Replace empty enums with Never type

[taiki-e]

We have two empty enums. It should be preferable to unify this.

[taiki-e]

Depending on the state of rust-lang/rust#57012, I think we can use Never instead of real ! type in FutureExt::never_error.

cc @Nemo157 Any thoughts?

[Nemo157]

rust-lang/rust#58733 is tempting...

[taiki-e]

rust-lang/rust#58733 is tempting...

It is certainly tempting but feels potentially unstable because it seems that some versions cannot compile (although they are not versions supported by this crate...).

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I was thinking of copying "Future compatibility" section of std::convert::Infallible's document, but in the first place, we can use Infallible instead of having our own Never enum here.

Use Infallible directly or type Never = core::convert::Infallible may be better, considering that compatibility is taken into account when never type stabilizes.
Then when Infallible becomes type Infallible = !, we can switch to #[cfg(never_type_stabilized)] type Never = !.

[taiki-e]

Use Infallible directly or type Never = core::convert::Infallible may be better, considering that compatibility is taken into account when never type stabilizes.
Then when Infallible becomes type Infallible = !, we can switch to #[cfg(never_type_stabilized)] type Never = !.

Currently this PR does this.

[taiki-e]

😕 This seems to happen frequently today.

• https://travis-ci.com/rust-lang-nursery/futures-rs/jobs/212712832
• https://travis-ci.com/rust-lang-nursery/futures-rs/jobs/212712842
• Compat wrapper methods #1705 (comment)

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

[taiki-e]

It seems that the total build time has been greatly reduced with a943d65.
before(master): 2 hrs 42 min 44 sec (https://travis-ci.com/rust-lang-nursery/futures-rs/builds/117603909)
after(this pr): 43 min 9 sec (https://travis-ci.com/rust-lang-nursery/futures-rs/builds/117710514)

[taiki-e]

r? @cramertj

[LucioFranco]

would it make sense to impl std::error::Error for this so that it can be used as a Never error type? I know so far we've used a bunch of bounds in tower like so S::Error: Into<Box<std::error::Error>> and this would make it compat with that.

[taiki-e]

@LucioFranco Infallible does not implement std::error::Error, but Into<Box<std::error::Error>> you mentioned seems to work. https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=cda064b0d4459549673d30c79d70c886

EDIT: see #1681 (comment)

[LucioFranco]

Awesome, thank you!

[taiki-e]

Infallible seems to implement std::error::Error via std::string::ParseError.

• playground
• std::string::ParseError