test that 50411 will not regress

[RalfJung]

Fixes #50411

[rust-highfive]

r? @aturon

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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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---
[00:57:14] 
[00:57:14] ------------------------------------------
[00:57:14] stderr:
[00:57:14] ------------------------------------------
[00:57:14] {"message":"librustc_mir/dataflow/move_paths/builder.rs:302: SetDiscriminant should not exist during borrowck","code":null,"level":"error: internal compiler error","spans":[{"file_name":"/checkout/src/libcore/iter/iterator.rs","byte_start":56708,"byte_end":56731,"line_start":1682,"line_end":1682,"column_start":15,"column_end":38,"is_primary":true,"text":[{"text":"        }) == LoopState::Continue(())","highlight_start":15,"highlight_end":38}],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"error: internal compiler error: librustc_mir/dataflow/move_paths/builder.rs:302: SetDiscriminant should not exist during borrowck\n  --> /checkout/src/libcore/iter/iterator.rs:1682:15\n   |\nLL |         }) == LoopState::Continue(())\n   |               ^^^^^^^^^^^^^^^^^^^^^^^\n\n"}
[00:57:14] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:57:14] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:57:14] {"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error\n\n"}
[00:57:14] note: the compiler unexpectedly panicked. this is a bug.
[00:57:14] 
[00:57:14] note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
[00:57:14] 
[00:57:14] 
[00:57:14] note: rustc 1.31.0-dev running on x86_64-unknown-linux-gnu
[00:57:14] 
[00:57:14] note: compiler flags: -Z ui-testing -Z unstable-options -Z mir-opt-level=3 -C prefer-dynamic -C rpath
[00:57:14] 
[00:57:14] ------------------------------------------
[00:57:14] 
[00:57:14] thread '[run-pass] run-pass/mir/mir-inlining/ice-issue-50411.rs' panicked at 'explicit panic', tools/compiletest/src/runtest.rs:3277:9
---
[00:57:14] 
[00:57:14] thread 'main' panicked at 'Some tests failed', tools/compiletest/src/main.rs:501:22
[00:57:14] 
[00:57:14] 
[00:57:14] 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/run-pass" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "run-pass" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-5.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options " "--target-rustcflags" "-Crpath 

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:10fcb2fa
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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[RalfJung]

Uh... What? I ran these tests locally!

[RalfJung]

The cause is likely that the bug goes way with test-miri = true, which enables -Zalways-encode-mir on libstd. Strange world...