Implement `Deref` for COM interface hierarchies defined with the `interface` macro (#2969)

crates/libs/interface/src/lib.rs

@@ -103,7 +103,12 @@ impl Interface {
                 const IID: ::windows_core::GUID = #guid;
             }
             impl ::windows_core::RuntimeName for #name {}
-
+            impl ::std::ops::Deref for #name {
+                type Target = #parent;
+                fn deref(&self) -> &Self::Target {
+                    unsafe { ::std::mem::transmute(self) }
+                }
+            }
             #com_trait
             #vtable
             #conversions

crates/tests/interface/tests/deref.rs

@@ -0,0 +1,56 @@
+#![allow(non_snake_case)]
+
+use windows_core::*;
+
+#[interface("7e75ffe0-2f8c-4040-953e-b1f83a48f77b")]
+unsafe trait IFirst: IUnknown {
+    unsafe fn First(&self) -> i32;
+}
+
+#[interface("fe43afb2-43a1-45f9-adbb-1079b410cb9a")]
+unsafe trait ISecond: IFirst {
+    unsafe fn Second(&self) -> i32;
+}
+
+#[interface("4b8c8b47-32dd-4aba-8c68-b0d14703b845")]
+unsafe trait IThird: ISecond {
+    unsafe fn Third(&self) -> i32;
+}
+
+#[implement(IFirst, ISecond, IThird)]
+struct Class;
+
+impl IFirst_Impl for Class {
+    unsafe fn First(&self) -> i32 {
+        1
+    }
+}
+
+impl ISecond_Impl for Class {
+    unsafe fn Second(&self) -> i32 {
+        2
+    }
+}
+
+impl IThird_Impl for Class {
+    unsafe fn Third(&self) -> i32 {
+        3
+    }
+}
+
+#[test]
+fn test() {
+    unsafe {
+        let third: IThird = Class.into();
+        assert_eq!(third.First(), 1);
+        assert_eq!(third.Second(), 2);
+        assert_eq!(third.Third(), 3);
+
+        let second: ISecond = third.cast().unwrap();
+        assert_eq!(second.First(), 1);
+        assert_eq!(second.Second(), 2);
+
+        let first: IFirst = third.cast().unwrap();
+        assert_eq!(first.First(), 1);
+    }
+}