Implement RFC 2539: cfg_attr with multiple attributes

[Havvy]

This PR implements RFC 2539, except for the lint.

The lint is non-trivial and will be done in its own PR.

The tracking issue 555666 used in this PR does not actually exist, since there's no tracking issue for this RFC at the time of this. Once there's a tracking issue, I'll update the URL.

I moved src/ui/cfg-attr-trailing-comma.rs into src/ui/conditional-compilation/parse.rs instead of having it be its own test file.

r? @petrochenkov

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There is one problem with getting the error message to say expected identifier or `)` for cfg_attr(pred, attr,,)). Instead, it just says expected identifier. It also says that when the feature gate is not enabled, which is definitely wrong and confusing. The expected identifier message comes from Parser::expected_ident_found which doesn't look at the `Parser::expected_tokens vec. I'm wondering if I should fix that in this PR as well.

[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.

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[00:51:39] ---- [ui] ui/feature-gates/feature-gate-cfg-attr-multi-2.rs stdout ----
[00:51:39] 
[00:51:39] error: ui test compiled successfully!
[00:51:39] status: exit code: 0
[00:51:39] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/feature-gates/feature-gate-cfg-attr-multi-2.rs" "--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/feature-gates/feature-gate-cfg-attr-multi-2/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/feature-gates/feature-gate-cfg-attr-multi-2/auxiliary" "-A" "unused"
[00:51:39] ------------------------------------------
[00:51:39] 
[00:51:39] ------------------------------------------
[00:51:39] stderr:
[00:51:39] stderr:
[00:51:39] ------------------------------------------
[00:51:39] 
[00:51:39] ------------------------------------------
[00:51:39] 
[00:51:39] thread '[ui] ui/feature-gates/feature-gate-cfg-attr-multi-2.rs' panicked at 'explicit panic', tools/compiletest/src/runtest.rs:3267:9
[00:kout!\(.*\)|\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
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travis_time:start:1ff3b324
travis_time:start:1ff3b324
$ 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:0103086c
$ 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)

[tmandry]

Maybe premature, but have you considered using SmallVec to save an allocation?

[Havvy]

I don't know how to measure whether or not that would be a win. And yes, it's a bit premature, since I want to get the code right first.

[petrochenkov]

Sorry for the delay, will review tomorrow.

[petrochenkov]

This is a fun example.
I didn't know that crate attributes are configured twice to get the correct feature list early.

[petrochenkov]

@bors r+

[bors]

📌 Commit bbe832d has been approved by petrochenkov