Made sure downcast_from_ptr() correctly consumes the Trap wrapper class

lib/api/src/js/trap.rs

@@ -57,6 +57,8 @@ impl Trap {
 
 #[wasm_bindgen]
 impl Trap {
+    /// A marker method to indicate that an object is an instance of the `Trap`
+    /// class.
     pub fn __wbg_wasmer_trap() {}
 }
 
@@ -108,17 +110,29 @@ fn downcast_from_ptr(value: &JsValue) -> Option<Trap> {
     let class = prototype.constructor();
     let key = JsValue::from_str("__wbg_wasmer_trap");
 
-    let _marker_func: js_sys::Function = Reflect::get(&class, &key)
+    let marker_func: Option<js_sys::Function> = Reflect::get(&class, &key)
         .and_then(|v: JsValue| v.dyn_into())
-        .ok()?;
+        .ok();
 
-    // Note: this assumes the wrapper class generated by #[wasm_bindgen] will
-    // always store the pointer in a field called "__wbg_ptr". This is valid
-    // as of wasm-bindgen version 0.2.87
-    let key = JsValue::from_str("__wbg_ptr");
-    let ptr = Reflect::get(value, &key).ok().and_then(|v| v.as_f64())?;
+    if marker_func.is_none() {
+        // We couldn't find the marker, so it's something else.
+        return None;
+    }
 
-    // Safety: The marker function exists, therefore it's safe to cast back to a trap.
+    // Note: this assumes the wrapper class generated by #[wasm_bindgen] will
+    // always have a `__destroy_into_raw()` method which consumes the `Trap`
+    // wrapper and returns a pointer.
+    //
+    // This is valid as of wasm-bindgen version 0.2.87
+    let key = JsValue::from_str("__destroy_into_raw");
+    let ptr = Reflect::get(value, &key)
+        .ok()
+        .and_then(|v| v.dyn_into::<js_sys::Function>().ok())
+        .and_then(|destroy_into_raw| destroy_into_raw.call0(value).ok())
+        .and_then(|ret| ret.as_f64())?;
+
+    // Safety: The marker function exists, therefore it's safe to convert back
+    // to a Trap.
     unsafe {
         Some(<Trap as wasm_bindgen::convert::FromWasmAbi>::from_abi(
             ptr as u32,