OIT shader crashes with DX12 backend

[akimakinai]

Bevy version

master

What you did

Run order_independent_transparency example with WGPU_BACKEND = dx12

What went wrong

2024-10-09T08:59:03.651096Z ERROR wgpu_core::device::global: Device::create_render_pipeline error: Internal error in ShaderStages(FRAGMENT) shader: FXC D3DCompile error (0x80004005): E:\src\bevy\Shader@0x000001FFACB89500(246,15-75): error X3507: 'fragment': Not all control paths return a value

2024-10-09T08:59:03.652003Z ERROR wgpu::backend::wgpu_core: Shader translation error for stage ShaderStages(FRAGMENT): FXC D3DCompile error (0x80004005): E:\src\bevy\Shader@0x000001FFACB89500(246,15-75): error X3507: 'fragment': Not all control paths return a value

2024-10-09T08:59:03.652636Z ERROR wgpu::backend::wgpu_core: Please report it to https://github.com/gfx-rs/wgpu
2024-10-09T08:59:03.652845Z ERROR wgpu::backend::wgpu_core: Handling wgpu errors as fatal by default
thread 'Async Compute Task Pool (3)' panicked at E:\.cargo\registry\src\index.crates.io-6f17d22bba15001f\wgpu-22.1.0\src\backend\wgpu_core.rs:3411:5:
wgpu error: Validation Error

Caused by:
  In Device::create_render_pipeline, label = 'oit_resolve_pipeline'
    Internal error in ShaderStages(FRAGMENT) shader: FXC D3DCompile error (0x80004005): E:\src\bevy\Shader@0x000001FFACB89500(246,15-75): error X3507: 'fragment': Not all control paths return a value



stack backtrace:
2024-10-09T08:59:04.650435Z ERROR wgpu_core::device::global: Device::create_render_pipeline error: Internal error in ShaderStages(FRAGMENT) shader: FXC D3DCompile error (0x80004005): E:\src\bevy\Shader@0x000001FFACBCB950(1922,69-104): error X3507: 'oit_drawX_naga_oil_mod_XMJSXM6K7MNXXEZK7OBUXAZLMNFXGKOR2N5UXIX': Not all control paths return a value

2024-10-09T08:59:04.650895Z ERROR wgpu::backend::wgpu_core: Shader translation error for stage ShaderStages(FRAGMENT): FXC D3DCompile error (0x80004005): E:\src\bevy\Shader@0x000001FFACBCB950(1922,69-104): error X3507: 'oit_drawX_naga_oil_mod_XMJSXM6K7MNXXEZK7OBUXAZLMNFXGKOR2N5UXIX': Not all control paths return a value

2024-10-09T08:59:04.651202Z ERROR wgpu::backend::wgpu_core: Please report it to https://github.com/gfx-rs/wgpu

Additional information

• Returning value from oit_draw was considered but not implemented? https://github.com/bevyengine/bevy/pull/14876/files#r1780356792
• Simply adding return after discard might not work due to an upstream issue: Spurious validation error due to discard statement gfx-rs/wgpu#4416