improve the error message when #[panic_implementation] is missing

[japaric]

closes #51341

r? @nagisa
cc @phil-opp

[rust-highfive]

The job x86_64-gnu-llvm-3.9 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.

[01:04:21] ......................................................................................i.............
[01:04:27] ...................................i................................................................
[01:04:34] ....................................................................................................
[01:04:39] ....................................................................................................
[01:04:43] ..........................................................................F.........................
[01:04:45] failures:
[01:04:45] 
[01:04:45] ---- [compile-fail] compile-fail/weak-lang-item.rs stdout ----
[01:04:45] 
[01:04:45] 
[01:04:45] error: error pattern ' language item required, but not found: `panic_impl`' not found!
[01:04:45] 
[01:04:45] error: error pattern ' language item required, but not found: `eh_personality`' not found!
[01:04:45] 
[01:04:45] error: multiple error patterns not found
[01:04:45] status: exit code: 101
[01:04:45] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/compile-fail/weak-lang-item.rs" "--target=x86_64-unknown-linux-gnu" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/compile-fail/weak-lang-item/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/compile-fail/weak-lang-item/auxiliary" "-A" "unused"
[01:04:45] ------------------------------------------
[01:04:45] 
[01:04:45] ------------------------------------------
[01:04:45] stderr:
[01:04:45] stderr:
[01:04:45] ------------------------------------------
[01:04:45] error[E0259]: the name `core` is defined multiple times
[01:04:45]   --> /checkout/src/test/compile-fail/weak-lang-item.rs:18:1
[01:04:45]    |
[01:04:45] LL | extern crate core;
[01:04:45]    | ^^^^^^^^^^^^^^^^^^ `core` reimported here
[01:04:45]    |
[01:04:45]    = note: `core` must be defined only once in the type namespace of this module
[01:04:45] help: You can use `as` to change the binding name of the import
[01:04:45]    |
[01:04:45] LL | extern crate core as other_core;
[01:04:45] 
[01:04:45] 
[01:04:45] error: `#[panic_implementation]` function required, but not found
[01:04:45] 
[01:04:45] error: language item required, but not found: `eh_personality`
[01:04:45] error: aborting due to 3 previous errors
[01:04:45] 
[01:04:45] For more information about this error, try `rustc --explain E0259`.
[01:04:45] 
---
[01:04:45] 
[01:04:45] thread 'main' panicked at 'Some tests failed', tools/compiletest/src/main.rs:498:22
[01:04:45] 
[01:04:45] 
[01:04:45] 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/compile-fail" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/compile-fail" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "compile-fail" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-3.9/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options " "--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" "3.9.1\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[01:04:45] 
[01:04:45] 
[01:04:45] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:04:45] Build completed unsuccessfully in 0:16:24
[01:04:45] Build completed unsuccessfully in 0:16:24
[01:04:45] Makefile:58: recipe for target 'check' failed
[01:04:45] make: *** [check] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:06365a9b
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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)

[nagisa]

@bors r+

[bors]

📌 Commit ee52862 has been approved by nagisa