Can non-nullable references be implemented as a library?

[Kagamin]

so Wrote:

> > This way or another, you need a null check. Why extra syntax?
> 
> This is also how it is done in those languages no?
> They also have null checks at every assignment, but since compiler puts  
> those lines, you don't know anything.
> 
> Maybe this is not what you mean?

Do you care about performance? If you zealously design with non-nulls, you won't have too many checks.