Implement static imports

This allows importing static objects like `document`, `window`, or an arbitrary
JS object from a module

src/lib.rs

@@ -9,8 +9,12 @@
 
 extern crate wasm_bindgen_macro;
 
+use std::cell::UnsafeCell;
+use std::ops::Deref;
 use std::ptr;
 
+use convert::WasmBoundary;
+
 /// A module which is typically glob imported from:
 ///
 /// ```
@@ -243,6 +247,49 @@ impl Drop for JsValue {
     }
 }
 
+/// Wrapper type for imported statics.
+///
+/// This type is used whenever a `static` is imported from a JS module, for
+/// example this import:
+///
+/// ```ignore
+/// #[wasm_bindgen]
+/// extern {
+///     static console: JsValue;
+/// }
+/// ```
+///
+/// will generate in Rust a value that looks like:
+///
+/// ```ignore
+/// static console: JsStatic<JsValue> = ...;
+/// ```
+///
+/// This type implements `Deref` to the inner type so it's typically used as if
+/// it were `&T`.
+pub struct JsStatic<T> {
+    #[doc(hidden)]
+    pub __inner: UnsafeCell<Option<T>>,
+    #[doc(hidden)]
+    pub __init: fn() -> T,
+}
+
+unsafe impl<T: Sync> Sync for JsStatic<T> {}
+unsafe impl<T: Send> Send for JsStatic<T> {}
+
+impl<T: WasmBoundary> Deref for JsStatic<T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unsafe {
+            (*self.__inner.get()).get_or_insert_with(|| {
+                assert!(T::DESCRIPTOR == JsValue::DESCRIPTOR,
+                        "only JS values can be imported as statics for now");
+                (self.__init)()
+            })
+        }
+    }
+}
+
 /// Throws a JS exception.
 ///
 /// This function will throw a JS exception with the message provided. The