initialize_adapter_from_env_or_default ignores backend_bits if WGPU_ADAPTER_NAME is not set

[kurtschelfthout]

Description
initialize_adapter_from_env_or_default ignores WGPU_BACKEND if WGPU_ADAPTER_NAME is not set.

Repro steps

        let instance = wgpu::Instance::default();

        let backends = wgpu::util::backend_bits_from_env().unwrap_or_else(wgpu::Backends::all);
        println!("backends: {backends:?}");
        let adapters = instance.enumerate_adapters(backends);

        for adapter in adapters {
            let info = adapter.get_info();
            println!("adapter: {:?}", info);
        }

        let adapter = wgpu::util::initialize_adapter_from_env_or_default(&instance, backends, None)
            .await
            .expect("No suitable GPU adapters found on the system!");
        let info = adapter.get_info();
        println!(
            "Using {:#?} {} with {:#?} backend",
            info.device_type, info.name, info.backend
        );

I have an Intel and a discrete Nvidia GPU. When I run this with env variables like:

WGPU_POWER_PREF="high"
WGPU_BACKEND = "DX12"

It prints:

backends: Backends(DX12)
adapter: AdapterInfo { name: "NVIDIA GeForce RTX 2070 with Max-Q Design", vendor: 4318, device: 7956, device_type: DiscreteGpu, driver: "", driver_info: "", backend: Dx12 }
adapter: AdapterInfo { name: "Intel(R) UHD Graphics", vendor: 32902, device: 39876, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Dx12 }
adapter: AdapterInfo { name: "Microsoft Basic Render Driver", vendor: 5140, device: 140, device_type: Cpu, driver: "", driver_info: "", backend: Dx12 }     
Using DiscreteGpu NVIDIA GeForce RTX 2070 with Max-Q Design with Vulkan backend (Env vars: WGPU_POWER_PREF (low|high), WGPU_ADAPTER_NAME, WGPU_BACKEND).  

Expected vs observed behavior
Despite setting the backend to DX12, wgpu uses Vulkan.

Extra materials
This happens because initialize_adapter_from_env_or_default calls initialize_adapter_from_env, which returns None if WGPU_ADAPTER_NAME is not set:

pub fn initialize_adapter_from_env(instance: &Instance, backend_bits: Backends) -> Option<Adapter> {
    let desired_adapter_name = std::env::var("WGPU_ADAPTER_NAME")
        .as_deref()
        .map(str::to_lowercase)
        .ok()?;
   // snip

Now initialize_adapter_from_env_or_default ignores the backend (but does respect power preference):

pub async fn initialize_adapter_from_env_or_default(
    instance: &Instance,
    backend_bits: wgt::Backends,
    compatible_surface: Option<&Surface>,
) -> Option<Adapter> {
    match initialize_adapter_from_env(instance, backend_bits) {
        Some(a) => Some(a),
        None => {
            instance
                .request_adapter(&RequestAdapterOptions {
                    power_preference: power_preference_from_env().unwrap_or_default(),
                    force_fallback_adapter: false,
                    compatible_surface,
                })
                .await
        }
    }
}

Platform
Windows 10, wgpu 0.16.

[Wumpf]

Resolved by #3904