ash: Add const STRUCTURE_TYPE to all Vulkan structures for matching with match_struct! macro (#614)

* ash: Add `const STRUCTURE_TYPE` to all Vulkan structures for matching with `match_struct!` macro

In Vulkan layers extracing a structure based on its `s_type` is a common
operation, but comparing against an enum value and subsequently casting
to the right type is verbose and error-prone.

By generating a `const STRUCTURE_TYPE` with the given value for every
Vulkan structure it becomes possible to implement a macro that abstracts
this logic away in a safer way.

* generator: Reuse `HasStructureType::STRUCTURE_TYPE` in `s_type` initializer

Changelog.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Added `VK_EXT_image_drm_format_modifier` device extension (#603)
 - Update Vulkan-Headers to 1.3.219 (#605, #608, #619)
+- Added `const STRUCTURE_TYPE` to all Vulkan structures for matching with `match_struct!` macro (#614)
 - Added `VK_EXT_sample_locations` device extension (#616)
 - Added `VK_NV_coverage_reduction_mode` device extension (#617)
 - Added `VK_KHR_ray_tracing_maintenance1` device extension (#620)

ash/src/lib.rs

@@ -72,6 +72,107 @@ impl<'r, T> RawPtr<T> for Option<&'r T> {
     }
 }
 
+/// Given a mutable raw pointer to a type with an `s_type` member such as [`vk::BaseOutStructure`],
+/// match on a set of Vulkan structures. The struct will be rebound to the given variable of the
+/// type of the given Vulkan structure.
+///
+/// Note that all match bodies have to be enclosed by curly braces due to macro parsing limitations.
+/// It is unfortunately not possible to write `x @ ash::vk::SomeStruct => one_line_expression(),`.
+///
+/// ```
+/// let mut info = ash::vk::DeviceCreateInfo::default();
+/// let info: *mut ash::vk::BaseOutStructure = <*mut _>::cast(&mut info);
+/// unsafe {
+///     ash::match_out_struct!(match info {
+///         info @ ash::vk::DeviceQueueCreateInfo => {
+///             dbg!(&info); // Unreachable
+///         }
+///         info @ ash::vk::DeviceCreateInfo => {
+///             dbg!(&info);
+///         }
+///     })
+/// }
+/// ```
+///
+/// In addition this macro propagates implicit return values just like normal `match` blocks, as
+/// long as a default value or expression is provided in the "any" match arm
+/// (`_ => { some_value() }`). For the time being said arm must be wrapped in curly braces; an
+/// expression like `_ => None` is not yet supported.
+///
+/// ```
+/// # let mut info = ash::vk::DeviceCreateInfo::default();
+/// # let info: *mut ash::vk::BaseOutStructure = <*mut _>::cast(&mut info);
+/// let device_create_flags: Option<ash::vk::DeviceCreateFlags> = unsafe {
+///     ash::match_out_struct!(match info {
+///         info @ ash::vk::DeviceQueueCreateInfo => {
+///             dbg!(&info); // Unreachable
+///             Some(ash::vk::DeviceCreateFlags::empty())
+///         }
+///         info @ ash::vk::DeviceCreateInfo => {
+///             dbg!(&info);
+///             Some(info.flags)
+///         }
+///         _ => {
+///             None
+///         }
+///     })
+/// };
+/// ```
+#[macro_export]
+macro_rules! match_out_struct {
+    (match $p:ident { $($bind:ident @ $ty:path => $body:block $(,)?)+ $(_ => $any:block $(,)?)? }) => {
+        match std::ptr::addr_of!((*$p).s_type).read() {
+            $(<$ty as $crate::vk::TaggedStructure>::STRUCTURE_TYPE => {
+                let $bind = $p
+                    .cast::<$ty>()
+                    .as_mut()
+                    .unwrap();
+                $body
+            }),+
+            _ => { $($any)? }
+        }
+    };
+}
+
+/// Given an immutable raw pointer to a type with an `s_type` member such as [`vk::BaseInStructure`],
+/// match on a set of Vulkan structures. The struct will be rebound to the given variable of the
+/// type of the given Vulkan structure.
+///
+/// Note that all match bodies have to be enclosed by curly braces due to macro parsing limitations.
+/// It is unfortunately not possible to write `x @ ash::vk::SomeStruct => one_line_expression(),`.
+///
+/// ```
+/// let info = ash::vk::DeviceCreateInfo::default();
+/// let info: *const ash::vk::BaseInStructure = <*const _>::cast(&info);
+/// unsafe {
+///     ash::match_in_struct!(match info {
+///         info @ ash::vk::DeviceQueueCreateInfo => {
+///             dbg!(&info); // Unreachable
+///         }
+///         info @ ash::vk::DeviceCreateInfo => {
+///             dbg!(&info);
+///         }
+///     })
+/// }
+/// ```
+///
+/// See the [`match_out_struct!`] documentation for an example with implicit return values.
+#[macro_export]
+macro_rules! match_in_struct {
+    (match $p:ident { $($bind:ident @ $ty:path => $body:block $(,)?)+ $(_ => $any:block $(,)?)? }) => {
+        match std::ptr::addr_of!((*$p).s_type).read() {
+            $(<$ty as $crate::vk::TaggedStructure>::STRUCTURE_TYPE => {
+                let $bind = $p
+                    .cast::<$ty>()
+                    .as_ref()
+                    .unwrap();
+                $body
+            }),+
+            _ => { $($any)? }
+        }
+    };
+}
+
 #[cfg(test)]
 mod tests {
     use super::vk;

ash/src/vk/prelude.rs

@@ -52,3 +52,10 @@ impl From<vk::Extent2D> for vk::Rect2D {
         }
     }
 }
+
+/// Structures implementing this trait are layout-compatible with [`vk::BaseInStructure`] and
+/// [`vk::BaseOutStructure`]. Such structures have an `s_type` field indicating its type, which
+/// must always match the value of [`TaggedStructure::STRUCTURE_TYPE`].
+pub unsafe trait TaggedStructure {
+    const STRUCTURE_TYPE: vk::StructureType;
+}

generator/src/lib.rs

@@ -497,7 +497,6 @@ pub trait FieldExt {
     /// array types (e.g. `[f32; 3]`) will be converted to pointer types (e.g. `&[f32; 3]`),
     /// which is needed for `C` function parameters. Set to `false` for struct definitions.
     fn type_tokens(&self, is_ffi_param: bool) -> TokenStream;
-    fn is_clone(&self) -> bool;
 
     /// Whether this is C's `void` type (not to be mistaken with a void _pointer_!)
     fn is_void(&self) -> bool;
@@ -632,10 +631,6 @@ fn discard_outmost_delimiter(stream: TokenStream) -> TokenStream {
 }
 
 impl FieldExt for vkxml::Field {
-    fn is_clone(&self) -> bool {
-        true
-    }
-
     fn param_ident(&self) -> Ident {
         let name = self.name.as_deref().unwrap_or("field");
         let name_corrected = match name {
@@ -1429,15 +1424,9 @@ pub fn derive_default(struct_: &vkxml::Struct, has_lifetime: bool) -> Option<Tok
     let default_fields = members.clone().map(|field| {
         let param_ident = field.param_ident();
         if is_structure_type(field) {
-            let ty = field
-                .type_enums
-                .as_ref()
-                .and_then(|ty| ty.split(',').next());
-            if let Some(variant) = ty {
-                let variant_ident = variant_ident("VkStructureType", variant);
-
+            if field.type_enums.is_some() {
                 quote! {
-                    #param_ident: StructureType::#variant_ident
+                    #param_ident: Self::STRUCTURE_TYPE
                 }
             } else {
                 quote! {
@@ -1567,6 +1556,13 @@ pub fn derive_setters(
         .clone()
         .find(|field| field.param_ident() == "p_next");
 
+    let structure_type_field = members
+        .clone()
+        .find(|field| field.param_ident() == "s_type");
+
+    // Must either have both, or none:
+    assert_eq!(next_field.is_some(), structure_type_field.is_some());
+
     let nofilter_count_members = [
         ("VkPipelineViewportStateCreateInfo", "pViewports"),
         ("VkPipelineViewportStateCreateInfo", "pScissors"),
@@ -1822,7 +1818,24 @@ pub fn derive_setters(
             quote!(unsafe impl #extends for #name<'_> {})
         });
 
+    let impl_structure_type_trait = structure_type_field.map(|s_type| {
+        let value = s_type
+            .type_enums
+            .as_deref()
+            .expect("s_type field must have a value in `vk.xml`");
+
+        assert!(!value.contains(','));
+
+        let value = variant_ident("VkStructureType", value);
+        quote! {
+            unsafe impl #lifetime TaggedStructure for #name #lifetime {
+                const STRUCTURE_TYPE: StructureType = StructureType::#value;
+            }
+        }
+    });
+
     let q = quote! {
+        #impl_structure_type_trait
         #(#impl_extend_trait)*
         #next_trait