Add regression test for #60629

[hellow554]

This bug was fixed, but I don't know which one. (I think it even doesn't matter at all).

Added a regression test.

op@OP ~/m/r/s/t/incremental> rustc --version
rustc 1.35.0-nightly (acd8dd6a5 2019-04-05)
op@OP ~/m/r/s/t/incremental> rustc -C incremental= --cfg rpass1 issue-60629.rs
warning: struct is never constructed: `A`
 --> issue-60629.rs:3:1
  |
3 | struct A;
  | ^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

op@OP ~/m/r/s/t/incremental> rustc -C incremental= --cfg rpass2 issue-60629.rs
error: internal compiler error: src/librustc/ty/query/plumbing.rs:1195: Cannot force dep node: coherent_trait(core[c27c]::ops[0]::drop[0]::Drop[0])

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:635:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: aborting due to previous error


note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.35.0-nightly (acd8dd6a5 2019-04-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C incremental

with latest nightly it does not crash anymore, so nothing more to do.

Fixes #60629

[rust-highfive]

r? @nikomatsakis

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[rust-highfive]

The job x86_64-gnu-llvm-6.0 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:0845f5bc:start=1557484333212618286,finish=1557484333961939217,duration=749320931
$ 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
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---

[00:04:34] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:35] tidy error: /checkout/src/test/incremental/issue-60629.rs: too many trailing newlines (2)
[00:04:39] some tidy checks failed
[00:04:39] 
[00:04:39] 
[00:04:39] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:39] 
[00:04:39] 
[00:04:39] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:39] Build completed unsuccessfully in 0:01:11
[00:04:39] Build completed unsuccessfully in 0:01:11
[00:04:39] make: *** [tidy] Error 1
[00:04:39] Makefile:67: recipe for target 'tidy' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0bc1c248
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Fri May 10 10:37:04 UTC 2019
---
travis_time:end:048baef0:start=1557484624966320558,finish=1557484624971148335,duration=4827777
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:05389742
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:0d57608e
travis_time:start:0d57608e
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:072f5f28
$ dmesg | grep -i kill

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[rust-highfive]

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.

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[hellow554]

Git is hard... :|