That's not what I meant. A language could give you first-class-citizen constructs for things like tests and constraints that could be compiler-enforced. Some

That's not what I meant. A language could give you first-class-citizen constructs for things like tests and constraints that could be compiler-enforced. Some of that can be a part of the type system such as the ability to express "positive odd prime integer between 7 and 197". Some of that could be in the form of other constructs such as function entry and exit guards. You can "implement" all of that on your own as part of the program but that is not the same thing because semantic meaning and intention is lost. However when it is a first-class language-level construct it has cascading benefits and tooling can also make use of it.