crates that provide a panic_handler are exempt from the unused_extern_crates lint

[japaric]

fixes the first false positive reported in #53964

[rust-highfive]

r? @cramertj

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[cramertj]

Seems reasonable to me!

@bors r+

[bors]

📌 Commit 9b764c3 has been approved by cramertj

[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.

[00:47:13] ....................................................................................................
[00:47:16] ....................i...............................................................................
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[00:47:25] ..................................................................F.................................
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[00:47:38] .......................................................................................i............
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---
[00:47:50] [00:47:50] ---- [ui] ui/removing-extern-crate.rs stdout ----
[00:47:50] diff of stderr:
[00:47:50] [00:47:50] - warning: unused extern crate
[00:47:50] + warning: `extern crate` is not idiomatic in the new edition
[00:47:50] 3 |
[00:47:50] 3 |
[00:47:50] 4 LL | extern crate std as foo;
[00:47:50] [00:47:50] - | ^^^^^^^^^^^ckout/src/test/ui/removing-extern-crate.rs:16:9\n |\nLL | #![warn(rust_2018_idioms)]\n | ^^^^^^^^^^^^^^^^\n = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]\n\n"}
[00:47:50] {"message":"unused extern crate","code":{"code":"unused_extern_crates","explanation":null},"level":"warning","spans":[{"file_name":"/checkout/src/test/ui/removing-extern-crate.rs","byte_start":632,"byte_end":650,"line_start":20,"line_end":20,"column_start":1,"column_end":19,"is_primary":true,"text":[{"text":"extern crate core;","highlight_start":1,"highlight_end":19}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"remove it","code":null,"level":"help","spans":[{"file_name":"/checkout/src/test/ui/removing-extern-crate.rs","byte_start":632,"byte_end":650,"line_start":20,"line_end":20,"column_start":1,"column_end":19,"is_primary":true,"text":[{"text":"extern crate core;","highlight_start":1,"highlight_end":19}],"label":null,"suggested_replacement":"","suggestion_applicability":"MachineApplicable","expansion":null}],"children":[],"rendered":null}],"rendered":"warning: unused extern crate\n --> /checkout/src/test/ui/removing-extern-crate.rs:20:1\n |\nLL | extern crate core;\n | ^^^^^^^^^^^^^^^^^^ help: remove it\n\n"}
[00:47:50] {"message":"`extern crate` is not idiomatic in the new edition","code":{"code":"unused_extern_crates","explanation":null},"level":"warning","spans":[{"file_name":"/checkout/src/test/ui/removing-extern-crate.rs","byte_start":670,"byte_end":694,"line_start":23,"line_end":23,"column_start":5,"column_end":29,"is_primary":tru":5,"column_end":22,"is_primary":true,"text":[{"text":" extern crate std;","highlight_start":5,"highlight_end":22}],"label":null,"suggested_replacement":"use std;","suggestion_applicability":"MachineApplicable","expansion":null}],"children":[],"rendered":null}],"rendered":"warning: `extern crate` is not idiomatic in the new edition\n --> /checkout/src/test/ui/removing-extern-crate.rs:24:5\n |\nLL | extern crate std;\n | ^^^^^^^^^^^^^^^^^ help: convert it to a `use`\n\n"}
[00:47:50] ------------------------------------------
[00:47:50] [00:47:50] thread '[ui] ui/removing-extern-crate.rs' panicked at 'explicit panic', tools/compiletest/src/runtest.rs:3196:9
[00:47:50] note: Run with `RUST_BACKTRACE=1` for a backtrace.
---
[00:47:50] [00:47:50] thread 'main' panicked at 'Some tests failed', tools/compiletest/src/main.rs:497:22
[00:47:50] [00:47:50] [00:47:50] 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" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linThu, 06 Sep 2018 20:17:12 GMT
The command "date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
" exited with 0.
travis_fold:start:after_failure.1

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)

[japaric]

bors r=cramertj

[japaric]

@bors r=cramertj

[bors]

📌 Commit 6c4f3f5 has been approved by cramertj

[bors]

☔ The latest upstream changes (presumably #54051) made this pull request unmergeable. Please resolve the merge conflicts.