`enum` fallback design allows for silent API breakages

[oguzkocer]

Consider the WpErrorCode implementation below:

fn showcase(error_code: WpErrorCode) {
    if let WpErrorCode::CustomError(error_code_as_string) = error_code {
        if error_code_as_string == "foo" {
            // handle the "foo" error
        }
    }

If we were to add a new WpErrorCode::Foo variant, it'll start to fail and has to be handled as such:

fn showcase(error_code: WpErrorCode) {
    if let WpErrorCode::Foo = error_code {
        // handle the "foo" error
    }
}

This kind of silent API breakages are very dangerous. Our design should allow us to add more error variants without breaking the existing implementation.

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The same issue can be seen in other enum fallbacks as well. For example, if we were to add a new variant to PostTypeCapabilities, it can break the same way.