[vk] Don't request portability enumeration without the extension.

VUID-VkInstanceCreateInfo-flags-06559 forbids us from passing
`vk::InstanceCreateFlags::ENUMERATE_PORTABILITY_KHR` unless we
actually found and requested the "VK_KHR_portability_enumeration"
layer.

Fixes #4004.

wgpu-hal/src/vulkan/instance.rs

@@ -609,6 +609,11 @@ impl crate::Instance<super::Api> for super::Instance {
         let obs_layer = CStr::from_bytes_with_nul(b"VK_LAYER_OBS_HOOK\0").unwrap();
         let has_obs_layer = find_layer(&instance_layers, obs_layer).is_some();
 
+        let portability_enumeration_layer =
+            CStr::from_bytes_with_nul(b"VK_KHR_portability_enumeration\0").unwrap();
+        let has_portability_enumeration_layer =
+            find_layer(&instance_layers, portability_enumeration_layer).is_some();
+
         let mut layers: Vec<&'static CStr> = Vec::new();
 
         // Request validation layer if asked.
@@ -657,6 +662,14 @@ impl crate::Instance<super::Api> for super::Instance {
         #[cfg(not(target_os = "android"))]
         let android_sdk_version = 0;
 
+        // Avoid VUID-VkInstanceCreateInfo-flags-06559: Only ask the instance to
+        // enumerate incomplete Vulkan implementations (which we need on Mac) if
+        // we managed to find the layer that provides the flag.
+        let mut flags = vk::InstanceCreateFlags::empty();
+        if has_portability_enumeration_layer {
+            flags |= vk::InstanceCreateFlags::ENUMERATE_PORTABILITY_KHR;
+        }
+
         let vk_instance = {
             let str_pointers = layers
                 .iter()
@@ -668,7 +681,7 @@ impl crate::Instance<super::Api> for super::Instance {
                 .collect::<Vec<_>>();
 
             let create_info = vk::InstanceCreateInfo::builder()
-                .flags(vk::InstanceCreateFlags::ENUMERATE_PORTABILITY_KHR)
+                .flags(flags)
                 .application_info(&app_info)
                 .enabled_layer_names(&str_pointers[..layers.len()])
                 .enabled_extension_names(&str_pointers[layers.len()..]);