cleanup upb.rs

We shouldn't give easy access to the zeroed block, so let's pass in pbi::private to deter misuse.
Also saw that arena wasn't used mutably in a repeatedfield test, so let mut -> let;

PiperOrigin-RevId: 574896460

rust/upb.rs

@@ -150,7 +150,7 @@ const UPB_SCRATCH_SPACE_BYTES: usize = 64_000;
 /// with readonly access.
 pub struct ScratchSpace;
 impl ScratchSpace {
-    pub fn zeroed_block() -> RawMessage {
+    pub fn zeroed_block(_private: Private) -> RawMessage {
         unsafe {
             INIT.call_once(|| {
                 let layout =
@@ -455,7 +455,7 @@ mod tests {
 
     #[test]
     fn i32_array() {
-        let mut arena = Arena::new();
+        let arena = Arena::new();
         let mut arr = RepeatedField::<i32>::new(&arena);
         assert_eq!(arr.len(), 0);
         arr.push(1);

src/google/protobuf/compiler/rust/accessors/singular_message.cc

@@ -51,7 +51,7 @@ void SingularMessage::InMsgImpl(Context<FieldDescriptor> field) const {
             // a null ptr received from upb manifests as Option::None
             match submsg {
                 // TODO:(b/304357029)
-                None => $prefix$View::new($pbi$::Private, $pbr$::ScratchSpace::zeroed_block()),
+                None => $prefix$View::new($pbi$::Private, $pbr$::ScratchSpace::zeroed_block($pbi$::Private)),
                 Some(field) => $prefix$View::new($pbi$::Private, field),
               }
           }