Guards example may be confusing

[skade]

In https://doc.rust-lang.org/rust-by-example/flow_control/match/guard.html, it is stated that:

Note that the compiler does not check arbitrary expressions for whether all possible conditions have been checked. Therefore, you must use the _ pattern at the end.

I was teaching a class today where someone interpreted this sentence as _ being mandatory when using guard expressions. They were weirded out because of an example where I bound to a variable instead (bc. the guards weren't covering) and didn't know that was possible.

[balroggg]

@skade In this case the compiler can not guarantee a matching pattern. Maybe the example should be

_ => unreachable!("Negative number is not supported"),

to indicate error case

[xiaochuanyu]

@skade / @balroggg I made a PR to try address this.
Take a look and let me know what you think.

[balroggg]

@xiaochuanyu LGTM

[marioidival]

Thank you guys!