Restrict SubpassData image dimension to Vulkan environments

SubpassData is a Vulkan-specific concept (input attachments). Reject
OpTypeImage with Dim SubpassData in non-Vulkan environments with
ImageTypeSubpassDataRequiresVulkan error.

Author: LegNeato

rust/spirv-tools-core/src/validation/error.rs

@@ -4143,6 +4143,14 @@ pub enum ValidationError {
         /// Actual operand count.
         actual: usize,
     },
+    /// OpTypeImage with SubpassData dimension requires Vulkan environment.
+    #[error("OpTypeImage {type_id:?} with SubpassData dimension is only allowed in Vulkan, current environment is {env:?}")]
+    ImageTypeSubpassDataRequiresVulkan {
+        /// The image type ID.
+        type_id: Option<TypeId>,
+        /// The current target environment.
+        env: crate::target_env::TargetEnv,
+    },
     /// OpTypeImage with SubpassData dimension must not be arrayed.
     #[error("OpTypeImage {type_id:?} with SubpassData dimension must not be arrayed")]
     ImageTypeSubpassDataMustNotBeArrayed {

rust/spirv-tools-core/src/validation/rules/image.rs

@@ -272,6 +272,17 @@ impl ValidationRule for ImageTypeRule {
                 );
             }
 
+            // SubpassData requires Vulkan environment
+            if dim == Dim::DimSubpassData && !ctx.env.is_vulkan() {
+                return Err(
+                    ValidationError::ImageTypeSubpassDataRequiresVulkan {
+                        type_id,
+                        env: ctx.env,
+                    }
+                    .into(),
+                );
+            }
+
             // SubpassData constraints
             if dim == Dim::DimSubpassData {
                 if arrayed != 0 {

rust/spirv-tools-core/src/validation/tests/misc.rs

@@ -7098,3 +7098,53 @@ OpFunctionEnd
     assemble_and_validate_with_env(text, TargetEnv::Vulkan1_2)
         .expect("Block var with per-member Locations should pass");
 }
+
+// ============================================================================
+// SubpassData Vulkan-only restriction
+// ============================================================================
+
+#[test]
+fn subpass_data_accepted_in_vulkan() {
+    let text = r#"
+OpCapability Shader
+OpCapability InputAttachment
+OpMemoryModel Logical GLSL450
+OpEntryPoint Fragment %main "main"
+OpExecutionMode %main OriginUpperLeft
+%void = OpTypeVoid
+%fn = OpTypeFunction %void
+%f32 = OpTypeFloat 32
+%img = OpTypeImage %f32 SubpassData 0 0 0 2 Unknown
+%main = OpFunction %void None %fn
+%entry = OpLabel
+OpReturn
+OpFunctionEnd
+"#;
+    assemble_and_validate_with_env(text, TargetEnv::Vulkan1_2)
+        .expect("SubpassData should be accepted in Vulkan");
+}
+
+#[test]
+fn subpass_data_rejected_in_non_vulkan() {
+    let text = r#"
+OpCapability Shader
+OpCapability InputAttachment
+OpMemoryModel Logical GLSL450
+OpEntryPoint Fragment %main "main"
+OpExecutionMode %main OriginUpperLeft
+%void = OpTypeVoid
+%fn = OpTypeFunction %void
+%f32 = OpTypeFloat 32
+%img = OpTypeImage %f32 SubpassData 0 0 0 2 Unknown
+%main = OpFunction %void None %fn
+%entry = OpLabel
+OpReturn
+OpFunctionEnd
+"#;
+    let err = assemble_and_validate_with_env(text, TargetEnv::Universal1_6)
+        .expect_err("SubpassData should be rejected in non-Vulkan");
+    assert!(
+        matches!(err, ValidationError::ImageTypeSubpassDataRequiresVulkan { .. }),
+        "expected ImageTypeSubpassDataRequiresVulkan, got {err:?}"
+    );
+}