Prevent using multiple push constant variables in one entry point.

src/valid/interface.rs

@@ -90,6 +90,8 @@ pub enum EntryPointError {
     ForbiddenStageOperations,
     #[error("Global variable {0:?} is used incorrectly as {1:?}")]
     InvalidGlobalUsage(Handle<crate::GlobalVariable>, GlobalUse),
+    #[error("More than 1 push constant variable is used")]
+    MoreThanOnePushConstantUsed(Handle<crate::GlobalVariable>),
     #[error("Bindings for {0:?} conflict with other resource")]
     BindingCollision(Handle<crate::GlobalVariable>),
     #[error("Argument {0} varying error")]
@@ -701,6 +703,28 @@ impl super::Validator {
             bg.clear();
         }
 
+        #[cfg(feature = "validate")]
+        {
+            let mut used_push_constants = module
+                .global_variables
+                .iter()
+                .filter(|&(_, var)| var.space == crate::AddressSpace::PushConstant)
+                .map(|(handle, _)| handle)
+                .filter(|&handle| !info[handle].is_empty());
+            // Check if there is one push constant. Some iterators can restart iteration after
+            // `next` returns None, so don't continue if the iterator returns None.
+            if used_push_constants.next().is_some() {
+                // Check if there is a second push constant.
+                match used_push_constants.next() {
+                    None => {}
+                    Some(handle) => {
+                        return Err(EntryPointError::MoreThanOnePushConstantUsed(handle)
+                            .with_span_handle(handle, &module.global_variables))
+                    }
+                }
+            }
+        }
+
         #[cfg(feature = "validate")]
         for (var_handle, var) in module.global_variables.iter() {
             let usage = info[var_handle];