RUST.md: iris's bindless texture array does not survive real Android GPUs

Iris asked whether the 'unknown number of images' approach even works on
mobile. It does not, measured with a new rig (rigs/gpu-probe, no APK
needed) and sourced rather than recalled: the emulator's software Vulkan
refuses iris's descriptor-indexing request outright, and on real hardware
the current Android Vulkan Profile baseline (80.1% of active devices)
does not require VK_EXT_descriptor_indexing either -- Arm's own docs say
only Valhall/5th-Gen Mali (2019+) support it.

iris already solved the identical problem for text in I1 (the glyph
atlas). The recommendation is to generalize it to images rather than
widen the binding array further; not yet implemented, since it changes
iris's render core.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

rigs/gpu-probe/src/vk.rs

@@ -0,0 +1,108 @@
+//! The raw Vulkan half of the probe.
+//!
+//! wgpu reports a feature only after a chain of its own decisions -- which
+//! physical device, which API version, which extension list -- so "wgpu says
+//! no" and "the driver says no" are different claims. This asks
+//! `vkGetPhysicalDeviceFeatures2` itself and prints the inputs to that chain,
+//! so a disagreement can be attributed rather than guessed at.
+
+use ash::{Entry, vk};
+use std::ffi::CStr;
+
+pub fn report() {
+    let entry = match unsafe { Entry::load() } {
+        Ok(e) => e,
+        Err(e) => {
+            println!("\nraw vulkan: cannot load loader -- {e}");
+            return;
+        }
+    };
+
+    let instance_version = match unsafe { entry.try_enumerate_instance_version() } {
+        Ok(Some(v)) => v,
+        Ok(None) => vk::API_VERSION_1_0,
+        Err(e) => {
+            println!("\nraw vulkan: enumerate_instance_version failed -- {e}");
+            return;
+        }
+    };
+    println!("\nraw vulkan:");
+    println!("  loader instance version: {}", ver(instance_version));
+
+    // Ask for the highest instance version the loader admits to: wgpu clamps
+    // the device version by the instance's, so an instance created at 1.0
+    // makes a 1.3 device look like 1.0.
+    let app_info = vk::ApplicationInfo::default().api_version(instance_version);
+    let create = vk::InstanceCreateInfo::default().application_info(&app_info);
+    let instance = match unsafe { entry.create_instance(&create, None) } {
+        Ok(i) => i,
+        Err(e) => {
+            println!("  create_instance failed -- {e}");
+            return;
+        }
+    };
+
+    let devices = unsafe { instance.enumerate_physical_devices() }.unwrap_or_default();
+    for phd in devices {
+        let props = unsafe { instance.get_physical_device_properties(phd) };
+        let name = unsafe { CStr::from_ptr(props.device_name.as_ptr()) };
+        println!("  device: {}", name.to_string_lossy());
+        println!("    device api version: {}", ver(props.api_version));
+
+        let exts =
+            unsafe { instance.enumerate_device_extension_properties(phd) }.unwrap_or_default();
+        let has_ext = exts.iter().any(|e| {
+            (unsafe { CStr::from_ptr(e.extension_name.as_ptr()) }) == c"VK_EXT_descriptor_indexing"
+        });
+        println!("    VK_EXT_descriptor_indexing advertised: {has_ext}");
+        println!("    device extensions: {}", exts.len());
+
+        let mut indexing = vk::PhysicalDeviceDescriptorIndexingFeatures::default();
+        let mut features2 = vk::PhysicalDeviceFeatures2::default().push_next(&mut indexing);
+        unsafe { instance.get_physical_device_features2(phd, &mut features2) };
+
+        for (name, v) in [
+            (
+                "shaderSampledImageArrayNonUniformIndexing",
+                indexing.shader_sampled_image_array_non_uniform_indexing,
+            ),
+            (
+                "descriptorBindingSampledImageUpdateAfterBind",
+                indexing.descriptor_binding_sampled_image_update_after_bind,
+            ),
+            (
+                "shaderStorageImageArrayNonUniformIndexing",
+                indexing.shader_storage_image_array_non_uniform_indexing,
+            ),
+            (
+                "descriptorBindingStorageImageUpdateAfterBind",
+                indexing.descriptor_binding_storage_image_update_after_bind,
+            ),
+            (
+                "shaderStorageBufferArrayNonUniformIndexing",
+                indexing.shader_storage_buffer_array_non_uniform_indexing,
+            ),
+            (
+                "descriptorBindingStorageBufferUpdateAfterBind",
+                indexing.descriptor_binding_storage_buffer_update_after_bind,
+            ),
+            (
+                "descriptorBindingPartiallyBound",
+                indexing.descriptor_binding_partially_bound,
+            ),
+        ] {
+            println!("    {name:48} {}", if v != 0 { "yes" } else { "NO" });
+        }
+    }
+
+    unsafe { instance.destroy_instance(None) };
+}
+
+fn ver(v: u32) -> String {
+    format!(
+        "{}.{}.{}",
+        vk::api_version_major(v),
+        vk::api_version_minor(v),
+        vk::api_version_patch(v)
+    )
+}