Panics without explanation when creating render pipeline with >4 fragment color targets

[Keavon]

Description
When creating a render pipeline, it is limited to 4 color attachments. This limit is set by MAX_COLOR_TARGETS. WGPU does not provide an error message explaining this, instead it proceeds and another crate (arrayvec) ends up panicking with a very unhelpful error message.

Repro steps
WGPU version 0.11

device.create_render_pipeline(&wgpu::RenderPipelineDescriptor {
		...
		fragment: Some(wgpu::FragmentState {
			...
			targets: // Provide 5 or more `ColorTargetState`s here

In wgpu-hal-0.11.4/src/vulkan/device.rs:1456, the following code tries to call ArrayVec::push() which panics if the array is full (and the array capacity is set to MAX_COLOR_TARGETS):

compatible_rp_key.colors.push(super::ColorAttachmentKey {
    base: super::AttachmentKey::compatible(
        vk_format,
        vk::ImageLayout::COLOR_ATTACHMENT_OPTIMAL,
    ),
    resolve: None,
});

Expected vs observed behavior
A size check against MAX_COLOR_TARGETS should occur and a helpful error message should be provided explaining the maximum of 4, and perhaps offering suggested workarounds.

Platform
Windows

[kvark]

This is a good thing to contribute!

[Anti-Alias]

Hit this same error myself today.
I was wondering if there was a plan to either increase this value or have it be queryable in WGPU.
I assume it's set to 4 for portability reasons.

Currently trying to implement deferred rendering with 5+ attachments:

• position (xyz),
• normal (xyz),
• diffuse/albedo (rgba),
• specular (rgb),
• emissive (rgb)

Not sure if this means I'll need to make multiple passes through the same geometry, or if there's an alternate method that doesn't require me to make 5 attachments.
I considered combining the specular and emissive into one target (emissive(rgb) + specular_intensity(a)), but I want to allow for colored specular highlights.
Still kinda a newbie at WGPU/modern graphics pipelines, so bear with me 😃.

[Keavon]

@Anti-Alias if you're in the Rust Gamedev server, check this conversation regarding how to more efficiently use the necessary channels in deferred rendering. I had the same problems before I realized many of those channels are actually unnecessary.

https://discord.com/channels/676678179678715904/676685797524766720/902044811492745216

[jinleili]

This issue was fixed by #2913