are local to an [`Instance`](crate::Instance) and are either immutable or
 /// mutable.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct GlobalType {
     content: ValType,
     mutability: Mutability,
@@ -295,7 +295,7 @@ impl GlobalType {
 /// Tables are contiguous chunks of a specific element, typically a `funcref` or
 /// an `anyref`. The most common use for tables is a function table through
 /// which `call_indirect` can invoke other functions.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct TableType {
     element: ValType,
     limits: Limits,
@@ -336,7 +336,7 @@ impl TableType {
 ///
 /// Memories are described in units of pages (64KB) and represent contiguous
 /// chunks of addressable memory.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct MemoryType {
     limits: Limits,
 }
@@ -366,7 +366,7 @@ impl MemoryType {
 /// [`Module::imports`](crate::Module::imports) API. Each [`ImportType`]
 /// describes an import into the wasm module with the module/name that it's
 /// imported from as well as the type of item that's being imported.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct ImportType {
     module: String,
     name: String,
@@ -409,7 +409,7 @@ impl ImportType {
 /// [`Module::exports`](crate::Module::exports) accessor and describes what
 /// names are exported from a wasm module and the type of the item that is
 /// exported.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
 pub struct ExportType {
     name: String,
     ty: ExternType,

crates/api/tests/linker.rs

@@ -0,0 +1,66 @@
+use anyhow::Result;
+use wasmtime::*;
+
+#[test]
+fn link_undefined() -> Result<()> {
+    let store = Store::default();
+    let linker = Linker::new(&store);
+    let module = Module::new(&store, r#"(module (import "" "" (func)))"#)?;
+    assert!(linker.instantiate(&module).is_err());
+    let module = Module::new(&store, r#"(module (import "" "" (global i32)))"#)?;
+    assert!(linker.instantiate(&module).is_err());
+    let module = Module::new(&store, r#"(module (import "" "" (memory 1)))"#)?;
+    assert!(linker.instantiate(&module).is_err());
+    let module = Module::new(&store, r#"(module (import "" "" (table 1 funcref)))"#)?;
+    assert!(linker.instantiate(&module).is_err());
+    Ok(())
+}
+
+#[test]
+fn link_twice_bad() -> Result<()> {
+    let store = Store::default();
+    let mut linker = Linker::new(&store);
+
+    // functions
+    linker.func("", "", || {})?;
+    assert!(linker.func("", "", || {}).is_err());
+    assert!(linker
+        .func("", "", || -> Result<(), Trap> { loop {} })
+        .is_err());
+    linker.func("", "", |_: i32| {})?;
+
+    // globals
+    let ty = GlobalType::new(ValType::I32, Mutability::Const);
+    let global = Global::new(&store, ty, Val::I32(0))?;
+    linker.define("", "", global.clone())?;
+    assert!(linker.define("", "", global.clone()).is_err());
+
+    let ty = GlobalType::new(ValType::I32, Mutability::Var);
+    let global = Global::new(&store, ty, Val::I32(0))?;
+    linker.define("", "", global.clone())?;
+    assert!(linker.define("", "", global.clone()).is_err());
+
+    let ty = GlobalType::new(ValType::I64, Mutability::Const);
+    let global = Global::new(&store, ty, Val::I64(0))?;
+    linker.define("", "", global.clone())?;
+    assert!(linker.define("", "", global.clone()).is_err());
+
+    // memories
+    let ty = MemoryType::new(Limits::new(1, None));
+    let memory = Memory::new(&store, ty);
+    linker.define("", "", memory.clone())?;
+    assert!(linker.define("", "", memory.clone()).is_err());
+    let ty = MemoryType::new(Limits::new(2, None));
+    let memory = Memory::new(&store, ty);
+    assert!(linker.define("", "", memory.clone()).is_err());
+
+    // tables
+    let ty = TableType::new(ValType::FuncRef, Limits::new(1, None));
+    let table = Table::new(&store, ty, Val::AnyRef(AnyRef::Null))?;
+    linker.define("", "", table.clone())?;
+    assert!(linker.define("", "", table.clone()).is_err());
+    let ty = TableType::new(ValType::FuncRef, Limits::new(2, None));
+    let table = Table::new(&store, ty, Val::AnyRef(AnyRef::Null))?;
+    assert!(linker.define("", "", table.clone()).is_err());
+    Ok(())
+}

crates/c-api/Cargo.toml

@@ -17,6 +17,7 @@ test = false
 doctest = false
 
 [dependencies]
+anyhow = "1.0"
 wasmtime = { path = "../api", default-features = false }
 wasi-common = { path = "../wasi-common" }
 wasmtime-wasi = { path = "../wasi" }

crates/c-api/include/wasmtime.h

@@ -4,6 +4,7 @@
 #define WASMTIME_API_H
 
 #include <wasm.h>
+#include <wasi.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -67,6 +68,39 @@ WASM_API_EXTERN bool wasmtime_wat2wasm(
     own wasm_byte_vec_t *error_message
 );
 
+#define WASMTIME_DECLARE_OWN(name) \
+  typedef struct wasmtime_##name##_t wasmtime_##name##_t; \
+  \
+  WASM_API_EXTERN void wasmtime_##name##_delete(own wasmtime_##name##_t*);
+
+WASMTIME_DECLARE_OWN(linker)
+
+WASM_API_EXTERN own wasmtime_linker_t* wasmtime_linker_new(wasm_store_t* store);
+
+WASM_API_EXTERN bool wasmtime_linker_define(
+    wasmtime_linker_t *linker,
+    const wasm_name_t *module,
+    const wasm_name_t *name,
+    const wasm_extern_t *item
+);
+
+WASM_API_EXTERN bool wasmtime_linker_define_wasi(
+    wasmtime_linker_t *linker,
+    const wasi_instance_t *instance
+);
+
+WASM_API_EXTERN bool wasmtime_linker_define_instance(
+    wasmtime_linker_t *linker,
+    const wasm_name_t *name,
+    const wasm_instance_t *instance
+);
+
+WASM_API_EXTERN wasm_instance_t* wasmtime_linker_instantiate(
+    const wasmtime_linker_t *linker,
+    const wasm_module_t *module,
+    own wasm_trap_t **trap
+);
+
 #undef own
 
 #ifdef __cplusplus

crates/c-api/src/ext.rs

@@ -1,9 +1,9 @@
 //! This file defines the extern "C" API extension, which are specific
 //! to the wasmtime implementation.
 
-use crate::{wasm_byte_vec_t, wasm_config_t, wasm_engine_t};
+use crate::*;
 use std::str;
-use wasmtime::{OptLevel, ProfilingStrategy, Strategy};
+use wasmtime::{Extern, Linker, OptLevel, ProfilingStrategy, Strategy};
 
 #[repr(u8)]
 #[derive(Clone)]
@@ -136,3 +136,80 @@ pub unsafe extern "C" fn wasmtime_wat2wasm(
         }
     }
 }
+
+#[repr(C)]
+pub struct wasmtime_linker_t {
+    linker: Linker,
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_new(store: *mut wasm_store_t) -> *mut wasmtime_linker_t {
+    Box::into_raw(Box::new(wasmtime_linker_t {
+        linker: Linker::new(&(*store).store.borrow()),
+    }))
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_delete(linker: *mut wasmtime_linker_t) {
+    drop(Box::from_raw(linker));
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_define(
+    linker: *mut wasmtime_linker_t,
+    module: *const wasm_name_t,
+    name: *const wasm_name_t,
+    item: *const wasm_extern_t,
+) -> bool {
+    let linker = &mut (*linker).linker;
+    let module = match str::from_utf8((*module).as_slice()) {
+        Ok(s) => s,
+        Err(_) => return false,
+    };
+    let name = match str::from_utf8((*name).as_slice()) {
+        Ok(s) => s,
+        Err(_) => return false,
+    };
+    let item = match &(*item).which {
+        ExternHost::Func(e) => Extern::Func(e.borrow().clone()),
+        ExternHost::Table(e) => Extern::Table(e.borrow().clone()),
+        ExternHost::Global(e) => Extern::Global(e.borrow().clone()),
+        ExternHost::Memory(e) => Extern::Memory(e.borrow().clone()),
+    };
+    linker.define(module, name, item).is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_define_wasi(
+    linker: *mut wasmtime_linker_t,
+    instance: *const wasi_instance_t,
+) -> bool {
+    let linker = &mut (*linker).linker;
+    (*instance).wasi.add_to_linker(linker).is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_define_instance(
+    linker: *mut wasmtime_linker_t,
+    name: *const wasm_name_t,
+    instance: *const wasm_instance_t,
+) -> bool {
+    let linker = &mut (*linker).linker;
+    let name = match str::from_utf8((*name).as_slice()) {
+        Ok(s) => s,
+        Err(_) => return false,
+    };
+    linker
+        .instance(name, &(*instance).instance.borrow())
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_instantiate(
+    linker: *const wasmtime_linker_t,
+    module: *const wasm_module_t,
+    trap: *mut *mut wasm_trap_t,
+) -> *mut wasm_instance_t {
+    let linker = &(*linker).linker;
+    handle_instantiate(linker.instantiate(&(*module).module.borrow()), trap)
+}

crates/c-api/src/lib.rs

@@ -5,6 +5,7 @@
 
 // TODO complete the C API
 
+use anyhow::Result;
 use std::cell::RefCell;
 use std::panic::{self, AssertUnwindSafe};
 use std::{mem, ptr, slice};
@@ -691,6 +692,15 @@ pub unsafe extern "C" fn wasm_instance_delete(instance: *mut wasm_instance_t) {
     let _ = Box::from_raw(instance);
 }
 
+impl wasm_instance_t {
+    fn new(instance: Instance) -> wasm_instance_t {
+        wasm_instance_t {
+            instance: HostRef::new(instance),
+            exports_cache: RefCell::new(None),
+        }
+    }
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn wasm_instance_new(
     store: *mut wasm_store_t,
@@ -722,12 +732,16 @@ pub unsafe extern "C" fn wasm_instance_new(
         }
         return ptr::null_mut();
     }
-    match Instance::new(module, &externs) {
+    handle_instantiate(Instance::new(module, &externs), result)
+}
+
+unsafe fn handle_instantiate(
+    instance: Result<Instance>,
+    result: *mut *mut wasm_trap_t,
+) -> *mut wasm_instance_t {
+    match instance {
         Ok(instance) => {
-            let instance = Box::new(wasm_instance_t {
+            let instance = Box::new(wasm_instance_t::new(instance));
-                instance: HostRef::new(instance),
-                exports_cache: RefCell::new(None),
-            });
             if !result.is_null() {
                 (*result) = ptr::null_mut();
             }

crates/c-api/src/wasi.rs

@@ -168,7 +168,7 @@ pub unsafe extern "C" fn wasi_config_preopen_dir(
 
 #[repr(C)]
 pub struct wasi_instance_t {
-    wasi: Wasi,
+    pub wasi: Wasi,
     export_cache: HashMap<String, Box<wasm_extern_t>>,
 }
 

crates/wasi-common/wig/src/wasi.rs

@@ -266,8 +266,10 @@ pub fn define_struct_for_wiggle(args: TokenStream) -> TokenStream {
     let mut get_exports = Vec::new();
     let mut ctor_externs = Vec::new();
     let mut ctor_fields = Vec::new();
+    let mut linker_add = Vec::new();
 
     for module in doc.modules() {
+        let module_name = module.name.as_str();
         let module_id = Ident::new(module.name.as_str(), Span::call_site());
         for func in module.funcs() {
             let name = func.name.as_str();
@@ -275,6 +277,9 @@ pub fn define_struct_for_wiggle(args: TokenStream) -> TokenStream {
             fields.push(quote! { pub #name_ident: wasmtime::Func });
             get_exports.push(quote! { #name => Some(&self.#name_ident) });
             ctor_fields.push(name_ident.clone());
+            linker_add.push(quote! {
+                linker.define(#module_name, #name, self.#name_ident.clone())?;
+            });
 
             let mut shim_arg_decls = Vec::new();
             let mut params = Vec::new();
@@ -501,6 +506,12 @@ pub fn define_struct_for_wiggle(args: TokenStream) -> TokenStream {
                     _ => None,
                 }
             }
+
+            /// Adds all wasi items to the specified `Linker`.
+            pub fn add_to_linker(&self, linker: &mut wasmtime::Linker) -> anyhow::Result<()> {
+                #(#linker_add)*
+                Ok(())
+            }
         }
     }
 }

examples/linking.c

@@ -64,68 +64,38 @@ int main() {
     exit(1);
   }
 
-  // Create imports for `linking2`
+  // Create our linker which will be linking our modules together, and then add
-  wasm_importtype_vec_t linking2_import_types;
+  // our WASI instance to it.
-  wasm_module_imports(linking2_module, &linking2_import_types);
+  wasmtime_linker_t *linker = wasmtime_linker_new(store);
-  const wasm_extern_t **linking2_imports = calloc(linking2_import_types.size, sizeof(void*));
+  bool ok = wasmtime_linker_define_wasi(linker, wasi);
-  assert(linking2_imports);
+  assert(ok);
-  for (int i = 0; i < linking2_import_types.size; i++) {
-    const wasm_extern_t *binding = wasi_instance_bind_import(wasi, linking2_import_types.data[i]);
-    if (binding != NULL) {
-      linking2_imports[i] = binding;
-    } else {
-      printf("> Failed to satisfy import\n");
-      exit(1);
-    }
-  }
-  wasm_importtype_vec_delete(&linking2_import_types);
 
-  // Instantiate `linking2`
+  // Instantiate `linking2` with our linker.
-  wasm_instance_t *linking2 = wasm_instance_new(store, linking2_module, linking2_imports, &trap);
+  wasm_instance_t *linking2 = wasmtime_linker_instantiate(linker, linking2_module, &trap);
   if (linking2 == NULL) {
+    if (trap == NULL) {
+      printf("> failed to link!\n");
+    } else {
       print_trap(trap);
+    }
     exit(1);
   }
-  free(linking2_imports);
-  wasm_extern_vec_t linking2_externs;
-  wasm_instance_exports(linking2, &linking2_externs);
-  wasm_exporttype_vec_t linking2_exports;
-  wasm_module_exports(linking2_module, &linking2_exports);
 
-  // Create imports for `linking1`
+  // Register our new `linking2` instance with the linker
-  wasm_importtype_vec_t linking1_import_types;
+  wasm_name_t linking2_name;
-  wasm_module_imports(linking1_module, &linking1_import_types);
+  linking2_name.data = "linking2";
-  const wasm_extern_t **linking1_imports = calloc(linking1_import_types.size, sizeof(void*));
+  linking2_name.size = strlen(linking2_name.data);
-  assert(linking1_imports);
+  ok = wasmtime_linker_define_instance(linker, &linking2_name, linking2);
-  for (int i = 0; i < linking1_import_types.size; i++) {
+  assert(ok);
-    const wasm_importtype_t *import = linking1_import_types.data[i];
-    const wasm_name_t *module = wasm_importtype_module(import);
-    const wasm_name_t *name = wasm_importtype_name(import);
-    if (strncmp(module->data, "linking2", module->size) == 0) {
-      const wasm_extern_t *e = NULL;
-      for (int j = 0; j < linking2_exports.size; j++) {
-        const wasm_name_t *export_name = wasm_exporttype_name(linking2_exports.data[j]);
-        if (name->size == export_name->size &&
-            strncmp(name->data, export_name->data, name->size) == 0) {
-          e = linking2_externs.data[j];
-          break;
-        }
-      }
-      if (e) {
-        linking1_imports[i] = e;
-        continue;
-      }
-    }
 
-    printf("> Failed to satisfy import\n");
+  // Instantiate `linking1` with the linker now that `linking2` is defined
-    exit(1);
+  wasm_instance_t *linking1 = wasmtime_linker_instantiate(linker, linking1_module, &trap);
-  }
-  wasm_importtype_vec_delete(&linking1_import_types);
-
-  // Instantiate `linking1`
-  wasm_instance_t *linking1 = wasm_instance_new(store, linking1_module, linking1_imports, &trap);
   if (linking1 == NULL) {
+    if (trap == NULL) {
+      printf("> failed to link!\n");
+    } else {
       print_trap(trap);
+    }
     exit(1);
   }
 
@@ -143,9 +113,9 @@ int main() {
 
   // Clean up after ourselves at this point
   wasm_extern_vec_delete(&linking1_externs);
-  wasm_extern_vec_delete(&linking2_externs);
   wasm_instance_delete(linking1);
   wasm_instance_delete(linking2);
+  wasmtime_linker_delete(linker);
   wasm_module_delete(linking1_module);
   wasm_module_delete(linking2_module);
   wasm_store_delete(store);

examples/linking.rs

@@ -9,47 +9,23 @@ use wasmtime_wasi::{Wasi, WasiCtx};
 fn main() -> Result<()> {
     let store = Store::default();
 
+    // First set up our linker which is going to be linking modules together. We
+    // want our linker to have wasi available, so we set that up here as well.
+    let mut linker = Linker::new(&store);
+    let wasi = Wasi::new(&store, WasiCtx::new(std::env::args())?);
+    wasi.add_to_linker(&mut linker)?;
+
     // Load and compile our two modules
     let linking1 = Module::from_file(&store, "examples/linking1.wat")?;
     let linking2 = Module::from_file(&store, "examples/linking2.wat")?;
 
-    // Instantiate the first, `linking2`, which uses WASI imports
+    // Instantiate our first module which only uses WASI, then register that
-    let wasi = Wasi::new(&store, WasiCtx::new(std::env::args())?);
+    // instance with the linker since the next linking will use it.
-    let mut imports = Vec::new();
+    let linking2 = linker.instantiate(&linking2)?;
-    for import in linking2.imports() {
+    linker.instance("linking2", &linking2)?;
-        if import.module() == "wasi_snapshot_preview1" {
-            if let Some(export) = wasi.get_export(import.name()) {
-                imports.push(Extern::from(export.clone()));
-                continue;
-            }
-        }
-        panic!(
-            "couldn't find import for `{}::{}`",
-            import.module(),
-            import.name()
-        );
-    }
-    let linking2 = Instance::new(&linking2, &imports)?;
 
-    // And using the previous instance we can create the imports for `linking1`,
+    // And with that we can perform the final link and the execute the module.
-    // using the previous exports.
+    let linking1 = linker.instantiate(&linking1)?;
-    let mut imports = Vec::new();
-    for import in linking1.imports() {
-        if import.module() == "linking2" {
-            if let Some(export) = linking2.get_export(import.name()) {
-                imports.push(export.clone());
-                continue;
-            }
-        }
-        panic!(
-            "couldn't find import for `{}::{}`",
-            import.module(),
-            import.name()
-        );
-    }
-    let linking1 = Instance::new(&linking1, &imports)?;
-
-    // And once everything is instantiated we can run!
     let run = linking1.get_export("run").and_then(|e| e.func()).unwrap();
     let run = run.get0::<()>()?;
     run()?;