Sorting, trigonometry, filesystem access, HTTP. All these libraries I took for granted until I created a programming language. Nontrivial!
Related Posts
Blogging about my programming language project: choosing the basic syntax!
https://www.garden-lang.org/blog/syntax.html
The niche of short, environment-specific scripts, and the consequences for programming language design choices: https://brrt-to-the-future.blogspot.com/2021/03/why-bother-with-scripting.html
It is remarkably hard to escape command line arguments safely on Windows, and the standard libraries of multiple languages have needed patching: https://flatt.tech/research/posts/batbadbut-you-cant-securely-execute-commands-on-windows/