[burntsushi]

I doubt epage is suggesting that. And note that in that case, the thing distinguishing the cause is not `std::io::Error`, but `std::io::ErrorKind`. The latter is not the error type, but something that forms a part of the I/O error type.

It's very rare that `pub enum Error { ... }` is something I'd put into the public API of a library. epage is absolutely correct that it is an extensibility hazard. But having a sub-ordinate "kind" error enum is totally fine (assuming you mark it `#[non_exhaustive]`).

[the8472]

It's not uncommon to have it on the error itself, rather than a details/kind auxiliary type. AWS SDK does it, nested even [0][1], diesel[2], password_hash[3].

[0] https://docs.rs/aws-smithy-runtime-api/1.9.3/aws_smithy_runt... [1] https://docs.rs/aws-sdk-s3/1.119.0/aws_sdk_s3/operation/get_... [2] https://docs.rs/diesel/2.3.5/diesel/result/enum.Error.html [3] https://docs.rs/password-hash/0.5.0/password_hash/errors/enu...

[burntsushi]

Sure, I've done it too: https://docs.rs/ignore/latest/ignore/enum.Error.html

It's not necessarily about frequency, but about extensibility. There's lot of grey area there. If you're very certain of your error domain and what kinds of details you want to offer, then the downside of a less extensible error type may never actualize. Similarly, if you're more open to more frequent semver incompatible releases, then the downside also may never actualize.

[IshKebab]

Why is it an extensibility hazard (assuming you mark the `pub enum Error` as non-exhaustive)?

I mean I don't see the difference between having the non-exhaustive enum at the top level vs in a subordinate 'kind'.

[epage]

Take the example at https://docs.rs/thiserror/latest/thiserror/

- Struct variant fields are public, limiting how you evolve the fields and types

- Struct variants need non_exhaustive

- It shows using `from` on an error. What happens if you want to include more context? Or change your impl which can change the source error type

None of this is syntactically unique to errors. This becomes people's first thought of what to do and libraries like thiserror make it easy and showcase it in their docs.

[IshKebab]

> Struct variant fields are public, limiting how you evolve the fields and types

But the whole point of thiserror style errors is to make the errors part of your public API. This is no different to having a normal struct (not error related) as part of your public API is it?

> Struct variants need non_exhaustive

Can't you just add that tag? I dunno, I've never actually used thiserror.

Your third point makes sense though.

[epage]

> > Struct variant fields are public, limiting how you evolve the fields and types

> But the whole point of thiserror style errors is to make the errors part of your public API. This is no different to having a normal struct (not error related) as part of your public API is it?

Likely you should use private fields with public accessors so you can evolve it.

[the8472]

Having private variants and fields would be useful, yeah. Std cheats a bit with its ErrorKind::Uncategorized unstable+hidden variant to have something unmatchable.

[IshKebab]

Probably because non-exhaustive enums didn't exist when it was written?