Fix web-sys Location test

crates/web-sys/src/lib.rs

@@ -19,13 +19,48 @@ use js_sys::Object;
 
 #[cfg(feature = "Window")]
 pub fn window() -> Option<Window> {
+    use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering::SeqCst};
     use wasm_bindgen::{JsValue, JsCast};
 
-    js_sys::Function::new_no_args("return this")
+    // Cached `Box<JsValue>`, if we've already executed this.
+    //
+    // 0 = not calculated
+    // 1 = `None`
+    // n = Some(n) == Some(Box<JsValue>)
+    static WINDOW: AtomicUsize = ATOMIC_USIZE_INIT;
+
+    match WINDOW.load(SeqCst) {
+        0 => {}
+        1 => return None,
+        n => return unsafe { Some((*(n as *const JsValue)).clone().unchecked_into()) },
+    }
+
+    // Ok we don't have a cached value, let's load one! Manufacture a function
+    // to get access to the `this` context and see if it's an instance of
+    // `Window`.
+    //
+    // Note that we avoid `unwrap()` on `call0` to avoid code size bloat, we
+    // just handle the `Err` case as returning `None`.
+    let window = js_sys::Function::new_no_args("return this")
         .call0(&JsValue::undefined())
-        .ok()?
-        .dyn_into::<Window>()
         .ok()
+        .and_then(|w| w.dyn_into::<Window>().ok());
+
+    match &window {
+        None => WINDOW.store(1, SeqCst),
+        Some(window) => {
+            let window: &JsValue = window.as_ref();
+            let ptr: *mut JsValue = Box::into_raw(Box::new(window.clone()));
+            match WINDOW.compare_exchange(0, ptr as usize, SeqCst, SeqCst) {
+                // We stored out value, relinquishing ownership of `ptr`
+                Ok(_) => {}
+                // Another thread one, drop our value
+                Err(_) => unsafe { drop(Box::from_raw(ptr)) },
+            }
+        }
+    }
+
+    window
 }
 
 include!(concat!(env!("OUT_DIR"), "/bindings.rs"));