Move the declaration of the upb map begin constant into rust

PiperOrigin-RevId: 656463679

rust/upb.rs

@@ -682,8 +682,7 @@ pub struct RawMapIter {
 
 impl RawMapIter {
     pub fn new(_private: Private, map: RawMap) -> Self {
-        // SAFETY: __rust_proto_kUpb_Map_Begin is never modified
-        RawMapIter { map, iter: unsafe { __rust_proto_kUpb_Map_Begin } }
+        RawMapIter { map, iter: UPB_MAP_BEGIN }
     }
 
     /// # Safety

rust/upb/lib.rs

@@ -27,7 +27,7 @@ pub use extension_registry::{upb_ExtensionRegistry, RawExtensionRegistry};
 mod map;
 pub use map::{
     upb_Map, upb_Map_Clear, upb_Map_Delete, upb_Map_Get, upb_Map_Insert, upb_Map_New, upb_Map_Next,
-    upb_Map_Size, MapInsertStatus, RawMap, __rust_proto_kUpb_Map_Begin,
+    upb_Map_Size, MapInsertStatus, RawMap, UPB_MAP_BEGIN,
 };
 
 mod message;

rust/upb/map.rs

@@ -21,9 +21,9 @@ pub enum MapInsertStatus {
     OutOfMemory = 2,
 }
 
-extern "C" {
-    pub static __rust_proto_kUpb_Map_Begin: usize;
+pub const UPB_MAP_BEGIN: usize = usize::MAX;
 
+extern "C" {
     pub fn upb_Map_New(arena: RawArena, key_type: CType, value_type: CType) -> RawMap;
     pub fn upb_Map_Size(map: RawMap) -> usize;
     pub fn upb_Map_Insert(

rust/upb/upb_api.c

@@ -21,5 +21,3 @@
 #include "upb/mini_table/message.h"  // IWYU pragma: keep
 #include "upb/text/debug_string.h"   // IWYU pragma: keep
 // go/keep-sorted end
-
-const size_t __rust_proto_kUpb_Map_Begin = kUpb_Map_Begin;