feat(crypto): Implement a hacky downcast function.

crates/matrix-sdk-crypto/src/js/mod.rs

@@ -7,3 +7,34 @@ pub mod machine;
 pub mod requests;
 pub mod responses;
 pub mod sync_events;
+
+use js_sys::{Object, Reflect};
+use wasm_bindgen::{convert::RefFromWasmAbi, prelude::*};
+
+/// A really hacky and dirty code to downcast a `JsValue` to `T:
+/// RefFromWasmAbi`, inspired by
+/// https://github.com/rustwasm/wasm-bindgen/issues/2231#issuecomment-656293288.
+///
+/// The returned value is a likely to be `wasm_bindgen::__ref::Ref<T>`.
+fn downcast<T>(value: &JsValue, classname: &str) -> Result<T::Anchor, JsError>
+where
+    T: RefFromWasmAbi<Abi = u32>,
+{
+    let constructor_name = Object::get_prototype_of(&value).constructor().name();
+
+    if constructor_name == classname {
+        let pointer = Reflect::get(value, &JsValue::from_str("ptr"))
+            .map_err(|_| JsError::new("Failed to read the `JsValue` pointer"))?;
+        let pointer = pointer
+            .as_f64()
+            .ok_or_else(|| JsError::new("Failed to read the `JsValue` pointer as a `f64`"))?
+            as u32;
+
+        Ok(unsafe { T::ref_from_abi(pointer) })
+    } else {
+        Err(JsError::new(&format!(
+            "Expect an `{}` instance, received `{}` instead",
+            classname, constructor_name,
+        )))
+    }
+}