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Some call points to a function that returns a Result will never return an Error.

Some call points to a function that returns an int will never return -2.

Sometimes you know things the type system does not know.

The difference is functions which return Result have explicitly chosen to return a Result because they can fail. Sure, it might not fail in the current implementation and/or configuration, but that could change later and you might not know until it causes problems. The type system is there to help you - why ignore it?

Because it would be a huge hassle to go into that library and write an alternate version that doesn't return a Result. So you're stuck with the type system being wrong in some way. You can add error-handling code upfront but it will be dead code at that point in time, which is also not good.

As a hypothetical example, when making a regex, I call `Regex::new(r"/d+")` which returns a result because my regex could be malformed and it could miscompile. It is entirely reasonable to unwrap this, though, as I will find out pretty quickly that it works or fails once I test the program.

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