wasmtime-c-api: Use a safe helper for initializing MaybeUninit out pointers

crates/c-api/src/func.rs

@@ -220,12 +220,7 @@ fn _wasmtime_func_call(
     match result {
         Ok(Ok(out)) => {
             for (slot, val) in results.iter_mut().zip(out.into_vec().into_iter()) {
-                unsafe {
+                crate::initialize(slot, wasm_val_t::from_val(val));
-                    // NB: The results array is likely uninitialized memory, so
-                    // use `ptr::write` rather than assignment (which tries to
-                    // run destructors).
-                    ptr::write(slot.as_mut_ptr(), wasm_val_t::from_val(val));
-                }
             }
             None
         }

crates/c-api/src/global.rs

@@ -74,9 +74,7 @@ pub extern "C" fn wasm_global_type(g: &wasm_global_t) -> Box<wasm_globaltype_t>
 
 #[no_mangle]
 pub extern "C" fn wasm_global_get(g: &wasm_global_t, out: &mut MaybeUninit<wasm_val_t>) {
-    unsafe {
+    crate::initialize(out, wasm_val_t::from_val(g.global().get()));
-        ptr::write(out.as_mut_ptr(), wasm_val_t::from_val(g.global().get()));
-    }
 }
 
 #[no_mangle]

crates/c-api/src/lib.rs

@@ -62,3 +62,14 @@ pub struct wasm_foreign_t {
 pub struct wasm_shared_module_t {
     _unused: [u8; 0],
 }
+
+/// Initialize a `MaybeUninit<T>`
+///
+/// TODO: Replace calls to this function with
+/// https://doc.rust-lang.org/nightly/std/mem/union.MaybeUninit.html#method.write
+/// once it is stable.
+pub(crate) fn initialize<T>(dst: &mut std::mem::MaybeUninit<T>, val: T) {
+    unsafe {
+        std::ptr::write(dst.as_mut_ptr(), val);
+    }
+}

crates/c-api/src/ref.rs

@@ -123,24 +123,23 @@ pub extern "C" fn wasmtime_externref_new_with_finalizer(
     finalizer: Option<wasmtime_externref_finalizer_t>,
     valp: &mut MaybeUninit<wasm_val_t>,
 ) {
-    unsafe {
+    crate::initialize(
-        ptr::write(
+        valp,
-            valp.as_mut_ptr(),
+        wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
-            wasm_val_t::from_val(Val::ExternRef(Some(ExternRef::new(CExternRef {
+            data,
-                data,
+            finalizer,
-                finalizer,
+        })))),
-            })))),
+    );
-        )
-    }
 }
 
 #[no_mangle]
-pub extern "C" fn wasmtime_externref_data(val: &wasm_val_t, datap: *mut *mut c_void) -> bool {
+pub extern "C" fn wasmtime_externref_data(
+    val: &wasm_val_t,
+    datap: &mut MaybeUninit<*mut c_void>,
+) -> bool {
     match val.val() {
         Val::ExternRef(None) => {
-            unsafe {
+            crate::initialize(datap, ptr::null_mut());
-                ptr::write(datap, ptr::null_mut());
-            }
             true
         }
         Val::ExternRef(Some(x)) => {
@@ -148,9 +147,7 @@ pub extern "C" fn wasmtime_externref_data(val: &wasm_val_t, datap: *mut *mut c_v
                 Some(r) => r.data,
                 None => x.data() as *const dyn Any as *mut c_void,
             };
-            unsafe {
+            crate::initialize(datap, data);
-                ptr::write(datap, data);
-            }
             true
         }
         _ => false,

crates/c-api/src/val.rs

@@ -124,8 +124,8 @@ impl wasm_val_t {
 
 #[no_mangle]
 pub unsafe extern "C" fn wasm_val_copy(out: &mut MaybeUninit<wasm_val_t>, source: &wasm_val_t) {
-    ptr::write(
+    crate::initialize(
-        out.as_mut_ptr(),
+        out,
         match into_valtype(source.kind) {
             ValType::I32
             | ValType::I64