IMO this is going up the spectrum from runtime checking to static checking, and it becomes more and more useful when you are writing libraries for mass consumpt

IMO this is going up the spectrum from runtime checking to static checking, and it becomes more and more useful when you are writing libraries for mass consumption or working in large codebases with a lot of developers. I wish I had stuff like this at my last job, so much boilerplate to basically guarantee similar stuff statically would be replaced by a few one liner 'ensure' clauses. I think we will see this feature added to production languages in 3-20 years, hopefully sooner. I think Eiffel had this feature too with their program by contract feature? It's basically baking in more and more parts of unit tests into the language itself, and a one liner 'ensures' clause is way more concise and stronger of a test than a unit test you have to run separately or an assert that does checks at runtime. The key issue with static verification is making sure it doesn't get in the way of build speeds, execution speeds and binary size.