add regression test for #60629
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
(accidentally removed the remote branch on github, therefore GH closed the other PR.)
r? @nikomatsakis
0b63f5530b