You keep trying to change the subject. But I have not promoted "subsetting" as a means to safety, and safety is anyway not interesting to real people. People want their programs to be useful. To be useful, a program must be correct, and every correct program is implicitly safe.
But the actual topic was not not that. The actual topic you have tried to steer away from is optimization. The point I made was that the author of a library can take up responsibilities that some people insist only the the language, via the compiler, can perform. The library author can perform optimizations the compiler fails to, and the library author can define interfaces that can only be used correctly, and safely. To the programmer using a library, it makes no difference, except that they may be unable to use some new, immature language, but can easily pick up and use a good library.
But the actual topic was not not that. The actual topic you have tried to steer away from is optimization. The point I made was that the author of a library can take up responsibilities that some people insist only the the language, via the compiler, can perform. The library author can perform optimizations the compiler fails to, and the library author can define interfaces that can only be used correctly, and safely. To the programmer using a library, it makes no difference, except that they may be unable to use some new, immature language, but can easily pick up and use a good library.