[dx12] Don't panic if MaxSupportedFeatureLevel wasn't set by query

CHANGELOG.md

@@ -57,6 +57,10 @@ Bottom level categories:
 - `DisplayHandle` should now be passed to `InstanceDescriptor` for correct EGL initialization on Wayland. By @MarijnS95 in [#8012](https://github.com/gfx-rs/wgpu/pull/8012)
   Note that the existing workaround to create surfaces before the adapter is no longer valid.
 
+#### DX12
+
+- Don't panic in adapter enumeration on older DX12 environments that don't support a high enough `D3D12FeatureLevel`. By @inner-daemons in [#8806](https://github.com/gfx-rs/wgpu/pull/8806).
+
 #### naga
 
 - Reject zero-value construction of a runtime-sized array with a validation error. Previously it would crash in the HLSL backend. By @mooori in [#8741](https://github.com/gfx-rs/wgpu/pull/8741).

wgpu-hal/src/dx12/adapter.rs

@@ -108,7 +108,7 @@ impl super::Adapter {
         let mut device_levels = Direct3D12::D3D12_FEATURE_DATA_FEATURE_LEVELS {
             NumFeatureLevels: d3d_feature_level.len() as u32,
             pFeatureLevelsRequested: d3d_feature_level.as_ptr().cast(),
-            MaxSupportedFeatureLevel: Default::default(),
+            MaxSupportedFeatureLevel: Direct3D::D3D_FEATURE_LEVEL(0),
         };
         unsafe {
             device.CheckFeatureSupport(
@@ -124,6 +124,9 @@ impl super::Adapter {
             Direct3D::D3D_FEATURE_LEVEL_12_0 => FeatureLevel::_12_0,
             Direct3D::D3D_FEATURE_LEVEL_12_1 => FeatureLevel::_12_1,
             Direct3D::D3D_FEATURE_LEVEL_12_2 => FeatureLevel::_12_2,
+            // Some older windows versions will leave the max feature level unset
+            // without returning an error if the feature level isn't supported.
+            Direct3D::D3D_FEATURE_LEVEL(0) => return None,
             _ => unreachable!(),
         };