Struct 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)?
            .ok_or_else(|| Error::msg("externref has no host data"))?
            .downcast_ref::<Cow<str>>()
            .ok_or_else(|| Error::msg("externref was not a string"))?
            .clone()
            .into_owned();
        let b = b
            .data(&caller)?
            .ok_or_else(|| Error::msg("externref has no host data"))?
            .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)?
        .expect("externref should have host data")
        .downcast_ref::<Cow<str>>()
        .expect("host data should be a `Cow<str>`"),
    "Hello, World!"
);

Implementations

impl ExternRef

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

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

Synchronously allocates a new ExternRef wrapping the given value.

The resulting value is automatically unrooted when the given context’s scope is exited. If you need to hold the reference past the context’s scope, convert the result into a ManuallyRooted<T>. See the documentation for Rooted<T> and ManuallyRooted<T> for more details.

Automatic Garbage Collection

If the GC heap is at capacity, and there isn’t room for allocating a new externref, this method will automatically trigger a synchronous collection in an attempt to free up space in the GC heap.

Errors

If the allocation cannot be satisfied because the GC heap is currently out of memory, then a GcHeapOutOfMemory<T> error is returned. The allocation might succeed on a second attempt if you drop some rooted GC references and try again.

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

Example

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

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

    // Allocate an `externref` wrapping a `String`.
    let externref = match ExternRef::new(&mut scope, "hello!".to_string()) {
        // The allocation succeeded.
        Ok(x) => x,
        // The allocation failed.
        Err(e) => match e.downcast::<GcHeapOutOfMemory<String>>() {
            // The allocation failed because the GC heap does not have
            // capacity for this allocation.
            Ok(oom) => {
                // Take back ownership of our `String`.
                let s = oom.into_inner();
                // Drop some rooted GC refs from our system to potentially
                // free up space for Wasmtime to make this allocation.
                drop_some_gc_refs();
                // Finally, try to allocate again, reusing the original
                // string.
                ExternRef::new(&mut scope, s)?
            }
            Err(e) => return Err(e),
        },
    };

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

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

Panics

Panics if the context is configured for async; use [ExternRef::new_async][crate::ExternRef::new_async] to perform asynchronous allocation instead.

pub fn convert_any( context: impl AsContextMut, anyref: Rooted<AnyRef>, ) -> Result<Rooted<ExternRef>, Error>

Convert an anyref into an externref.

This is equivalent to the extern.convert_any instruction in Wasm.

You can get the underlying anyref again via the AnyRef::convert_extern method or the any.convert_extern Wasm instruction.

The resulting ExternRef will not have any host data associated with it, so ExternRef::data and ExternRef::data_mut will both return None.

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

Example

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

// Create an `anyref`.
let i31 = I31::wrapping_u32(0x1234);
let anyref = AnyRef::from_i31(&mut store, i31);

// Convert that `anyref` into an `externref`.
let externref = ExternRef::convert_any(&mut store, anyref)?;

// This `externref` doesn't have any associated host data.
assert!(externref.data(&store)?.is_none());

// We can convert it back to an `anyref` and get its underlying `i31`
// data.
let anyref = AnyRef::convert_extern(&mut store, externref)?;
assert_eq!(anyref.unwrap_i31(&store)?.get_u32(), 0x1234);

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

where T: 'static,

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

Returns None if this is an externref wrapper of an anyref created by the extern.convert_any instruction or the ExternRef::convert_any method.

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)?.ok_or_else(|| Error::msg("no host data"))?;
// 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<Option<&'a mut (dyn Any + Send + Sync + 'static)>, Error>

where T: 'static,

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

Returns None if this is an externref wrapper of an anyref created by the extern.convert_any instruction or the ExternRef::convert_any constructor.

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)?.ok_or_else(|| Error::msg("no host data"))?;
// 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, Error>

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<(), Error>

Formats the value using the given formatter.