[WIP] UNUSED_EXTERN_CRATES -> Deny

[Centril]

Continuing on from #59482 with the rust_2018_idioms group, we have unused_extern_crates to progress.

r? @oli-obk
(I'm primarily assigning you because the PR needs a reviewer for the code changes, not for the T-Lang policy questions.. ^^)

[Centril]

Likely blocked on #58702 and #58828.

[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:0c646b52:start=1553770199581150818,finish=1553770200679817968,duration=1098667150
$ 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
Setting environment variables from .travis.yml
---
18940 ./obj/build/x86_64-unknown-linux-gnu/stage0-codegen
17940 ./src/llvm-project/lldb/www/python_reference
travis_time:end:034c1d09:start=1553771999465456737,finish=1553772000416824880,duration=951368143
travis_fold:end:after_failure.1
trabuild/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:00a3c5be
$ dmesg | grep -i kill

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[Centril]

Justified by https://github.com/rust-lang/rfcs/blob/master/text/2126-path-clarity.md#basic-changes. In particular, this is only for extern crates that the compiler thinks are unnecessary.

[petrochenkov]

This lint has multiple open issues:

• Remove unused_extern_crates warning for crates used by nested code #44294
• strange rust_2018_idioms warning on code comment in clippy codebase #51881
• 2018 idioms: rustc-internal crates linted to be removed when they can't #56038
• edition idioms: Incorrect span in extern crate removal #56328
• 2018: fix lint on unused dependencies #57274
• unused_extern_crates: suggests removing sysroot crate when used inside a submodule #57424
• unused_extern_crates: wrong suggestion for aliased crate #57672
  , they need to be triaged and possibly fixed before proceeding with this PR.

[petrochenkov]

Also, this is a usual unused lint, they are not normally deny-ed, only warn-ed about.

[Centril]

@petrochenkov Deny is for crater purposes to measure before turning back into Warn.

[Centril]

We discussed this on this weeks lang meeting; the general consensus was to crater and see how things fall out and move on from there.

However, I think we should be cognizant of #59483 (comment).

[Dylan-DPC-zz]

ping from triage @Centril Closing this as inactive