Add wgpu crate features for backends (#4815)

* Introduce `dx12` and `metal` crate features to `wgpu`

* Implement dummy `Context` to allow compilation with `--no-default-features`

* Address review

* Remove `dummy::Context` in favor of `hal::api::Empty`

* Add changelog entry

* Panic early in `Instance::new()` if no backend is enabled

Co-Authored-By: Andreas Reich <1220815+Wumpf@users.noreply.github.com>

---------

Co-authored-by: Andreas Reich <1220815+Wumpf@users.noreply.github.com>

CHANGELOG.md

@@ -55,6 +55,10 @@ Previously, `DeviceExt::create_texture_with_data` only allowed data to be provid
 
 This feature allowed you to call `global_id` on any wgpu opaque handle to get a unique hashable identity for the given resource. This is now available without the feature flag. By @cwfitzgerald in [#4841](https://github.com/gfx-rs/wgpu/pull/4841)
 
+### `dx12` and `metal` backend crate features
+
+Wgpu now exposes backend feature for the Direct3D 12 (`dx12`) and Metal (`metal`) backend. These are enabled by default, but don't do anything when not targetting the corresponding OS. By @daxpedda in [#4815](https://github.com/gfx-rs/wgpu/pull/4815)
+
 ### New Features
 
 #### General

wgpu-core/src/lib.rs

@@ -222,6 +222,10 @@ define_backend_caller! { gfx_if_vulkan, gfx_if_vulkan_hidden, "vulkan" if all(fe
 define_backend_caller! { gfx_if_metal, gfx_if_metal_hidden, "metal" if all(feature = "metal", any(target_os = "macos", target_os = "ios")) }
 define_backend_caller! { gfx_if_dx12, gfx_if_dx12_hidden, "dx12" if all(feature = "dx12", windows) }
 define_backend_caller! { gfx_if_gles, gfx_if_gles_hidden, "gles" if feature = "gles" }
+define_backend_caller! { gfx_if_empty, gfx_if_empty_hidden, "empty" if all(
+    not(any(feature = "metal", feature = "vulkan", feature = "gles")),
+    any(target_os = "macos", target_os = "ios"),
+) }
 
 /// Dispatch on an [`Id`]'s backend to a backend-generic method.
 ///
@@ -276,6 +280,7 @@ macro_rules! gfx_select {
             wgt::Backend::Metal => $crate::gfx_if_metal!($global.$method::<$crate::api::Metal>( $($param),* )),
             wgt::Backend::Dx12 => $crate::gfx_if_dx12!($global.$method::<$crate::api::Dx12>( $($param),* )),
             wgt::Backend::Gl => $crate::gfx_if_gles!($global.$method::<$crate::api::Gles>( $($param),+ )),
+            wgt::Backend::Empty => $crate::gfx_if_empty!($global.$method::<$crate::api::Empty>( $($param),+ )),
             other => panic!("Unexpected backend {:?}", other),
         }
     };

wgpu-hal/src/vulkan/instance.rs

@@ -507,7 +507,7 @@ impl super::Instance {
         Ok(self.create_surface_from_vk_surface_khr(surface))
     }
 
-    #[cfg(any(target_os = "macos", target_os = "ios"))]
+    #[cfg(all(any(target_os = "macos", target_os = "ios"), feature = "metal"))]
     fn create_surface_from_view(
         &self,
         view: *mut c_void,
@@ -805,13 +805,13 @@ impl crate::Instance<super::Api> for super::Instance {
                 let hinstance = unsafe { GetModuleHandleW(std::ptr::null()) };
                 self.create_surface_from_hwnd(hinstance as *mut _, handle.hwnd.get() as *mut _)
             }
-            #[cfg(target_os = "macos")]
+            #[cfg(all(target_os = "macos", feature = "metal"))]
             (Rwh::AppKit(handle), _)
                 if self.shared.extensions.contains(&ext::MetalSurface::name()) =>
             {
                 self.create_surface_from_view(handle.ns_view.as_ptr())
             }
-            #[cfg(target_os = "ios")]
+            #[cfg(all(target_os = "ios", feature = "metal"))]
             (Rwh::UiKit(handle), _)
                 if self.shared.extensions.contains(&ext::MetalSurface::name()) =>
             {

wgpu/Cargo.toml

@@ -24,7 +24,7 @@ targets = [
 [lib]
 
 [features]
-default = ["wgsl"]
+default = ["wgsl", "dx12", "metal"]
 # Apply run-time checks, even in release builds. These are in addition
 # to the validation carried out at public APIs in all builds.
 strict_asserts = ["wgc?/strict_asserts", "wgt/strict_asserts"]
@@ -33,11 +33,6 @@ glsl = ["naga/glsl-in"]
 wgsl = ["wgc?/wgsl"]
 trace = ["serde", "wgc/trace"]
 replay = ["serde", "wgc/replay"]
-# Enables the GLES backend on Windows & macOS
-angle = ["wgc/gles"]
-webgl = ["hal", "wgc/gles"]
-# Enables the Vulkan backend on macOS & iOS
-vulkan-portability = ["wgc/vulkan"]
 # Implement `Send` and `Sync` on Wasm.
 fragile-send-sync-non-atomic-wasm = [
     "hal/fragile-send-sync-non-atomic-wasm",
@@ -46,6 +41,17 @@ fragile-send-sync-non-atomic-wasm = [
 ]
 # Log all API entry points at info instead of trace level.
 api_log_info = ["wgc/api_log_info"]
+# Backends
+# Enables the DX12 backend on Windows
+dx12 = ["wgc?/dx12"]
+# Enables the Metal backend on macOS & iOS
+metal = ["wgc?/metal"]
+# Enables the GLES backend on Windows & macOS
+angle = ["wgc?/gles"]
+# Enables the Vulkan backend on macOS & iOS
+vulkan-portability = ["wgc?/vulkan"]
+# Enables the GLES backend on Wasm (currently will also enable GLES dependencies on any other target)
+webgl = ["hal", "wgc/gles"]
 
 # wgpu-core is always available as an optional dependency, "wgc".
 # Whenever wgpu-core is selected, we want raw window handle support.
@@ -60,15 +66,15 @@ features = ["raw-window-handle"]
 workspace = true
 features = ["raw-window-handle"]
 
-# We want the wgpu-core Metal backend on macOS and iOS.
+# Enable `wgc` by default on macOS and iOS to allow the `metal` crate feature to
+# enable the Metal backend while being no-op on other targets.
 [target.'cfg(any(target_os = "macos", target_os = "ios"))'.dependencies.wgc]
 workspace = true
-features = ["metal"]
 
 # We want the wgpu-core Direct3D backend and OpenGL (via WGL) on Windows.
 [target.'cfg(windows)'.dependencies.wgc]
 workspace = true
-features = ["dx12", "gles"]
+features = ["gles"]
 
 # We want the wgpu-core Vulkan backend on Unix (but not emscripten, macOS, iOS) and Windows.
 [target.'cfg(any(windows, all(unix, not(target_os = "emscripten"), not(target_os = "ios"), not(target_os = "macos"))))'.dependencies.wgc]

wgpu/src/backend/direct.rs

@@ -229,7 +229,7 @@ impl Context {
         self.0.generate_report()
     }
 
-    #[cfg(any(target_os = "ios", target_os = "macos"))]
+    #[cfg(all(any(target_os = "ios", target_os = "macos"), feature = "metal"))]
     pub unsafe fn create_surface_from_core_animation_layer(
         &self,
         layer: *mut std::ffi::c_void,
@@ -265,7 +265,7 @@ impl Context {
         })
     }
 
-    #[cfg(target_os = "windows")]
+    #[cfg(all(target_os = "windows", feature = "dx12"))]
     pub unsafe fn create_surface_from_visual(&self, visual: *mut std::ffi::c_void) -> Surface {
         let id = unsafe { self.0.instance_create_surface_from_visual(visual, ()) };
         Surface {
@@ -274,7 +274,7 @@ impl Context {
         }
     }
 
-    #[cfg(target_os = "windows")]
+    #[cfg(all(target_os = "windows", feature = "dx12"))]
     pub unsafe fn create_surface_from_surface_handle(
         &self,
         surface_handle: *mut std::ffi::c_void,
@@ -289,7 +289,7 @@ impl Context {
         }
     }
 
-    #[cfg(target_os = "windows")]
+    #[cfg(all(target_os = "windows", feature = "dx12"))]
     pub unsafe fn create_surface_from_swap_chain_panel(
         &self,
         swap_chain_panel: *mut std::ffi::c_void,

wgpu/src/lib.rs

@@ -1786,19 +1786,62 @@ impl Default for Instance {
     /// Creates a new instance of wgpu with default options.
     ///
     /// Backends are set to `Backends::all()`, and FXC is chosen as the `dx12_shader_compiler`.
+    ///
+    /// # Panics
+    ///
+    /// If no backend feature for the active target platform is enabled,
+    /// this method will panic, see [`Instance::any_backend_feature_enabled()`].
     fn default() -> Self {
         Self::new(InstanceDescriptor::default())
     }
 }
 
 impl Instance {
+    /// Returns `true` if any backend feature is enabled for the current build configuration.
+    ///
+    /// Which feature makes this method return true depends on the target platform:
+    /// * MacOS/iOS: `metal`, `vulkan-portability` or `angle`
+    /// * All other: Always returns true
+    ///
+    /// TODO: Right now it's otherwise not possible yet to opt-out of all features on most platforms.
+    /// See <https://github.com/gfx-rs/wgpu/issues/3514>
+    /// * Windows: always enables Vulkan and GLES with no way to opt out
+    /// * Linux: always enables Vulkan and GLES with no way to opt out
+    /// * Web: either targets WebGPU backend or, if `webgl` enabled, WebGL
+    ///   * TODO: Support both WebGPU and WebGL at the same time, see <https://github.com/gfx-rs/wgpu/issues/2804>
+    pub const fn any_backend_feature_enabled() -> bool {
+        // Method intentionally kept verbose to keep it a bit easier to follow!
+
+        // On macOS and iOS, at least one of Metal, Vulkan or GLES backend must be enabled.
+        let is_mac_or_ios = cfg!(target_os = "macos") || cfg!(target_os = "ios");
+        if is_mac_or_ios {
+            cfg!(feature = "metal")
+                || cfg!(feature = "vulkan-portability")
+                || cfg!(feature = "angle")
+        } else {
+            true
+        }
+    }
+
     /// Create an new instance of wgpu.
     ///
     /// # Arguments
     ///
     /// - `instance_desc` - Has fields for which [backends][Backends] wgpu will choose
     ///   during instantiation, and which [DX12 shader compiler][Dx12Compiler] wgpu will use.
+    ///
+    /// # Panics
+    ///
+    /// If no backend feature for the active target platform is enabled,
+    /// this method will panic, see [`Instance::any_backend_feature_enabled()`].
     pub fn new(instance_desc: InstanceDescriptor) -> Self {
+        if !Self::any_backend_feature_enabled() {
+            panic!(
+                "No wgpu backend feature that is implemented for the target platform was enabled. \
+                 See `wgpu::Instance::any_backend_feature_enabled()` for more information."
+            );
+        }
+
         Self {
             context: Arc::from(crate::backend::Context::init(instance_desc)),
         }
@@ -2029,7 +2072,7 @@ impl Instance {
     /// # Safety
     ///
     /// - layer must be a valid object to create a surface upon.
-    #[cfg(any(target_os = "ios", target_os = "macos"))]
+    #[cfg(all(any(target_os = "ios", target_os = "macos"), feature = "metal"))]
     pub unsafe fn create_surface_from_core_animation_layer(
         &self,
         layer: *mut std::ffi::c_void,
@@ -2055,7 +2098,7 @@ impl Instance {
     /// # Safety
     ///
     /// - visual must be a valid IDCompositionVisual to create a surface upon.
-    #[cfg(target_os = "windows")]
+    #[cfg(all(target_os = "windows", feature = "dx12"))]
     pub unsafe fn create_surface_from_visual(
         &self,
         visual: *mut std::ffi::c_void,
@@ -2081,7 +2124,7 @@ impl Instance {
     /// # Safety
     ///
     /// - surface_handle must be a valid SurfaceHandle to create a surface upon.
-    #[cfg(target_os = "windows")]
+    #[cfg(all(target_os = "windows", feature = "dx12"))]
     pub unsafe fn create_surface_from_surface_handle(
         &self,
         surface_handle: *mut std::ffi::c_void,
@@ -2107,7 +2150,7 @@ impl Instance {
     /// # Safety
     ///
     /// - visual must be a valid SwapChainPanel to create a surface upon.
-    #[cfg(target_os = "windows")]
+    #[cfg(all(target_os = "windows", feature = "dx12"))]
     pub unsafe fn create_surface_from_swap_chain_panel(
         &self,
         swap_chain_panel: *mut std::ffi::c_void,