wasmtime-c-api: Make wasmtime_externref_new write to an out pointer

The C API prefers not to return structs by value.

Same for `wasmtime_externref_new_with_finalizer`.

crates/c-api/include/wasmtime.h

@@ -839,13 +839,14 @@ WASM_API_EXTERN wasmtime_error_t *wasmtime_funcref_table_grow(
 /**
  * \brief Create a new `externref` value.
  *
- * Creates a new `externref` value wrapping the provided data.
+ * Creates a new `externref` value wrapping the provided data, and writes it to
+ * `valp`.
  *
  * This function does not take an associated finalizer to clean up the data when
  * the reference is reclaimed. If you need a finalizer to clean up the data,
  * then use #wasmtime_externref_new_with_finalizer.
  */
-WASM_API_EXTERN wasm_val_t wasmtime_externref_new(void *data);
+WASM_API_EXTERN void wasmtime_externref_new(void *data, wasm_val_t *valp);
 
 /**
  * \brief A finalizer for an `externref`'s wrapped data.
@@ -859,15 +860,17 @@ typedef void (*wasmtime_externref_finalizer_t)(void*);
 /**
  * \brief Create a new `externref` value with a finalizer.
  *
- * Creates a new `externref` value wrapping the provided data.
+ * Creates a new `externref` value wrapping the provided data, and writes it to
+ * `valp`.
  *
  * When the reference is reclaimed, the wrapped data is cleaned up with the
  * provided finalizer. If you do not need to clean up the wrapped data, then use
  * #wasmtime_externref_new.
  */
-WASM_API_EXTERN wasm_val_t wasmtime_externref_new_with_finalizer(
+WASM_API_EXTERN void wasmtime_externref_new_with_finalizer(
     void *data,
-    wasmtime_externref_finalizer_t finalizer
+    wasmtime_externref_finalizer_t finalizer,
+    wasm_val_t *valp
 );
 
 /**

crates/c-api/src/ref.rs

@@ -1,5 +1,6 @@
 use crate::wasm_val_t;
 use std::any::Any;
+use std::mem::MaybeUninit;
 use std::os::raw::c_void;
 use std::ptr;
 use wasmtime::{ExternRef, Func, Val};
@@ -112,19 +113,25 @@ impl Drop for CExternRef {
 }
 
 #[no_mangle]
-pub extern "C" fn wasmtime_externref_new(data: *mut c_void) -> wasm_val_t {
+pub extern "C" fn wasmtime_externref_new(data: *mut c_void, valp: &mut MaybeUninit<wasm_val_t>) {
-    wasmtime_externref_new_with_finalizer(data, None)
+    wasmtime_externref_new_with_finalizer(data, None, valp)
 }
 
 #[no_mangle]
 pub extern "C" fn wasmtime_externref_new_with_finalizer(
     data: *mut c_void,
     finalizer: Option<wasmtime_externref_finalizer_t>,
-) -> wasm_val_t {
+    valp: &mut MaybeUninit<wasm_val_t>,
+) {
+    unsafe {
+        ptr::write(
+            valp.as_mut_ptr(),
             wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
                 data,
                 finalizer,
-    }))))
+            })))),
+        )
+    }
 }
 
 #[no_mangle]

examples/externref.c

@@ -82,7 +82,8 @@ int main() {
   printf("Creating new `externref`...\n");
 
   // Create a new `externref` value.
-  wasm_val_t externref = wasmtime_externref_new("Hello, World!");
+  wasm_val_t externref;
+  wasmtime_externref_new("Hello, World!", &externref);
   assert(externref.kind == WASM_ANYREF);
 
   // The `externref`'s wrapped data should be the string "Hello, World!".