Error types: methods or variants?

[clarfonthey]

Basically: should error types expose what they are via an enum such as std:io::ErrorKind or via accessor methods like reqwest::Error::is_http?

This seems like there should be some guideline for which is preferred and when.

[KodrAus]

I personally prefer methods to variants, because you can introduce subtle runtime bugs by changing the conditions under which a variant is produced. Say we have an error with a Http variant:

#[non_exhaustive]
enum MyError {
    Http,
    OtherVariant,
}

Now say we want to introduce a specific Timeout variant, which used to be under Http:

#[non_exhaustive]
enum MyError {
    Http,
    Timeout,
    OtherVariant,
}

Anyone depending on timeouts caught by a pattern on Http won't anymore. But with methods the new timeout error could still return true from is_http, but a new is_timeout method is more specific.