Improve bug! message for impossible case in Relate

[phansch]

Hitting this branch in Clippy and I think it makes sense to print
both values here in case other users hit this branch, too.

(I'm not really sure what type x would be here)

[rust-highfive]

r? @varkor

(rust_highfive has picked a reviewer for you, use r? to override)

[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.

[00:07:58]    Compiling syntax_ext v0.0.0 (file:///checkout/src/libsyntax_ext)
[00:08:29] error[E0308]: mismatched types
[00:08:29]    --> librustc/ty/relate.rs:705:66
[00:08:29]     |
[00:08:29] 705 |             (UnpackedKind::Lifetime(a), b) | (UnpackedKind::Type(a), b) => {
[00:08:29]     |                                                                  ^ expected enum `ty::sty::RegionKind`, found struct `ty::TyS`
[00:08:29]     |
[00:08:29]     = note: expected type `&ty::sty::RegionKind`
[00:08:29]                found type `&ty::TyS<'_>`
[00:08:31] error: aborting due to previous error
[00:08:31] 
[00:08:31] For more information about this error, try `rustc --explain E0308`.
[00:08:31] error: Could not compile `rustc`.
[00:08:31] error: Could not compile `rustc`.
[00:08:31] 
[00:08:31] To learn more, run the command again with --verbose.
[00:08:31] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" " jemalloc" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json"
[00:08:31] expected success, got: exit code: 101
[00:08:31] thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1112:9
[00:08:31] travis_fold:end:stage0-rustc

[00:08:31] travis_time:end:stage0-rustc:start=1538201806045186247,finish=1538201988538998808,duration=182493812561


[00:08:31] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:08:31] Build completed unsuccessfully in 0:04:02
[00:08:31] Makefile:28: recipe for target 'all' failed
[00:08:31] make: *** [all] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0f0fef88
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:29c1f58b:start=1538201989168133185,finish=1538201989172166309,duration=4033124
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0aacf08c
$ 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:031190c9
travis_time:start:031190c9
$ 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:089dd640
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[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.

[00:08:13] 
[00:08:13] To learn more, run the command again with --verbose.
[00:08:13] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" " jemalloc" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json"
[00:08:13] expected success, got: exit code: 101
[00:08:13] thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1112:9
[00:08:13] travis_fold:end:stage0-rustc

[00:08:13] travis_time:end:stage0-rustc:start=1538204258249848143,finish=1538204429150607837,duration=170900759694


[00:08:13] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:08:13] Build completed unsuccessfully in 0:03:46
[00:08:13] make: *** [all] Error 1
[00:08:13] Makefile:28: recipe for target 'all' failed
ers || 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:01a5a0b8
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[phansch]

Will open a new PR once I get the tests to run locally again.