hide --explain hint if error has no extended info

[euclio]

Fixes#59848.

r? @estebank

[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:0843304c:start=1555522398370705725,finish=1555522399467573317,duration=1096867592
$ 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
---
travis_time:start:test_assembly
Check compiletest suite=assembly mode=assembly (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:14:51] [01:14:51] running 9 tests
[01:14:51] iiiiiiiii
[01:14:51] [01:14:51] finished in 0.164
[01:14:51] travis_fold:end:test_assembly
---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:15:08] [01:15:08] running 121 tests
[01:15:36] .iiiii...i.....i..i...i..i.i.i..i.ii...i.....i..i....i..........iiii..........i...ii...i.......ii.i. 100/121
[01:15:42] i.i......iii.i.....ii
[01:15:42] [01:15:42] finished in 33.849
[01:15:42] travis_fold:end:test_debuginfo
---
[01:15:42] [01:15:42] running 22 tests
[01:15:50] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:517:22
[01:15:50] note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
[01:15:50] ....F..............F..
[01:15:50] [01:15:50] ---- [ui] ui-fulldeps/hash-stable-is-unstable.rs stdout ----
[01:15:50] diff of stderr:
[01:15:50] [01:15:50] [01:15:50] 49 [01:15:50] 50 error: aborting due to 6 previous errors
[01:15:50] 51 [01:15:50] - Some errors occurred: E0601, E0658.
[01:15:50] + Some errors have detailed explanations: E0601, E0658.
[01:15:50] 54 [01:15:50] [01:15:50] [01:15:50] The actual stderr differed from the expected stderr.
[01:15:50] The actual stderr differed from the expected stderr.
[01:15:50] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/hash-stable-is-unstable/hash-stable-is-unstable.stderr
[01:15:50] To update references, rerun the tests and pass the `--bless` flag
[01:15:50] To only update this specific test, also pass `--test-args hash-stable-is-unstable.rs`
[01:15:50] error: 1 errors occurred comparing output.
[01:15:50] status: exit code: 1
[01:15:50] status: exit code: 1
[01:15:50] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui-fulldeps/hash-stable-is-unstable.rs" "-Zthreads=1" "--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-fulldeps/hash-stable-is-unstable/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/hash-stable-is-unstable/auxiliary" "-A" "unused"
[01:15:50] ------------------------------------------
[01:15:50] [01:15:50] ------------------------------------------
[01:15:50] stderr:
[01:15:50] stderr:
[01:15:50] ------------------------------------------
[01:15:50] error[E0601]: `main` function not found in crate `hash_stable_is_unstable`
[01:15:50] |
[01:15:50] = note: consider adding a `main` function to `/checkout/src/test/ui-fulldeps/hash-stable-is-unstable.rs`
[01:15:50] error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
[01:15:50] --> /checkout/src/test/ui-fulldeps/hash-stable-is-unstable.rs:3:1
[01:15:50] |
[01:15:50] LL | extern crate rustc_data_structures;
---
[01:15:50] [01:15:50] error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
[01:15:50] --> /checkout/src/test/ui-fulldeps/hash-stable-is-unstable.rs:13:10
[01:15:50] |
[01:15:50] LL | #[derive(HashStable)]
[01:15:50] |
[01:15:50] = note: for more information, see https://github.com/rust-lang/rust/issues/27812
[01:15:50] = help: add #![feature(rustc_private)] to the crate attributes to enable
[01:15:50] ---
[01:15:50] 10 [01:15:50] [01:15:50] [01:15:50] The actual stderr differed from the expected stderr.
[01:15:50] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/macro-crate-rlib/macro-crate-rlib.stderr
[01:15:50] To update references, rerun the tests and pass the `--bless` flag
[01:15:50] To only update this specific test, also pass `--test-args macro-crate-rlib.rs`
[01:15:50] error: 1 errors occurred comparing output.
[01:15:50] status: exit code: 1
[01:15:50] status: exit code: 1
[01:15:50] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui-fulldeps/macro-crate-rlib.rs" "-Zthreads=1" "--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-fulldeps/macro-crate-rlib/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/macro-crate-rlib/auxiliary" "-A" "unused"
[01:15:50] ------------------------------------------
[01:15:50] [01:15:50] ------------------------------------------
[01:15:50] stderr:
[01:15:50] stderr:
[01:15:50] ------------------------------------------
[01:15:50] error[E0457]: plugin `rlib_crate_test` only found in rlib format, but must be available in dylib format
[01:15:50] |
[01:15:50] |
[01:15:50] LL | #![plugin(rlib_crate_test)]
[01:15:50] [01:15:50] error: aborting due to previous error
[01:15:50] [01:15:50] ---
[01:15:50] test result: FAILED. 20 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out
[01:15:50] [01:15:50] [01:15:50] [01:15:50] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui-fulldeps" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-6.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Zunstable-options -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "6.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[01:15:50] [01:15:50] [01:15:50] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:15:50] Build completed unsuccessfully in 0:13:21
[01:15:50] Build completed unsuccessfully in 0:13:21
[01:15:50] Makefile:48: recipe for target 'check' failed
[01:15:50] make: *** [check] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1cc2bdfc
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Apr 17 18:49:34 UTC 2019
---
travis_time:end:00ed2916:start=1555526976803274022,finish=1555526976861656309,duration=58382287
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:09626ad4
$ 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:18dba0ac
$ 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)

[estebank]

r=me after rebase

[euclio]

@estebank Should be good to go.

[estebank]

@bors r+ p=1

[bors]

📌 Commit b6f148c has been approved by estebank

[bors]

⌛ Testing commit b6f148c with merge e928e94...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: estebank
Pushing e928e94 to master...