Struct wasmtime::ExternRef

pub struct ExternRef { /* private fields */ }

An opaque, GC-managed reference to some host data that can be passed to WebAssembly.

The ExternRef type represents WebAssembly externref values. Wasm can’t do anything with the externrefs other than put them in tables, globals, and locals or pass them to other functions (such as imported functions from the host). Unlike anyrefs, Wasm guests cannot directly allocate new externrefs; only the host can.

You can use ExternRef to give access to host objects and control the operations that Wasm can perform on them via what functions you allow Wasm to import.

Like all WebAssembly references, these are opaque and unforgeable to Wasm: they cannot be faked and Wasm cannot, for example, cast the integer 0x12345678 into a reference, pretend it is a valid externref, and trick the host into dereferencing it and segfaulting or worse.

Note that you can also use Rooted<ExternRef> and ManuallyRooted<ExternRef> as a type parameter with Func::typed- and Func::wrap-style APIs.

Example

let engine = Engine::default();
let mut store = Store::new(&engine, ());

// Define some APIs for working with host strings from Wasm via `externref`.
let mut linker = Linker::new(&engine);
linker.func_wrap(
    "host-string",
    "new",
    |caller: Caller<'_, ()>| -> Result<Rooted<ExternRef>> {
        ExternRef::new(caller, Cow::from(""))
    },
)?;
linker.func_wrap(
    "host-string",
    "concat",
    |mut caller: Caller<'_, ()>, a: Rooted<ExternRef>, b: Rooted<ExternRef>| -> Result<Rooted<ExternRef>> {
        let mut s = a
            .data(&caller)?
            .downcast_ref::<Cow<str>>()
            .ok_or_else(|| Error::msg("externref was not a string"))?
            .clone()
            .into_owned();
        let b = b
            .data(&caller)?
            .downcast_ref::<Cow<str>>()
            .ok_or_else(|| Error::msg("externref was not a string"))?;
        s.push_str(&b);
        ExternRef::new(&mut caller, s)
    },
)?;

// Here is a Wasm module that uses those APIs.
let module = Module::new(
    &engine,
    r#"
        (module
            (import "host-string" "concat" (func $concat (param externref externref)
                                                         (result externref)))
            (func (export "run") (param externref externref) (result externref)
                local.get 0
                local.get 1
                call $concat
            )
        )
    "#,
)?;

// Create a couple `externref`s wrapping `Cow<str>`s.
let hello = ExternRef::new(&mut store, Cow::from("Hello, "))?;
let world = ExternRef::new(&mut store, Cow::from("World!"))?;

// Instantiate the module and pass the `externref`s into it.
let instance = linker.instantiate(&mut store, &module)?;
let result = instance
    .get_typed_func::<(Rooted<ExternRef>, Rooted<ExternRef>), Rooted<ExternRef>>(&mut store, "run")?
    .call(&mut store, (hello, world))?;

// The module should have concatenated the strings together!
assert_eq!(
    result.data(&store)?.downcast_ref::<Cow<str>>().unwrap(),
    "Hello, World!"
);

Implementations

impl ExternRef

pub fn new<T>(context: impl AsContextMut, value: T) -> Result<Rooted<ExternRef>>

where T: 'static + Any + Send + Sync,

Creates a new instance of ExternRef wrapping the given value.

The resulting value is automatically unrooted when the given context’s scope is exited. See Rooted<T>’s documentation for more details.

This method will not automatically trigger a GC to free up space in the GC heap; instead it will return an error. This gives you more precise control over when collections happen and allows you to choose between performing synchronous and asynchronous collections.

Errors

If the allocation cannot be satisfied because the GC heap is currently out of memory, but performing a garbage collection might free up space such that retrying the allocation afterwards might succeed, then a GcHeapOutOfMemory<T> error is returned.

The GcHeapOutOfMemory<T> error contains the host value that the externref would have wrapped. You can extract that value from this error and reuse it when attempting to allocate an externref again after GC or otherwise do with it whatever you see fit.

Example

let mut store = Store::<()>::default();

{
    let mut scope = RootScope::new(&mut store);

    // Create an `externref` wrapping a `str`.
    let externref = match ExternRef::new(&mut scope, "hello!") {
        Ok(x) => x,
        // If the heap is out of memory, then do a GC and try again.
        Err(e) if e.is::<GcHeapOutOfMemory<&'static str>>() => {
            // Do a GC! Note: in an async context, you'd want to do
            // `scope.as_context_mut().gc_async().await`.
            scope.as_context_mut().gc();

            // Extract the original host value from the error.
            let host_value = e
                .downcast::<GcHeapOutOfMemory<&'static str>>()
                .unwrap()
                .into_inner();

            // Try to allocate the `externref` again, now that the GC
            // has hopefully freed up some space.
            ExternRef::new(&mut scope, host_value)?
        }
        Err(e) => return Err(e),
    };

    // Use the `externref`, pass it to Wasm, etc...
}

// The `externref` is automatically unrooted when we exit the scope.

pub fn new_manually_rooted<T>( store: impl AsContextMut, value: T, ) -> Result<ManuallyRooted<ExternRef>>

where T: 'static + Any + Send + Sync,

Creates a new, manually-rooted instance of ExternRef wrapping the given value.

The resulting value must be manually unrooted, or else it will leak for the entire duration of the store’s lifetime. See ManuallyRooted<T>’s documentation for more details.

Errors

This function returns the same errors in the same scenarios as ExternRef::new.

Example

let mut store = Store::<()>::default();

// Create a manually-rooted `externref` wrapping a `str`.
let externref = ExternRef::new_manually_rooted(&mut store, "hello!")?;

// Use `externref` a bunch, pass it to Wasm, etc...

// Don't forget to explicitly unroot the `externref` when you're done
// using it!
externref.unroot(&mut store);

pub fn data<'a, T>( &self, store: impl Into<StoreContext<'a, T>>, ) -> Result<&'a (dyn Any + Send + Sync)>

where T: 'a,

Get a shared borrow of the underlying data for this ExternRef.

Returns an error if this externref GC reference has been unrooted (eg if you attempt to use a Rooted<ExternRef> after exiting the scope it was rooted within). See the documentation for Rooted<T> for more details.

Example

let mut store = Store::<()>::default();

let externref = ExternRef::new(&mut store, "hello")?;

// Access the `externref`'s host data.
let data = externref.data(&store)?;
// Dowcast it to a `&str`.
let data = data.downcast_ref::<&str>().ok_or_else(|| Error::msg("not a str"))?;
// We should have got the data we created the `externref` with!
assert_eq!(*data, "hello");

pub fn data_mut<'a, T>( &self, store: impl Into<StoreContextMut<'a, T>>, ) -> Result<&'a mut (dyn Any + Send + Sync)>

where T: 'a,

Get an exclusive borrow of the underlying data for this ExternRef.

Returns an error if this externref GC reference has been unrooted (eg if you attempt to use a Rooted<ExternRef> after exiting the scope it was rooted within). See the documentation for Rooted<T> for more details.

Example

let mut store = Store::<()>::default();

let externref = ExternRef::new::<usize>(&mut store, 0)?;

// Access the `externref`'s host data.
let data = externref.data_mut(&mut store)?;
// Dowcast it to a `usize`.
let data = data.downcast_mut::<usize>().ok_or_else(|| Error::msg("not a usize"))?;
// We initialized to zero.
assert_eq!(*data, 0);
// And we can mutate the value!
*data += 10;

pub unsafe fn from_raw( store: impl AsContextMut, raw: u32, ) -> Option<Rooted<ExternRef>>

Creates a new strongly-owned ExternRef from the raw value provided.

This is intended to be used in conjunction with Func::new_unchecked, Func::call_unchecked, and ValRaw with its externref field.

This function assumes that raw is an externref value which is currently rooted within the Store.

Unsafety

This function is particularly unsafe because raw not only must be a valid externref value produced prior by to_raw but it must also be correctly rooted within the store. When arguments are provided to a callback with Func::new_unchecked, for example, or returned via Func::call_unchecked, if a GC is performed within the store then floating externref values are not rooted and will be GC’d, meaning that this function will no longer be safe to call with the values cleaned up. This function must be invoked before possible GC operations can happen (such as calling wasm).

When in doubt try to not use this. Instead use the safe Rust APIs of TypedFunc and friends.

pub unsafe fn to_raw(&self, store: impl AsContextMut) -> Result<u32>

Converts this ExternRef to a raw value suitable to store within a ValRaw.

Returns an error if this externref has been unrooted.

Unsafety

Produces a raw value which is only safe to pass into a store if a GC doesn’t happen between when the value is produce and when it’s passed into the store.

Trait Implementations

impl Clone for ExternRef

fn clone(&self) -> ExternRef

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ExternRef

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.