Fix bug in COM interface chain support

crates/libs/interface/src/lib.rs

@@ -211,6 +211,17 @@ impl Interface {
         let parent_vtable_generics = if self.parent_is_iunknown() { quote!(Identity, OFFSET) } else { quote!(Identity, Impl, OFFSET) };
         let parent_vtable = self.parent_vtable();
 
+        // or_parent_matches will be `|| parent::matches(iid)` if this interface inherits from another
+        // interface (except for IUnknown) or will be empty if this is not applicable. This is what allows
+        // QueryInterface to work correctly for all interfaces in an inheritance chain, e.g.
+        // IFoo3 derives from IFoo2 derives from IFoo.
+        //
+        // We avoid matching IUnknown because object identity depends on the uniqueness of the IUnknown pointer.
+        let or_parent_matches = match parent_vtable.as_ref() {
+            Some(parent) if !self.parent_is_iunknown() => quote! (|| <#parent>::matches(iid)),
+            _ => quote!(),
+        };
+
         let functions = self
             .methods
             .iter()
@@ -287,8 +298,10 @@ impl Interface {
                         Self { base__: #parent_vtable::new::<#parent_vtable_generics>(), #(#entries),* }
                     }
 
-                    pub fn matches(iid: &windows_core::GUID) -> bool {
-                        iid == &<#name as ::windows_core::Interface>::IID
+                    #[inline(always)]
+                    pub fn matches(iid: &::windows_core::GUID) -> bool {
+                        *iid == <#name as ::windows_core::Interface>::IID
+                        #or_parent_matches
                     }
                 }
             }

crates/tests/implement_core/src/com_chain.rs

@@ -0,0 +1,50 @@
+use windows_core::*;
+
+#[interface("cccccccc-0000-0000-0000-000000000001")]
+unsafe trait IFoo: IUnknown {}
+
+#[interface("cccccccc-0000-0000-0000-000000000002")]
+unsafe trait IFoo2: IFoo {}
+
+#[interface("cccccccc-0000-0000-0000-000000000003")]
+unsafe trait IFoo3: IFoo2 {}
+
+// ObjectA implements a single interface chain, which consists of 3 different
+// interfaces: IFoo3, IFoo2, and IFoo. You do not need to explicitly list all
+// of the interfaces in the interface chain. Listing all of the interfaces is
+// less efficient because it generates redundant interface chains (pointer
+// fields in the the generated ObjectA_Impl type), which will never be used.
+#[implement(IFoo3)]
+struct ObjectWithChains {}
+
+impl IFoo_Impl for ObjectWithChains {}
+impl IFoo2_Impl for ObjectWithChains {}
+impl IFoo3_Impl for ObjectWithChains {}
+
+#[test]
+fn interface_chain_query() {
+    let object = ComObject::new(ObjectWithChains {});
+    let unknown: IUnknown = object.to_interface();
+    let _foo: IFoo = unknown.cast().expect("QueryInterface for IFoo");
+    let _foo2: IFoo2 = unknown.cast().expect("QueryInterface for IFoo2");
+    let _foo3: IFoo3 = unknown.cast().expect("QueryInterface for IFoo3");
+}
+
+// ObjectRedundantChains implements the same interfaces as ObjectWithChains,
+// but it defines more than one interface chain. This is unnecessary because it
+// is redundant, but we are verifying that this works.
+#[implement(IFoo3, IFoo2, IFoo)]
+struct ObjectRedundantChains {}
+
+impl IFoo_Impl for ObjectRedundantChains {}
+impl IFoo2_Impl for ObjectRedundantChains {}
+impl IFoo3_Impl for ObjectRedundantChains {}
+
+#[test]
+fn redundant_interface_chains() {
+    let object = ComObject::new(ObjectRedundantChains {});
+    let unknown: IUnknown = object.to_interface();
+    let _foo: IFoo = unknown.cast().expect("QueryInterface for IFoo");
+    let _foo2: IFoo2 = unknown.cast().expect("QueryInterface for IFoo2");
+    let _foo3: IFoo3 = unknown.cast().expect("QueryInterface for IFoo3");
+}

crates/tests/implement_core/src/lib.rs

@@ -3,4 +3,5 @@
 
 #![cfg(test)]
 
+mod com_chain;
 mod com_object;