The whole point is that we want a guarantee that the input has been checked at build-time. There is a huge difference between:
- Definitely checked at build-time, and
- Maybe checked at build-time, otherwise crashes at runtime
You can read the previous thread if you want; what you are suggesting is what the Foundation team suggested, and I explained why I think a build-time plugin is actually superior and delivers something closer to what developers are actually asking for.
Also, it is worth noting: expecting the compiler's automatic constant-folding to statically evaluate an entire URL parser is unrealistic. Instead, we can just ship this little utility (call it a "macro" or "linting plugin" or whatever), and the compiler can call out to that for some automated checks.
In some cases, it may be able to avoids parsing the string at runtime at all, in other cases, it might just diagnose some obviously invalid inputs, and in yet other cases, it might be possible to do something in-between: to generate some kind of internal data structure which optimises the runtime parsing in a library-specific way (e.g. marking the locations of important sections within the string). There is no way that the compiler could automatically perform the latter.