Obtain accurate alignment for empty slice of opaque Rust type
diff --git a/gen/src/builtin.rs b/gen/src/builtin.rs
index 409a5d1..2e75666 100644
--- a/gen/src/builtin.rs
+++ b/gen/src/builtin.rs
@@ -146,6 +146,8 @@
         out.next_section();
         writeln!(out, "template <typename T>");
         writeln!(out, "::std::size_t size_of();");
+        writeln!(out, "template <typename T>");
+        writeln!(out, "::std::size_t align_of();");
     }
 
     ifndef::write(out, builtin.rust_string, "CXXBRIDGE1_RUST_STRING");