[WIP] Fix the early drop in UnsafeFutureObj::into_raw for Pin<Box<T>>

[Kroisse]

It blocks rust-lang/futures-rs#1266.

[rust-highfive]

r? @Mark-Simulacrum

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[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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[01:09:46]     Finished release [optimized] target(s) in 44.55s
[01:09:46]      Running build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/alloc-e8d35b7f9eb87295
[01:09:46] 
[01:09:46] running 101 tests
[01:09:46] .............F...................................................................................... 100/101
[01:09:46] failures:
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[01:09:46] 
[01:09:46] ---- boxed_test::unsafe_future_obj_into_raw_pinned stdout ----
[01:09:46] thread 'boxed_test::unsafe_future_obj_into_raw_pinned' panicked at 'pinned value should not be dropped', liballoc/boxed_test.rs:179:5
[01:09:46] 
[01:09:46] 
[01:09:46] failures:
[01:09:46] failures:
[01:09:46]     boxed_test::unsafe_future_obj_into_raw_pinned
[01:09:46] test result: FAILED. 100 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
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[01:09:46] error: test failed, to rerun pass '--lib'
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[01:09:46] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "alloc" "--" "--quiet"
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[01:09:46] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:09:46] Build completed unsuccessfully in 0:24:20
[01:09:46] Build completed unsuccessfully in 0:24:20
[01:09:46] Makefile:58: recipe for target 'check' failed
[01:09:46] make: *** [check] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1b4c54f0
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:097cb336:start=1537542337101640836,finish=1537542337106427249,duration=4786413
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:27ecfa78
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\

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[cramertj]

FutureObj is moving out of std in #54339, so I'm going to close this since we can just implement this correctly in futures once that has landed.