how_to_ontop example broken on latest 1.34.0-nightly

[apatrushev]

x86_64 macOS
rustc 1.34.0-nightly (02c4c2892 2019-02-26)

Everything's set up!
Resuming => Yielding 0 => Got 0
Resuming => Yielding 1 => Got 1
Resuming => Yielding 2 => Got 2
Resuming => Yielding 3 => Got 3
Resuming => Yielding 4 => Got 4
Resuming => Yielding 5 => Got 5
Resuming => Yielding 6 => Got 6
Resuming => Yielding 7 => Got 7
Resuming => Yielding 8 => Got 8
Resuming => Yielding 9 => Got 9
Resuming context with unwind_stack() ontop!
Unwinding stack by panicking!
Illegal instruction: 4

[zonyitoo]

Debugging with LLDB, I saw it crashes at ud2 instruction..

    0x100001480 <+80>:  jmp    0x100001484               ; <+84> at how_to_ontop.rs
->  0x100001482 <+82>:  ud2
    0x100001484 <+84>:  leaq   -0x68(%rbp), %rdi
    0x100001488 <+88>:  callq  0x10000a360               ; _print at stdio.rs:743
    0x10000148d <+93>:  jmp    0x10000148f               ; <+95> at how_to_ontop.rs:43
    0x10000148f <+95>:  movq   -0x98(%rbp), %rax
    0x100001496 <+102>: movq   %rax, -0x38(%rbp)
    0x10000149a <+106>: movq   -0x90(%rbp), %rax
    0x1000014a1 <+113>: movq   %rax, -0x30(%rbp)
    0x1000014a5 <+117>: movq   -0x30(%rbp), %rax
    0x1000014a9 <+121>: movq   %rax, -0x28(%rbp)
    0x1000014ad <+125>: movq   -0x38(%rbp), %rax
    0x1000014b1 <+129>: movq   %rax, -0x20(%rbp)
    0x1000014b5 <+133>: movq   -0x28(%rbp), %rax
    0x1000014b9 <+137>: movq   -0x20(%rbp), %rcx
    0x1000014bd <+141>: movq   %rcx, (%rax)
    0x1000014c0 <+144>: xorl   %edx, %edx
    0x1000014c2 <+146>: movl   %edx, %edi
    0x1000014c4 <+148>: movl   $0x1, %esi
    0x1000014c9 <+153>: callq  0x100003050               ; alloc::alloc::exchange_malloc::hde2e5a8106125038 at alloc.rs:180
    0x1000014ce <+158>: movq   %rax, -0xa0(%rbp)
    0x1000014d5 <+165>: jmp    0x1000014d7               ; <+167> at boxed.rs
    0x1000014d7 <+167>: movq   -0xa0(%rbp), %rax
    0x1000014de <+174>: movq   %rax, -0xa8(%rbp)
    0x1000014e5 <+181>: leaq   0x2ddac(%rip), %rsi
    0x1000014ec <+188>: movq   -0xa8(%rbp), %rdi
    0x1000014f3 <+195>: callq  0x10000afe0               ; resume_unwind at panic.rs:420
    0x1000014f8 <+200>: jmp    0x100001508               ; <+216> at how_to_ontop.rs
    0x1000014fa <+202>: movl   %edx, %ecx
    0x1000014fc <+204>: movq   %rax, -0x10(%rbp)
    0x100001500 <+208>: movl   %ecx, -0x8(%rbp)
    0x100001503 <+211>: jmp    0x100001482               ; <+82> at how_to_ontop.rs:38
    0x100001508 <+216>: ud2
    0x10000150a <+218>: nopw   (%rax,%rax)

So LLVM thought there was a undefined behavior right here.

[zonyitoo]

The example crashes at this line. resume_unwind seems to trigger LLVM' sentinel of undefined behavior.