The normal case of this is already rejected by the .proto parser, this additional check covers legacy_closed_enum cases. Those cases will otherwise reach a CHECK-fail, this just makes for a clearer error message that this is simply a disallowed combination.
PiperOrigin-RevId: 696648744
To implement extensions in Rust, we are going to need a symbol we can depend on
to force-link the C++ extension registration code. The `descriptor_table_xxxxx`
variable seems ideal for this: it already exists and is not public-facing, so
we can reuse it without increasing code size. This CL marks it `extern "C"` to
ensure that we can safely access it from Rust.
PiperOrigin-RevId: 696615312
The intent of these checks was that if the C++20 version of this feature
are enabled to execute the top codepath. libstdc++ 12 defines this
feature as:
```cpp
// in: /usr/include/c++/12/bits/basic_string.h
#ifdef __cpp_lib_is_constant_evaluated
// Support P0980R1 in C++20.
# define __cpp_lib_constexpr_string 201907L
#elif __cplusplus >= 201703L && _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
// Support P0426R1 changes to char_traits in C++17.
# define __cpp_lib_constexpr_string 201611L
#endif
```
So this codepath was always being hit even with -std=c++17 and then
resulted in a failure because the string wasn't actually constinit. This
matches the other use of this feature check in `inlined_string_field.h`
Closes#18890
COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/18890 from keith:ks/fix-uses-of-__cpp_lib_constexpr_string edb4f5f6b6
PiperOrigin-RevId: 696526963
Protobuf Team Bot
Protobuf Team Bot
Hong Shin
Adam Cozzette
Keith Smiley