From 2fe7702957c7fe58b2ec725adc907a7ebaf9d308 Mon Sep 17 00:00:00 2001
From: chyyran <ronny@ronnychan.ca>
Date: Sat, 28 Sep 2024 01:07:55 -0400
Subject: [PATCH] rt(d3d11): avoid QueryInterface in GetSize

---
 librashader-common/src/d3d11.rs | 36 +++++++++++++++++++++++++++++----
 1 file changed, 32 insertions(+), 4 deletions(-)

diff --git a/librashader-common/src/d3d11.rs b/librashader-common/src/d3d11.rs
index cb7015d..dc1b0d5 100644
--- a/librashader-common/src/d3d11.rs
+++ b/librashader-common/src/d3d11.rs
@@ -1,6 +1,7 @@
 use crate::{FilterMode, GetSize, Size, WrapMode};
-use windows::core::Interface;
+use windows::Win32::Foundation::E_NOINTERFACE;
 use windows::Win32::Graphics::Direct3D11;
+use windows::Win32::Graphics::Direct3D11::{ID3D11Texture2D, D3D11_RESOURCE_DIMENSION_TEXTURE2D};
 
 impl From<WrapMode> for Direct3D11::D3D11_TEXTURE_ADDRESS_MODE {
     fn from(value: WrapMode) -> Self {
@@ -26,7 +27,21 @@ impl GetSize<u32> for Direct3D11::ID3D11RenderTargetView {
     type Error = windows::core::Error;
 
     fn size(&self) -> Result<Size<u32>, Self::Error> {
-        let parent = unsafe { self.GetResource()?.cast::<Direct3D11::ID3D11Texture2D>()? };
+        let parent: ID3D11Texture2D = unsafe {
+            let resource = self.GetResource()?;
+            if resource.GetType() != D3D11_RESOURCE_DIMENSION_TEXTURE2D {
+                return Err(windows::core::Error::new(
+                    E_NOINTERFACE,
+                    "expected ID3D11Texture2D as the resource for the view.",
+                ));
+            }
+            // SAFETY: We know tha the resource is an `ID3D11Texture2D`.
+            // This downcast is safe because ID3D11Texture2D has ID3D11Resource in its
+            // inheritance chain.
+            //
+            // This check + transmute is cheaper than doing `.cast` (i.e. `QueryInterface`).
+            std::mem::transmute(resource)
+        };
 
         let mut desc = Default::default();
         unsafe {
@@ -44,8 +59,21 @@ impl GetSize<u32> for Direct3D11::ID3D11ShaderResourceView {
     type Error = windows::core::Error;
 
     fn size(&self) -> Result<Size<u32>, Self::Error> {
-        let parent = unsafe { self.GetResource()?.cast::<Direct3D11::ID3D11Texture2D>()? };
-
+        let parent: ID3D11Texture2D = unsafe {
+            let resource = self.GetResource()?;
+            if resource.GetType() != D3D11_RESOURCE_DIMENSION_TEXTURE2D {
+                return Err(windows::core::Error::new(
+                    E_NOINTERFACE,
+                    "expected ID3D11Texture2D as the resource for the view.",
+                ));
+            }
+            // SAFETY: We know tha the resource is an `ID3D11Texture2D`.
+            // This downcast is safe because ID3D11Texture2D has ID3D11Resource in its
+            // inheritance chain.
+            //
+            // This check + transmute is cheaper than doing `.cast` (i.e. `QueryInterface`).
+            std::mem::transmute(resource)
+        };
         let mut desc = Default::default();
         unsafe {
             parent.GetDesc(&mut desc);