dbg!() without parameters

[clarfonthey]

Fixes #57845.

[rust-highfive]

r? @Kimundi

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[zackmdavis]

I argue that the new behavior should be exercised in the expected-behavior test file.

[clarfonthey]

Added test for this, so, it should be ready to merge. It seems that an FCP is required so we probably want to start that now.

[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:008ec2db:start=1548913547281438917,finish=1548913620486458746,duration=73205019829
$ 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
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
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[01:04:02] ......i........................................................................F.................... 4600/5355
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---
[01:04:28] ------------------------------------------
[01:04:28] stderr:
[01:04:28] ------------------------------------------
[01:04:28] thread 'main' panicked at 'assertion failed: `(left == right)`
[01:04:28]   left: `[":21] Unit = Unit", ":22] a = Unit", ":28] Point{x: 42, y: 24,} = Point {", "    x: 42,", "    y: 24", "}", ":29] b = Point {", "    x: 42,", "    y: 24", "}", ":37]", ":41] &a = NoCopy(", "    1337", ")", ":41] dbg!(& a) = NoCopy(", "    1337", ")", ":46] f(&42) = 42", "before", ":51] { foo += 1; eprintln!(\"before\"); 7331 } = 7331"]`,
[01:04:28]  right: `[":21] Unit = Unit", ":22] a = Unit", ":28] Point{x: 42, y: 24,} = Point {", "    x: 42,", "    y: 24", "}", ":29] b = Point {", "    x: 42,", "    y: 24", "}", ":37]", ":40] &a = NoCopy(", "    1337", ")", ":40] dbg!(& a) = NoCopy(", "    1337", ")", ":45] f(&42) = 42", "before", ":50] { foo += 1; eprintln!(\"before\"); 7331 } = 7331"]`', /checkout/src/test/ui/rfc-2361-dbg-macro/dbg-macro-expected-behavior.rs:60:5
[01:04:28] 
[01:04:28] ------------------------------------------
[01:04:28] 
[01:04:28] thread '[ui] ui/rfc-2361-dbg-macro/dbg-macro-expected-behavior.rs' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:3291:9
---
[01:04:28] 
[01:04:28] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:495:22
[01:04:28] 
[01:04:28] 
[01:04:28] 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-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"
[01:04:28] 
[01:04:28] 
[01:04:28] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:04:28] Build completed unsuccessfully in 0:04:04
[01:04:28] Build completed unsuccessfully in 0:04:04
[01:04:28] make: *** [check] Error 1
[01:04:28] Makefile:48: recipe for target 'check' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0d289130
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Thu Jan 31 06:51:37 UTC 2019
---
travis_time:end:007dc580:start=1548917498628774313,finish=1548917498633131201,duration=4356888
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:18a81c5b
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!che

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)

[Dylan-DPC-zz]

r? @zackmdavis

ping from triage @clarcharr you have failing tests to resolve

[clarfonthey]

@Dylan-DPC ...I resolved the tests. This is ready for FCP.

[Centril]

r? @SimonSapin

Test was added

[SimonSapin]

This seems like an obvious improvement to me, but it adds a stable API:

@rfcbot fcp merge

[rfcbot]

Team member @SimonSapin has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Kimundi
• @SimonSapin
• @alexcrichton
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank @SimonSapin for their work and everyone else who contributed.

The RFC will be merged soon.

[Dylan-DPC-zz]

ping from triage @simonpersson / @rust-lang/libs any updates on this?

[Centril]

Based on team approval...

r? @Centril @bors r+ rollup

[bors]

📌 Commit ea9e2c4 has been approved by Centril