diff --git a/gcc/rust/checks/errors/rust-ast-validation.cc b/gcc/rust/checks/errors/rust-ast-validation.cc
index 0ba73c67415..b06b027f0a5 100644
--- a/gcc/rust/checks/errors/rust-ast-validation.cc
+++ b/gcc/rust/checks/errors/rust-ast-validation.cc
@@ -62,4 +62,23 @@ ASTValidation::visit (AST::ConstantItem &const_item)
   AST::ContextualASTVisitor::visit (const_item);
 }
 
+void
+ASTValidation::visit (AST::ExternalFunctionItem &item)
+{
+  auto &params = item.get_function_params ();
+
+  if (params.size () == 1)
+    rust_error_at (
+      params[0].get_locus (),
+      "C-variadic function must be declared with at least one named argument");
+
+  for (auto it = params.begin (); it != params.end (); it++)
+    if (it->is_variadic () && it + 1 != params.end ())
+      rust_error_at (
+	it->get_locus (),
+	"%<...%> must be the last argument of a C-variadic function");
+
+  AST::ContextualASTVisitor::visit (item);
+}
+
 } // namespace Rust
diff --git a/gcc/rust/checks/errors/rust-ast-validation.h b/gcc/rust/checks/errors/rust-ast-validation.h
index a21bcc44f68..6028b21ff00 100644
--- a/gcc/rust/checks/errors/rust-ast-validation.h
+++ b/gcc/rust/checks/errors/rust-ast-validation.h
@@ -36,6 +36,7 @@ class ASTValidation : public AST::ContextualASTVisitor
   virtual void visit (AST::ConstantItem &const_item);
   virtual void visit (AST::Lifetime &lifetime);
   virtual void visit (AST::LoopLabel &label);
+  virtual void visit (AST::ExternalFunctionItem &item);
 };
 
 } // namespace Rust