assert_eq!(font_load_res, Err(Error::MissingUfoDir(...))) or something maybe?

Tried. Compiler won't let me perform equality comparisons on the Result Error type

Two things:

As written, I would do this as,

assert!(matches!(font_load_res, Err(Error::MissingUfoDir(_))));

but there's probably a better option: explicitly testing for a panic with the should_panic attribute.

but there's probably a better option: explicitly testing for a panic with the should_panic attribute.

Ah nice, I didn't realize that you can add an expected string with should_panic. I tried this approach but I can't elicit a panic from the new custom Error. The attribute doesn't seem to catch errors like it does panics. I see this in the docs that you linked:

You can’t use the #[should_panic] annotation on tests that use Result<T, E>. Instead, you should return an Err value directly when the test should fail.

Let me tinker with the test Result approach a bit. If I can't find a better solution, I'll transition things over the matches! macro approach above.

Maybe it would work if you just unwrap the Result?

(but matches! is totally reasonable too)

It looks like the use of a Result return type in unit tests is good if you expect an Ok() but there isn't a way to expect an Err. We could return a test error when the function being tested returns Ok 💥 but that seems worse than the match pattern matching approach :)

I went with the matches! macro. That allows us to confirm the custom error type and works well!

b1c17ab