What is the preferred phrasing of messages in .expect() panics?

Example from the official Rust docs in the recent release:

use std::fs;

assert!(!fs::exists("does_not_exist.txt").expect("Can't check existence of file does_not_exist.txt"));
assert!(fs::exists("/root/secret_file.txt").is_err());

Source

Doesn't the name of the method "expect" indicate what you expected to happen? You already know what didn't or couldn't happen. Because your program crashed. Aren't you supposed to say what you thought was supposed to happen?

I see every different kind of variation of these messages even in the official Rust docs and code itself. So what should it be?