Add SettableValue for generic field setting
This is a prerequisite for optional types to have proper setters.
PiperOrigin-RevId: 542588427
diff --git a/rust/internal.rs b/rust/internal.rs
index ae07849..a96437d 100644
--- a/rust/internal.rs
+++ b/rust/internal.rs
@@ -34,6 +34,9 @@
use std::slice;
+/// Used to protect internal-only items from being used accidentally.
+pub struct Private;
+
/// Represents an ABI-stable version of `NonNull<[u8]>`/`string_view` (a
/// borrowed slice of bytes) for FFI use only.
///
