C might not include those specific examples, but POSIX + libc already include plenty of stuff.
C++ surely does include regex, with serialisation and http scheduled for C++23, or with luck with a TR as intermediate delivery.
Although serialisation depends on static reflection being finalized as well.
Still, it implies C fails, because you don't get POSIX on all available platforms.
I think the definition is meaningless. Rust spends huge amount of resources to test itself and its surrounding ecosystem on tier 1 platforms.
I'm fairly certain regex from c++ std will run like crap, if at all on something with 4MB of RAM.
Except the small detail that C's POSIX support is much wider than Rust's tier 1 platforms.
I am fairly certain that without profiling and defining a test configuration for a set of specific C++ compilers / standard C++ library I won't assert anything about std::regex performance with 4 MB of RAM.
libc is barely a standard library, it's almost nothing.