Testability of errors

[dtolnay]

From rust-lang/rust#39381.

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In order to test the error behaviour of a function you need an error struct to compare with. For example:
since ParseIntError is private, I have to generate one by doing something wrong, like u8::from_str("257"). This leads to the following test:

let pie_overflow = u8::from_str("257").unwrap_err();
assert_eq!(Missions::parse_enc_opt("ascii,256,U+41..U+67",8).unwrap_err(), pie_overflow);

If error types where public I would rather instantiate one and write:

let pie_overflow = ParseIntError { kind: std::num::Overflow };
assert_eq!(Missions::parse_enc_opt("ascii,256,U+41..U+67",8).unwrap_err(), pie_overflow);

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Let's come up with a guideline that empowers library consumers to test failure cases conveniently.

[dtolnay]

I feel like this is too diverse of a problem to address with a guideline. It is also not much of a backward compatibility risk -- if a crate is stabilized with untestable errors, exposing a way to test them will typically be additive. I would let crate authors figure this out case by case.