> aren't they just annotations? proper use of mutexes and lock ordering aren't that hard, they just require a little bit of discipline and consistency. Spatial

> aren't they just annotations? proper use of mutexes and lock ordering aren't that hard, they just require a little bit of discipline and consistency. Spatial memory safety is easy, just check the bounds before indexing an array. Temporal memory safety is easy, just free memory only after you've finished using it, and not too early or too late. As you say, thread safety is easy. Except we have loads of empirical evidence--from widespread failures of software--that it's not easy in practice. Especially in large codebases, remembering the remote conditions you need to uphold to maintain memory safety and thread safety can be difficult. I've written loads of code that created issues like "oops, I forgot to account for the possibility that someone might use this notification to immediately tell me to shut down." What these annotations provide is a way to have the compiler bop you in the head when you accidentally screw something up, in the same way the compiler bops you in the head if you fucked up a type or the name of something. And my experience is that many people do go through a phase with the borrow checker where they complain about it being incorrect, only to later discover that it was correct, and the pattern they thought was safe wasn't.