add test for deriving Debug on uninhabited enum

[durka]

Adds a test to close #38885.

[rust-highfive]

r? @nikomatsakis

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[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:1cff5dee:start=1541042911170445266,finish=1541042966331713766,duration=55161268500
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
---
[00:48:31] .................................................................................................... 500/4984
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[00:48:45] ..................................................................i...........i..................... 800/4984
[00:48:48] ...F.................................................................................iiiii.......... 900/4984
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---
[00:50:49] .......................................................i............................................ 4600/4984
[00:50:52] .................................................................................................... 4700/4984
00] +   --> $DIR/derive-uninhabited-enum-38885.rs:21:1
[00:51:00] 11    |
[00:51:00] 12 LL | enum Foo { //~ WARN never used
[00:51:00] 
[00:51:00] 
[00:51:00] The actual stderr differed from the expected stderr.
[00:51:00] The actual stderr differed from the expected stderr.
[00:51:00] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/derive-uninhabited-enum-38885/derive-uninhabited-enum-38885.stderr
[00:51:00] To update references, rerun the tests and pass the `--bless` flag
[00:51:00] To only update this specific test, also pass `--test-args derive-uninhabited-enum-38885.rs`
[00:51:00] error: 1 errors occurred comparing output.
[00:51:00] status: exit code: 0
[00:51:00] status: exit code: 0
[00:51:00] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/derive-uninhabited-enum-38885.rs" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/derive-uninhabited-enum-38885/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-Wunused" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/derive-uninhabited-enum-38885/auxiliary" "-A" "unused"
[00:51:00] ------------------------------------------
[00:51:00] 
[00:51:00] ------------------------------------------
[00:51:00] stderr:
[00:51:00] stderr:
[00:51:00] ------------------------------------------
[00:51:00] {"message":"enum is never used: `Void`","code":{"code":"dead_code","explanation":null},"level":"warning","spans":[{"file_name":"/checkout/src/test/ui/derive-uninhabit4743480 .
2476256 ./obj/build
1847548 ./obj/build/x86_64-unknown-linux-gnu
1194432 ./.git
1072228 ./src

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[nikomatsakis]

@durka looks like you need to run --bless

[durka]

Should be fixed.

[nikomatsakis]

@bors r+ rollup

[bors]

📌 Commit 2279f62 has been approved by nikomatsakis

[nikomatsakis]

Usually I just do #[allow(dead_code)], but it's fine this way too.