SPARK is a real-world system that allows some degree of formal proofs in the programming language and related tools:
https://learn.adacore.com/courses/intro-to-spark/index.html
https://docs.adacore.com/spark2014-docs/html/lrm/introductio...
https://learn.adacore.com/courses/intro-to-spark/index.html
https://docs.adacore.com/spark2014-docs/html/lrm/introductio...
Although, usually the people who use SPARK use it because it is certified for their industry (such as aerospace).