Update enabled_backend_features to take improved backend feature flags into account (#7195)

Author: Wumpf

wgpu/build.rs

@@ -1,16 +1,45 @@
 fn main() {
     cfg_aliases::cfg_aliases! {
         native: { not(target_arch = "wasm32") },
-        webgl: { all(target_arch = "wasm32", not(target_os = "emscripten"), feature = "webgl") },
-        webgpu: { all(target_arch = "wasm32", not(target_os = "emscripten"), feature = "webgpu") },
         Emscripten: { all(target_arch = "wasm32", target_os = "emscripten") },
-        wgpu_core: { any(native, webgl, Emscripten) },
+
         send_sync: { any(
-            not(target_arch = "wasm32"),
+            native,
             all(feature = "fragile-send-sync-non-atomic-wasm", not(target_feature = "atomics"))
         ) },
+
+        // Backends - keep this in sync with `wgpu-core/Cargo.toml` & docs in `wgpu/Cargo.toml`
+        webgpu: { all(not(native), not(Emscripten), feature = "webgpu") },
+        webgl: { all(not(native), not(Emscripten), feature = "webgl") },
         dx12: { all(target_os = "windows", feature = "dx12") },
         metal: { all(target_vendor = "apple", feature = "metal") },
+        vulkan: { any(
+            // The `vulkan` feature enables the Vulkan backend only on "native Vulkan" platforms, i.e. Windows/Linux/Android
+            all(any(windows, target_os = "linux", target_os = "android"), feature = "vulkan"),
+            // On Apple platforms, however, we require the `vulkan-portability` feature
+            // to explicitly opt-in to Vulkan since it's meant to be used with MoltenVK.
+            all(target_vendor = "apple", feature = "vulkan-portability")
+        ) },
+        gles: { any(
+            // The `gles` feature enables the OpenGL/GLES backend only on "native OpenGL" platforms, i.e. Windows, Linux, Android, and Emscripten.
+            // (Note that WebGL is also not included here!)
+            all(any(windows, target_os = "linux", target_os = "android", Emscripten), feature = "gles"),
+            // On Apple platforms, however, we require the `angle` feature to explicitly opt-in to OpenGL
+            // since its meant to be used with ANGLE.
+            all(target_vendor = "apple", feature = "angle")
+        ) },
+        noop: { feature = "noop" },
+
+        wgpu_core: {
+            any(
+                // On native, wgpu_core is currently always enabled, even if there's no backend enabled at all.
+                native,
+                // `wgpu_core` is implied if any backend other than WebGPU is enabled.
+                // (this is redundant except for `gles` and `noop`)
+                webgl, dx12, metal, vulkan, gles, noop
+            )
+        },
+
         // This alias is _only_ if _we_ need naga in the wrapper. wgpu-core provides
         // its own re-export of naga, which can be used in other situations
         naga: { any(feature = "naga-ir", feature = "spirv", feature = "glsl") },

wgpu/src/api/instance.rs

@@ -62,45 +62,27 @@ impl Instance {
     ///
     /// `InstanceDescriptor::backends` does not need to be a subset of this,
     /// but any backend that is not in this set, will not be picked.
-    ///
-    /// TODO: Right now it's otherwise not possible yet to opt-out of all features on some platforms.
-    /// See <https://github.com/gfx-rs/wgpu/issues/3514>
-    /// * Windows/Linux/Android: always enables Vulkan and GLES with no way to opt out
     pub const fn enabled_backend_features() -> Backends {
         let mut backends = Backends::empty();
-
-        if cfg!(native) {
-            if cfg!(metal) {
-                backends = backends.union(Backends::METAL);
-            }
-            if cfg!(dx12) {
-                backends = backends.union(Backends::DX12);
-            }
-
-            // Windows, Android, Linux currently always enable Vulkan and OpenGL.
-            // See <https://github.com/gfx-rs/wgpu/issues/3514>
-            if cfg!(target_os = "windows") || cfg!(unix) {
-                backends = backends.union(Backends::VULKAN).union(Backends::GL);
-            }
-
-            // Vulkan on Mac/iOS is only available through vulkan-portability.
-            if cfg!(target_vendor = "apple") && cfg!(feature = "vulkan-portability") {
-                backends = backends.union(Backends::VULKAN);
-            }
-
-            // GL on Mac is only available through angle.
-            if cfg!(target_os = "macos") && cfg!(feature = "angle") {
-                backends = backends.union(Backends::GL);
-            }
-        } else {
-            if cfg!(webgpu) {
-                backends = backends.union(Backends::BROWSER_WEBGPU);
-            }
-            if cfg!(webgl) {
-                backends = backends.union(Backends::GL);
-            }
+        // `.set` and `|=` don't work in a `const` context.
+        if cfg!(noop) {
+            backends = backends.union(Backends::NOOP);
+        }
+        if cfg!(vulkan) {
+            backends = backends.union(Backends::VULKAN);
+        }
+        if cfg!(any(gles, webgl)) {
+            backends = backends.union(Backends::GL);
+        }
+        if cfg!(metal) {
+            backends = backends.union(Backends::METAL);
+        }
+        if cfg!(dx12) {
+            backends = backends.union(Backends::DX12);
+        }
+        if cfg!(webgpu) {
+            backends = backends.union(Backends::BROWSER_WEBGPU);
         }
-
         backends
     }