any_backend_feature_enabled method to panic on

wgpu/src/lib.rs

@@ -1785,20 +1785,54 @@ impl error::Error for SurfaceError {}
 impl Default for Instance {
     /// Creates a new instance of wgpu with default options.
     ///
+    /// If no backend feature for the active target platform is enabled,
+    /// this method will panic, see [`Instance::any_backend_feature_enabled`].
+    /// 
     /// Backends are set to `Backends::all()`, and FXC is chosen as the `dx12_shader_compiler`.
     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` or `vulkan`
+    /// * 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 `gles` and `vulkan` with no way to opt out
+    /// * Linux: always enables `gles` and `vulkan` with no way to opt out
+    /// * Web: either targets WebGPU backend or, if `gles` 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` or `vulkan` feature 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")
+        } else {
+            true
+        }
+    }
+
     /// Create an new instance of wgpu.
+    /// 
+    /// If no backend feature for the active target platform is enabled,
+    /// this method will panic, see [`Instance::any_backend_feature_enabled`].
     ///
     /// # Arguments
     ///
     /// - `instance_desc` - Has fields for which [backends][Backends] wgpu will choose
     ///   during instantiation, and which [DX12 shader compiler][Dx12Compiler] wgpu will use.
     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)),
         }