wasmtime-c-api: Add Wasmtime-specific APIs for externrefs

This commit adds APIs to create new `externref` values (with and without
finalizers) and to get an `externref`'s wrapped data.

crates/c-api/src/ref.rs

@@ -1,4 +1,7 @@
+use crate::wasm_val_t;
+use std::any::Any;
 use std::os::raw::c_void;
+use std::ptr;
 use wasmtime::{ExternRef, Func, Val};
 
 /// `*mut wasm_ref_t` is a reference type (`externref` or `funcref`), as seen by
@@ -92,3 +95,60 @@ pub extern "C" fn wasm_ref_set_host_info_with_finalizer(
     eprintln!("`wasm_ref_set_host_info_with_finalizer` is not implemented");
     std::process::abort();
 }
+
+type wasmtime_externref_finalizer_t = extern "C" fn(*mut c_void);
+
+struct CExternRef {
+    data: *mut c_void,
+    finalizer: Option<wasmtime_externref_finalizer_t>,
+}
+
+impl Drop for CExternRef {
+    fn drop(&mut self) {
+        if let Some(f) = self.finalizer {
+            f(self.data);
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_externref_new(data: *mut c_void) -> wasm_val_t {
+    wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
+        data,
+        finalizer: None,
+    }))))
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_externref_new_with_finalizer(
+    data: *mut c_void,
+    finalizer: Option<wasmtime_externref_finalizer_t>,
+) -> wasm_val_t {
+    wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
+        data,
+        finalizer,
+    }))))
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_externref_data(val: &wasm_val_t, datap: *mut *mut c_void) -> bool {
+    match val.val() {
+        Val::ExternRef(None) => {
+            unsafe {
+                ptr::write(datap, ptr::null_mut());
+            }
+            true
+        }
+        Val::ExternRef(Some(x)) => {
+            let data = match x.data().downcast_ref::<CExternRef>() {
+                Some(r) => r.data,
+                None => x.data() as *const dyn Any as *mut c_void,
+            };
+            unsafe {
+                ptr::write(datap, data);
+            }
+            true
+        }
+        _ => false,
+    }
+}