Add the wasmtime::NoneRef host API type for Wasm's `(ref null? none…

…)` instances

crates/wasmtime/src/runtime/func.rs

@@ -615,6 +615,8 @@ impl Func {
     /// | `Rooted<StructRef>`               | `(ref struct)`                            |
     /// | `Option<Rooted<ArrayRef>>`        | `(ref null array)`                        |
     /// | `Rooted<ArrayRef>`                | `(ref array)`                             |
+    /// | `Option<NoneRef>`                 | `nullref` aka `(ref null none)`           |
+    /// | `NoneRef`                         | `(ref none)`                              |
     ///
     /// Note that anywhere a `Rooted<T>` appears, a `ManuallyRooted<T>` may also
     /// be used.
@@ -1444,6 +1446,8 @@ impl Func {
     /// | `(ref struct)`                            | `Rooted<StructRef>`                   |
     /// | `arrayref` aka `(ref null array)`         | `Option<Rooted<ArrayRef>>`            |
     /// | `(ref array)`                             | `Rooted<ArrayRef>`                    |
+    /// | `nullref` aka `(ref null none)`           | `Option<NoneRef>`                     |
+    /// | `(ref none)`                              | `NoneRef`                             |
     /// | `funcref` aka `(ref null func)`           | `Option<Func>`                        |
     /// | `(ref func)`                              | `Func`                                |
     /// | `(ref null <func type index>)`            | `Option<Func>`                        |

crates/wasmtime/src/runtime/gc.rs

@@ -11,6 +11,9 @@ pub use disabled::*;
 mod noextern;
 pub use noextern::NoExtern;
 
+mod none_ref;
+pub use none_ref::NoneRef;
+
 use core::fmt;
 use core::ops::Deref;
 

crates/wasmtime/src/runtime/gc/none_ref.rs

@@ -0,0 +1,154 @@
+use crate::{
+    store::{AutoAssertNoGc, StoreOpaque},
+    HeapType, Ref, RefType, Result, Uninhabited, Val, ValRaw, ValType, WasmTy,
+};
+use core::mem::MaybeUninit;
+
+/// A reference to the abstract `none` heap value.
+///
+/// The are no instances of `(ref none)`: it is an uninhabited type.
+///
+/// There is precisely one instance of `(ref null none)`, aka `nullref`: the
+/// null reference.
+///
+/// This `NoneRef` Rust type's sole purpose is for use with
+/// [`Func::wrap`][crate::Func::wrap]- and
+/// [`Func::typed`][crate::Func::typed]-style APIs for statically typing a
+/// function as taking or returning a `(ref null none)` (aka `Option<NoneRef>`)
+/// which is always `None`.
+///
+/// # Example
+///
+/// ```
+/// # use wasmtime::*;
+/// # fn _foo() -> Result<()> {
+/// let mut config = Config::new();
+/// config.wasm_function_references(true);
+/// config.wasm_gc(true);
+/// let engine = Engine::new(&config)?;
+///
+/// let module = Module::new(
+///     &engine,
+///     r#"
+///         (module
+///             (func (export "f") (param (ref null none))
+///                 ;; If the reference is null, return.
+///                 local.get 0
+///                 ref.is_null none
+///                 br_if 0
+///
+///                 ;; If the reference was not null (which is impossible)
+///                 ;; then raise a trap.
+///                 unreachable
+///             )
+///         )
+///     "#,
+/// )?;
+///
+/// let mut store = Store::new(&engine, ());
+/// let instance = Instance::new(&mut store, &module, &[])?;
+/// let f = instance.get_func(&mut store, "f").unwrap();
+///
+/// // We can cast a `(ref null none)`-taking function into a typed function that
+/// // takes an `Option<NoneRef>` via the `Func::typed` method.
+/// let f = f.typed::<Option<NoneRef>, ()>(&store)?;
+///
+/// // We can call the typed function, passing the null `none` reference.
+/// let result = f.call(&mut store, NoneRef::null());
+///
+/// // The function should not have trapped, because the reference we gave it was
+/// // null (as it had to be, since `NoneRef` is uninhabited).
+/// assert!(result.is_ok());
+/// # Ok(())
+/// # }
+/// ```
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct NoneRef {
+    _inner: Uninhabited,
+}
+
+impl NoneRef {
+    /// Get the null `(ref null none)` (aka `nullexternref`) reference.
+    #[inline]
+    pub fn null() -> Option<Self> {
+        None
+    }
+
+    /// Get the null `(ref null none)` (aka `nullexternref`) reference as a
+    /// [`Ref`].
+    #[inline]
+    pub fn null_ref() -> Ref {
+        Ref::Extern(None)
+    }
+
+    /// Get the null `(ref null none)` (aka `nullexternref`) reference as a
+    /// [`Val`].
+    #[inline]
+    pub fn null_val() -> Val {
+        Val::ExternRef(None)
+    }
+}
+
+unsafe impl WasmTy for NoneRef {
+    #[inline]
+    fn valtype() -> ValType {
+        ValType::Ref(RefType::new(false, HeapType::None))
+    }
+
+    #[inline]
+    fn compatible_with_store(&self, _store: &StoreOpaque) -> bool {
+        match self._inner {}
+    }
+
+    #[inline]
+    fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
+        match self._inner {}
+    }
+
+    #[inline]
+    fn is_vmgcref_and_points_to_object(&self) -> bool {
+        match self._inner {}
+    }
+
+    fn store(self, _store: &mut AutoAssertNoGc<'_>, _ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
+        match self._inner {}
+    }
+
+    unsafe fn load(_store: &mut AutoAssertNoGc<'_>, _ptr: &ValRaw) -> Self {
+        unreachable!("NoneRef is uninhabited")
+    }
+}
+
+unsafe impl WasmTy for Option<NoneRef> {
+    #[inline]
+    fn valtype() -> ValType {
+        ValType::Ref(RefType::new(true, HeapType::None))
+    }
+
+    #[inline]
+    fn compatible_with_store(&self, _store: &StoreOpaque) -> bool {
+        true
+    }
+
+    #[inline]
+    fn dynamic_concrete_type_check(
+        &self,
+        _store: &StoreOpaque,
+        _nullable: bool,
+        _ty: &HeapType,
+    ) -> Result<()> {
+        unreachable!()
+    }
+
+    #[inline]
+    fn store(self, _store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
+        ptr.write(ValRaw::externref(0));
+        Ok(())
+    }
+
+    #[inline]
+    unsafe fn load(_store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
+        debug_assert_eq!(ptr.get_externref(), 0);
+        None
+    }
+}

tests/all/func.rs

@@ -413,6 +413,8 @@ fn func_constructors() {
     Func::wrap(&mut store, || -> Option<NoFunc> { None });
     Func::wrap(&mut store, || -> NoExtern { loop {} });
     Func::wrap(&mut store, || -> Option<NoExtern> { None });
+    Func::wrap(&mut store, || -> NoneRef { loop {} });
+    Func::wrap(&mut store, || -> Option<NoneRef> { None });
 
     Func::wrap(&mut store, || -> Result<()> { loop {} });
     Func::wrap(&mut store, || -> Result<i32> { loop {} });
@@ -444,6 +446,8 @@ fn func_constructors() {
     Func::wrap(&mut store, || -> Result<Option<NoFunc>> { loop {} });
     Func::wrap(&mut store, || -> Result<NoExtern> { loop {} });
     Func::wrap(&mut store, || -> Result<Option<NoExtern>> { loop {} });
+    Func::wrap(&mut store, || -> Result<NoneRef> { loop {} });
+    Func::wrap(&mut store, || -> Result<Option<NoneRef>> { loop {} });
 }
 
 #[test]