Windows: use FXC by default for DirectX 12 startup (#613)

Squash of PR #613 by @PedroGuille.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/cadcraft/src/graphics.rs

@@ -1,10 +1,14 @@
 //! Windows graphics instance configuration, applied before wgpu can initialize any driver.
 
-use eframe::{NativeOptions, egui_wgpu::WgpuSetup, wgpu::Backends};
+use eframe::{
+    NativeOptions,
+    egui_wgpu::WgpuSetup,
+    wgpu::{Backends, Dx12Compiler},
+};
 
-/// Restrict the wgpu instance to DirectX 12 unless `backend_override` (normally
-/// `Backends::from_env()`, i.e. `WGPU_BACKEND`) names other backends.
-pub fn configure(options: &mut NativeOptions, backend_override: Option<Backends>) {
+/// Default to DirectX 12 with FXC before creating the wgpu instance. Explicit
+/// `WGPU_BACKEND` and `WGPU_DX12_COMPILER` choices, read by the caller, take precedence.
+pub fn configure(options: &mut NativeOptions, backend_override: Option<Backends>, compiler_override: Option<Dx12Compiler>) {
     if let WgpuSetup::CreateNew(create) = &mut options.wgpu_options.wgpu_setup {
         // eframe's default includes GL. Creating that backend can crash inside AMD's
         // atio6axx.dll before adapter selection or any Rust error handling runs, so the
@@ -12,6 +16,15 @@ pub fn configure(options: &mut NativeOptions, backend_override: Option<Backends>
         // out of the instance avoids it. This matches the verified WGPU_BACKEND=dx12
         // workaround without mutating process environment variables.
         create.instance_descriptor.backends = backend_override.unwrap_or(Backends::DX12);
+        // Auto can load another program's dxcompiler.dll without a matching dxil.dll.
+        // That fails wgpu's indirect-validation pipeline and loses the device before
+        // the first frame (issue #608). FXC uses the Windows-provided compiler runtime.
+        create.instance_descriptor.backend_options.dx12.shader_compiler = compiler_override.unwrap_or(Dx12Compiler::Fxc);
+        log::info!(
+            "Windows graphics: backends={:?}, DX12 compiler={:?}",
+            create.instance_descriptor.backends,
+            create.instance_descriptor.backend_options.dx12.shader_compiler
+        );
     }
 }
 
@@ -21,7 +34,7 @@ mod tests {
 
     fn configured_backends(backend_override: Option<Backends>) -> Backends {
         let mut options = NativeOptions::default();
-        configure(&mut options, backend_override);
+        configure(&mut options, backend_override, None);
         let WgpuSetup::CreateNew(create) = options.wgpu_options.wgpu_setup else {
             panic!("default native options must create a wgpu instance");
         };
@@ -38,6 +51,44 @@ mod tests {
         assert_eq!(configured_backends(None), Backends::DX12);
     }
 
+    #[test]
+    fn windows_default_uses_fxc_instead_of_searching_for_dxc() {
+        let mut options = NativeOptions::default();
+        if let WgpuSetup::CreateNew(create) = &mut options.wgpu_options.wgpu_setup {
+            // Auto can find an unrelated DXC without its matching DXIL runtime (issue #608).
+            create.instance_descriptor.backend_options.dx12.shader_compiler = Dx12Compiler::Auto;
+        }
+        configure(&mut options, None, None);
+        let WgpuSetup::CreateNew(create) = options.wgpu_options.wgpu_setup else {
+            panic!("default native options must create a wgpu instance");
+        };
+        assert!(matches!(create.instance_descriptor.backend_options.dx12.shader_compiler, Dx12Compiler::Fxc));
+    }
+
+    #[test]
+    fn explicit_compiler_and_backend_overrides_are_preserved() {
+        let backends = Backends::VULKAN | Backends::DX12;
+        for compiler in
+            [Dx12Compiler::Fxc, Dx12Compiler::Auto, Dx12Compiler::StaticDxc, Dx12Compiler::DynamicDxc { dxc_path: "custom/dxcompiler.dll".into() }]
+        {
+            let mut options = NativeOptions::default();
+            configure(&mut options, Some(backends), Some(compiler.clone()));
+            let WgpuSetup::CreateNew(create) = options.wgpu_options.wgpu_setup else {
+                panic!("default native options must create a wgpu instance");
+            };
+            assert_eq!(create.instance_descriptor.backends, backends);
+            match (compiler, create.instance_descriptor.backend_options.dx12.shader_compiler) {
+                (Dx12Compiler::Fxc, Dx12Compiler::Fxc)
+                | (Dx12Compiler::Auto, Dx12Compiler::Auto)
+                | (Dx12Compiler::StaticDxc, Dx12Compiler::StaticDxc) => {}
+                (Dx12Compiler::DynamicDxc { dxc_path: expected }, Dx12Compiler::DynamicDxc { dxc_path: actual }) => {
+                    assert_eq!(actual, expected);
+                }
+                (expected, actual) => panic!("compiler override changed from {expected:?} to {actual:?}"),
+            }
+        }
+    }
+
     #[test]
     fn explicit_backend_override_is_preserved() {
         for backends in [Backends::VULKAN, Backends::GL, Backends::DX12, Backends::VULKAN | Backends::DX12, Backends::empty()] {

apps/cadcraft/src/main.rs

@@ -237,9 +237,9 @@ fn main() -> eframe::Result {
     if let Some(icon) = app_icon() {
         options.viewport = options.viewport.with_icon(icon);
     }
-    // Before eframe creates the wgpu instance: default Windows to DirectX 12 only (see graphics.rs).
+    // Before eframe creates the wgpu instance: default Windows to DirectX 12 with FXC (see graphics.rs).
     #[cfg(target_os = "windows")]
-    graphics::configure(&mut options, eframe::wgpu::Backends::from_env());
+    graphics::configure(&mut options, eframe::wgpu::Backends::from_env(), eframe::wgpu::Dx12Compiler::from_env());
     // winit has no file drag-and-drop on Wayland (only on X11), so dropping files from the file
     // manager showed a "no" cursor. Run through XWayland when it's there; CADCRAFT_WAYLAND=1
     // keeps the native Wayland backend.