Deriving Debug on an enum with uninhabited type triggers warning

[seanmonstar]

// no warning
#[derive(Debug)]
enum Void {}

// warns about unreachable pattern
#[derive(Debug)]
enum Foo {
    Bar(u8),
    Void(Void),
}

Caused by #38069

[scottmcm]

Not seeing this in playground. Maybe it got fixed somewhere along the way?

[Havvy]

The code gives the following compiler output (with a #[crate_type="lib"]\n\n prefix)

warning: enum is never used: `Void`
 --> src/lib.rs:5:1
  |
5 | enum Void {}
  | ^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

warning: enum is never used: `Foo`
 --> src/lib.rs:9:1
  |
9 | enum Foo {
  | ^^^^^^^^

There's no warning about an unreachable pattern anymore. As such, proposing this be closed.

[Havvy]

cc @estebank

[estebank]

@Havvy thanks for the heads up! Lets close with a test to avoid regressions from the current behavior.

[LukasKalbertodt]

This problem resurfaced, but only when actually using !. This code (Playground):

#![feature(never_type)]

#[derive(Debug)]
pub struct Foo(!);

#[derive(Debug)]
pub struct Bar(Empty);

#[derive(Debug)]
pub enum Empty {}

Leads to this output (with nightly-2019-02-06):

warning: unreachable expression
 --> src/lib.rs:5:16
  |
5 | pub struct Foo(!);
  |                ^
  |
  = note: #[warn(unreachable_code)] on by default

So should we reopen this issue?

[estebank]

Yes, reopen it or refile.

…

On Thu, Feb 7, 2019, 4:03 PM Lukas Kalbertodt ***@***.***> wrote: This problem resurfaced, but only when actually using !. This code ( Playground <https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=76b116e886c0a1d1af7e87b52d1b33e6> ): #![feature(never_type)] #[derive(Debug)]pub struct Foo(!); #[derive(Debug)]pub struct Bar(Empty); #[derive(Debug)]pub enum Empty {} Leads to this output (with nightly-2019-02-06): warning: unreachable expression --> src/lib.rs:5:16 | 5 | pub struct Foo(!); | ^ | = note: #[warn(unreachable_code)] on by default So should we reopen this issue? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#38885 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABiDIhaypq1z1kmjXnqwRGQcMxTJYowEks5vLEAtgaJpZM4LdAcT> .