warn(unreachable_code) on empty error enums

[Kyuuhachi]

Using an empty enum as source produces an unreachable_code warning. Which is certainly true, but should it do that? Sometimes you want to return a Result for forward compatibility or whatever.

use snafu::ResultExt as _;

#[derive(Debug, snafu::Snafu)]
enum Outer {
	Inner { source: Inner },
}

#[derive(Debug, snafu::Snafu)]
enum Inner {}

fn infallible() -> Result<(), Inner> {
	Ok(())
}

fn main() -> Result<(), Outer> {
	infallible().context(InnerSnafu)
}

warning: unreachable expression
 --> src/main.rs:3:17
  |
3 | #[derive(Debug, snafu::Snafu)]
  |                 ^^^^^^^^^^^^
  |                 |
  |                 unreachable expression
  |                 any code following this expression is unreachable
  |
note: this expression has type `Inner`, which is uninhabited
 --> src/main.rs:3:17
  |
3 | #[derive(Debug, snafu::Snafu)]
  |                 ^^^^^^^^^^^^
  = note: `#[warn(unreachable_code)]` on by default
  = note: this warning originates in the derive macro `snafu::Snafu` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `snafu-unreachable` (bin "snafu-unreachable") generated 1 warning

[Enet4]

Would you be able to use Infallible instead of declaring Inner?

[Kyuuhachi]

That gives the same error, but is less forward compatible.

[Stargateur]

if Inner is hidden from user as its seem to be the thing you want why not use empty struct ? or even nothing inside Inner variant.