Add back cast but make it unsafe

Author: rylev

crates/libs/implement/src/lib.rs

@@ -8,6 +8,7 @@ use tokens::*;
 pub fn implement(attributes: proc_macro::TokenStream, original_type: proc_macro::TokenStream) -> proc_macro::TokenStream {
     let attributes = syn::parse_macro_input!(attributes as ImplementAttributes);
     let generics = attributes.generics();
+    let interfaces_len = Literal::usize_unsuffixed(attributes.implement.len());
 
     let constraints = quote! {
         #(#generics: ::windows::core::RuntimeType + 'static,)*
@@ -144,6 +145,7 @@ pub fn implement(attributes: proc_macro::TokenStream, original_type: proc_macro:
             }
         }
         impl <#constraints> #original_ident::<#(#generics,)*> {
+            /// Box and pin `self` and then try to cast it as the supplied interface
             fn alloc<I: ::windows::core::Interface>(self) -> ::windows::core::Result<I> {
                 let this = #impl_ident::<#(#generics,)*>::new(self);
                 let boxed = ::core::mem::ManuallyDrop::new(::std::boxed::Box::new(this));
@@ -157,6 +159,18 @@ pub fn implement(attributes: proc_macro::TokenStream, original_type: proc_macro:
                 }
                 result
             }
+
+            /// Try casting as the provided interface
+            ///
+            /// # Safety
+            ///
+            /// This function can only be safely called if `self` has been heap allocated and pinned using
+            /// the mechanisms provided by `implement` macro.
+            unsafe fn cast<I: ::windows::core::Interface>(&self) -> ::windows::core::Result<I> {
+                let boxed = (self as *const _ as *const ::windows::core::RawPtr).sub(2 + #interfaces_len) as *mut #impl_ident::<#(#generics,)*>;
+                let mut result = None;
+                <#impl_ident::<#(#generics,)*> as ::windows::core::IUnknownImpl>::QueryInterface(&*boxed, &I::IID, &mut result as *mut _ as _).and_some(result)
+            }
         }
         impl <#constraints> ::windows::core::Compose for #original_ident::<#(#generics,)*> {
             unsafe fn compose<'a>(implementation: Self) -> (::windows::core::IInspectable, &'a mut ::core::option::Option<::windows::core::IInspectable>) {

crates/tests/nightly_implement/tests/cast_self.rs

@@ -10,7 +10,7 @@ struct App;
 #[allow(non_snake_case)]
 impl IApplicationOverrides_Impl for App {
     fn OnLaunched(&self, _: &Option<LaunchActivatedEventArgs>) -> Result<()> {
-        let app: Application = self.cast()?;
+        let app: Application = unsafe { self.cast()? };
         assert!(app.FocusVisualKind()? == FocusVisualKind::DottedLine);
         Ok(())
     }

crates/tests/nightly_implement/tests/into_impl.rs

@@ -54,7 +54,7 @@ where
 #[allow(non_snake_case)]
 impl<T: RuntimeType + 'static> IIterable_Impl<T> for Iterable<T> {
     fn First(&self) -> Result<IIterator<T>> {
-        Ok(Iterator::<T>((self.cast()?, 0).into()).into())
+        Ok(Iterator::<T>((unsafe { self.cast()? }, 0).into()).into())
     }
 }
 

crates/tests/nightly_vector/tests/test.rs

@@ -61,7 +61,7 @@ impl<T: ::windows::core::RuntimeType + 'static> IVector_Impl<T> for Vector<T> {
         self.Size()
     }
     fn GetView(&self) -> Result<windows::Foundation::Collections::IVectorView<T>> {
-        self.cast()
+        unsafe { self.cast() }
     }
     fn IndexOf(&self, value: &T::DefaultType, result: &mut u32) -> Result<bool> {
         self.IndexOf(value, result)