The builtin itself is fine. It works exactly as it's intended. It says "I've double and tripple checked this. Trust me compiler. Just go fast". But you should not use it to construct an assert.
The nice thing about this approach is that the assertion provides value both in debug and release mode. In debug mode, it checks your invariants. And in release mode, it makes your program smaller and faster.
Personally I quite like rust's choice to have a pair of assert functions: assert!() and debug_assert!(). The standard assert function still does its check in both debug and release mode. And honestly thats a fine default these days. Sure, it makes the binary slightly bigger and the program slightly slower, but on modern computers it usually doesn't matter. And when it does matter (like your assertion check is expensive), we have debug_assert instead.
Usually in release mode you want to log the core dump and then fix the bug.
The unreachable pragma suggested by the author is just a more extreme version of the latter choice.