generator: Replace Extends{Root} with StructExtends<Root> generic

Instead of emitting a new `trait` for every `Root` struct that is being
implemented by one or more "child" structs (those that have `Root` in
their `structextends`), create one trait that takes the root struct as a
generic parameter, and implement that directly instead.

This not only saves on having to define the `trait` for every
`Root` struct but also paves the way towards providing default trait
implementations for any pair of root and child struct, such as the
`p_next` builder methods.

Changelog.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     - `get_pipeline_executable_statistics()`.
   The expected length of this array can be queried with the respective `*_len()` variant of these functions.
 - `push_next()` has been renamed to `extend()` and marked as `unsafe`. Users are encouraged to call `push()` for singular structs instead. (#909)
+- All `Extends{Root}` traits have been replaced with a single `Extends<Root>` trait using generics. (#971)
 
 ### Removed
 

ash/src/lib.rs

@@ -214,4 +214,57 @@ mod tests {
         };
         assert_eq!(chain, chain2);
     }
+    #[test]
+    #[should_panic]
+    fn disallow_nested_ptr_chains() {
+        let mut generated_commands =
+            vk::PhysicalDeviceDeviceGeneratedCommandsFeaturesEXT::default();
+        let mut private_data = vk::PhysicalDevicePrivateDataFeatures {
+            p_next: <*mut _>::cast(&mut generated_commands),
+            ..Default::default()
+        };
+        let _device_create_info = vk::DeviceCreateInfo::default().push(&mut private_data);
+    }
+    #[test]
+    fn test_nested_ptr_chains() {
+        let mut generated_commands =
+            vk::PhysicalDeviceDeviceGeneratedCommandsFeaturesEXT::default();
+        let mut private_data = vk::PhysicalDevicePrivateDataFeatures {
+            p_next: <*mut _>::cast(&mut generated_commands),
+            ..Default::default()
+        };
+        let mut variable_pointers = vk::PhysicalDeviceVariablePointerFeatures::default();
+        let mut corner = vk::PhysicalDeviceCornerSampledImageFeaturesNV::default();
+        let chain = alloc::vec![
+            <*mut _>::cast(&mut private_data),
+            <*mut _>::cast(&mut generated_commands),
+            <*mut _>::cast(&mut variable_pointers),
+            <*mut _>::cast(&mut corner),
+        ];
+        let mut device_create_info = vk::DeviceCreateInfo::default()
+            .push(&mut corner)
+            .push(&mut variable_pointers);
+        // Insert private_data->generated_commands into the chain, such that generate_commands->variable_pointers->corner:
+        device_create_info = unsafe { device_create_info.extend(&mut private_data) };
+        let chain2: Vec<*mut vk::BaseOutStructure<'_>> = unsafe {
+            vk::ptr_chain_iter(&mut device_create_info)
+                .skip(1)
+                .collect()
+        };
+        assert_eq!(chain, chain2);
+    }
+    #[test]
+    fn test_dynamic_add_to_ptr_chain() {
+        let mut variable_pointers = vk::PhysicalDeviceVariablePointerFeatures::default();
+        let variable_pointers: &mut dyn vk::Extends<vk::DeviceCreateInfo<'_>> =
+            &mut variable_pointers;
+        let chain = alloc::vec![<*mut _>::cast(variable_pointers)];
+        let mut device_create_info = vk::DeviceCreateInfo::default().push(variable_pointers);
+        let chain2: Vec<*mut vk::BaseOutStructure<'_>> = unsafe {
+            vk::ptr_chain_iter(&mut device_create_info)
+                .skip(1)
+                .collect()
+        };
+        assert_eq!(chain, chain2);
+    }
 }