Add ManuallyDrop::take

[CAD97]

Tracking issue: #55422

Proposed in this form in https://internals.rust-lang.org/t/mini-rfc-manuallydrop-take/8679,
see that thread for some history.

A small convenience wrapper for ManuallyDrop that makes a pattern (taking ownership of the contained data in drop) more obvious.

[rust-highfive]

r? @kennytm

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[CAD97]

Tracking issue currently points to this PR, that will need to be updated if this is accepted to be merged.

cc @scottmcm

[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.

travis_time:end:027d4d70:start=1540667294449047803,finish=1540667295441949262,duration=992901459
$ 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 SCCACHE_BUCKET=rust-lang-ci-sccache2
---
[00:04:06] tidy error: /checkout/src/libcore/mem.rs:976: trailing whitespace
[00:04:07] some tidy checks failed
[00:04:07] 
[00:04:07] 
[00:04:07] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:07] 
[00:04:07] 
[00:04:07] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:07] Build completed unsuccessfully in 0:00:49
[00:04:07] Build completed unsuccessfully in 0:00:49
[00:04:07] make: *** [tidy] Error 1
[00:04:07] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:2f326b80
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:1f68a157:start=1540667553143689352,finish=1540667553148061163,duration=4371811
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0caa2906
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\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
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:1e95a768
travis_time:start:1e95a768
$ 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:11d08ba6
$ 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)

[kennytm]

Just some documentation issues otherwise LGTM 🙂. cc @scottmcm.

[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.

travis_time:end:0a9cae5c:start=1540668777872306777,finish=1540668778909990888,duration=1037684111
$ 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 SCCACHE_BUCKET=rust-lang-ci-sccache2
---
[00:04:04] tidy error: /checkout/src/libcore/mem.rs:976: trailing whitespace
[00:04:05] some tidy checks failed
[00:04:05] 
[00:04:05] 
[00:04:05] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:05] 
[00:04:05] 
[00:04:05] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:05] Build completed unsuccessfully in 0:00:46
[00:04:05] Build completed unsuccessfully in 0:00:46
[00:04:05] Makefile:79: recipe for target 'tidy' failed
[00:04:05] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0a577ea4
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:104849c8:start=1540669034745397128,finish=1540669034749983643,duration=4586515
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0a875e34
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\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
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:04c79d90
travis_time:start:04c79d90
$ 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:1202e6a4
$ 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)

[scottmcm]

Thanks for the ccs, but I don't have anything to add 😅 Seems logical to me.

[kennytm]

r=me after squashing everything down to 1 commit.

cc @rust-lang/libs and @rust-lang/wg-unsafe-code-guidelines in case there's any problems caused by this function.

[CAD97]

Squashed.

[kennytm]

@bors r+ rollup

[bors]

📌 Commit 0757c0f has been approved by kennytm

[SimonSapin]

I expected the definition to be ptr::read(&mut slot.value). Reading a ManuallyDrop then unpacking it seems unnecessarily convoluted (even though it probably compiles to the same).

[scottmcm]

@SimonSapin I think it ended up this way because the code snippit on IRLO that became this was written as an external function. The alternative implementation sounds good too.