Use u32 instead of isize as the discriminant of the Rust oneof 'tagge…

…d union' enum type (which is used for the getter that gives you both which field is set and the value of that field).

Note that the oneof case enum (the enum to signals which case is set and not the value of that case) remains a repr(C) enum with no explicit storage type to be ABI compatible with the C++ case enum.

PiperOrigin-RevId: 708315860

src/google/protobuf/compiler/rust/oneof.cc

@@ -151,7 +151,6 @@ void GenerateOneofDefinition(Context& ctx, const OneofDescriptor& oneof) {
              }
            }},
       },
-      // TODO: Revisit if isize is the optimal repr for this enum.
       // Note: This enum deliberately has a 'msg lifetime associated with it
       // even if all fields were scalars; we could conditionally exclude the
       // lifetime under that case, but it would mean changing the .proto file
@@ -161,7 +160,7 @@ void GenerateOneofDefinition(Context& ctx, const OneofDescriptor& oneof) {
       #[non_exhaustive]
       #[derive(Debug, Clone, Copy)]
       #[allow(dead_code)]
-      #[repr(isize)]
+      #[repr(u32)]
       pub enum $view_enum_name$<'msg> {
         $view_fields$