example.ll

; ModuleID = 'example.0.rs'
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"

%"2.std::str::Chars" = type { %"2.std::slice::Iter<u8>" }
%"2.std::slice::Iter<u8>" = type { i8*, i8*, %"2.std::marker::PhantomData<&'static u8>" }
%"2.std::marker::PhantomData<&'static u8>" = type {}
%"2.std::option::Option<char>" = type { i32, [0 x i32], [1 x i32] }
%"2.std::option::Option<u32>" = type { i32, [0 x i32], [1 x i32] }

; Function Attrs: argmemonly nounwind
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1) #1

declare i32 @__CxxFrameHandler3(...) unnamed_addr

; Function Attrs: inlinehint uwtable
define internal i64 @something() unnamed_addr #2 personality i32 (...)* @__CxxFrameHandler3 {
entry-block:
  ret i64 0
}

; Function Attrs: inlinehint uwtable
define i64 @_ZN4core4iter8iterator8Iterator4fold17h0471ccca27326883E(%"2.std::str::Chars"* noalias nocapture dereferenceable(16), i64) unnamed_addr #2 personality i32 (...)* @__CxxFrameHandler3 {
entry-block:
  %arg1 = alloca i64
  %self = alloca %"2.std::str::Chars"
  %init = alloca i64
  %accum = alloca i64
  %_result = alloca {}
  %iter = alloca %"2.std::str::Chars"
  %x = alloca i32
  %temp1 = alloca {}
  %temp3 = alloca %"2.std::str::Chars"
  %temp4 = alloca %"2.std::str::Chars"
  %temp5 = alloca %"2.std::option::Option<char>"
  %temp7 = alloca %"2.std::str::Chars"*
  %temp10 = alloca { i64, i32 }
  %temp15 = alloca i8
  %temp16 = alloca i8
  store i64 %1, i64* %arg1
  br label %start

start:                                            ; preds = %entry-block
  store i8 0, i8* %temp16
  store i8 0, i8* %temp15
  store i8 1, i8* %temp15
  %2 = bitcast %"2.std::str::Chars"* %0 to i8*
  %3 = bitcast %"2.std::str::Chars"* %self to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 8, i1 false)
  %4 = load i64, i64* %arg1
  store i64 %4, i64* %init
  %5 = load i64, i64* %init
  store i8 1, i8* %temp16
  store i64 %5, i64* %accum
  store i8 0, i8* %temp15
  %6 = bitcast %"2.std::str::Chars"* %self to i8*
  %7 = bitcast %"2.std::str::Chars"* %temp4 to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %7, i8* %6, i64 16, i32 8, i1 false)
  br label %bb3

bb1:                                              ; preds = %bb15, %bb16
  cleanupret from %cleanuppad9 unwind to caller

bb2:                                              ; preds = %bb17, %bb18
  cleanupret from %cleanuppad7 unwind label %bb16

bb3:                                              ; preds = %start
  %8 = bitcast %"2.std::str::Chars"* %temp3 to i8*
  %9 = bitcast %"2.std::str::Chars"* %iter to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %9, i8* %8, i64 16, i32 8, i1 false)
  br label %bb4

bb4:                                              ; preds = %bb28, %bb22, %bb3
  store %"2.std::str::Chars"* %iter, %"2.std::str::Chars"** %temp7
  %10 = load %"2.std::str::Chars"*, %"2.std::str::Chars"** %temp7, !nonnull !0
  %11 = invoke i64 @something()
          to label %bb7 unwind label %bb24

bb5:                                              ; preds = %bb8
  br label %bb12

bb6:                                              ; preds = %bb26, %bb29, %bb19, %bb20_cleanup_trampoline_bb6, %bb24
  %cleanuppad5 = cleanuppad within none []
  cleanupret from %cleanuppad5 unwind label %bb18

bb7:                                              ; preds = %bb4
  %12 = bitcast %"2.std::option::Option<char>"* %temp5 to i64*
  store i64 %11, i64* %12, align 4
  %13 = getelementptr inbounds %"2.std::option::Option<char>", %"2.std::option::Option<char>"* %temp5, i32 0, i32 0
  %14 = load i32, i32* %13, !range !1
  switch i32 %14, label %unreachable [
    i32 0, label %bb8
    i32 1, label %bb9
  ]

bb8:                                              ; preds = %bb7
  br label %bb5

bb9:                                              ; preds = %bb7
  %15 = bitcast %"2.std::option::Option<char>"* %temp5 to { i32, i32 }*
  %16 = getelementptr inbounds { i32, i32 }, { i32, i32 }* %15, i32 0, i32 1
  %17 = load i32, i32* %16, !range !2
  store i32 %17, i32* %x
  store i8 0, i8* %temp16
  %18 = load i64, i64* %accum
  %19 = load i32, i32* %x, !range !2
  %20 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 0
  store i64 %18, i64* %20
  %21 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 1
  store i32 %19, i32* %21
  %22 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 0
  %23 = load i64, i64* %22
  %24 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 1
  %25 = load i32, i32* %24
  %26 = invoke i64 @something()
          to label %bb10 unwind label %bb20

bb10:                                             ; preds = %bb9
  store i8 1, i8* %temp16
  store i64 %26, i64* %accum
  br label %bb11

bb11:                                             ; preds = %bb10
  br label %bb22

bb12:                                             ; preds = %bb5
  store i8 0, i8* %temp16
  %27 = load i64, i64* %accum
  br label %bb13

bb13:                                             ; preds = %bb12
  br label %bb14

bb14:                                             ; preds = %bb13
  ret i64 %27

bb15:                                             ; preds = %bb16
  store i8 0, i8* %temp15
  br label %bb1

bb16:                                             ; preds = %bb27, %bb2
  %cleanuppad9 = cleanuppad within none []
  %28 = load i8, i8* %temp15, !range !3
  %29 = trunc i8 %28 to i1
  br i1 %29, label %bb15, label %bb1

bb17:                                             ; preds = %bb18
  store i8 0, i8* %temp16
  br label %bb2

bb18:                                             ; preds = %bb25, %bb6, %bb23
  %cleanuppad7 = cleanuppad within none []
  %30 = load i8, i8* %temp16, !range !3
  %31 = trunc i8 %30 to i1
  br i1 %31, label %bb17, label %bb2

bb19:                                             ; preds = %bb20
  cleanupret from %cleanuppad2 unwind label %bb6

bb20:                                             ; preds = %bb9
  %cleanuppad2 = cleanuppad within none []
  %32 = getelementptr inbounds %"2.std::option::Option<char>", %"2.std::option::Option<char>"* %temp5, i32 0, i32 0
  %33 = load i32, i32* %32, !range !1
  switch i32 %33, label %unreachable [
    i32 0, label %bb19
    i32 1, label %bb20_cleanup_trampoline_bb6
  ]

bb21:                                             ; preds = %bb22
  br label %bb28

bb22:                                             ; preds = %bb11
  %34 = getelementptr inbounds %"2.std::option::Option<char>", %"2.std::option::Option<char>"* %temp5, i32 0, i32 0
  %35 = load i32, i32* %34, !range !1
  switch i32 %35, label %unreachable [
    i32 0, label %bb21
    i32 1, label %bb4
  ]

bb23:                                             ; preds = %start
  %cleanuppad = cleanuppad within none []
  cleanupret from %cleanuppad unwind label %bb18

bb24:                                             ; preds = %bb4
  %cleanuppad1 = cleanuppad within none []
  cleanupret from %cleanuppad1 unwind label %bb6

bb28:                                             ; preds = %bb21
  br label %bb4

unreachable:                                      ; preds = %bb22, %bb20, %bb7
  unreachable

bb20_cleanup_trampoline_bb6:                      ; preds = %bb20
  cleanupret from %cleanuppad2 unwind label %bb6
}

; Function Attrs: nounwind
declare void @llvm.assume(i1) #3

attributes #0 = { uwtable }
attributes #1 = { argmemonly nounwind }
attributes #2 = { inlinehint uwtable }
attributes #3 = { nounwind }

!0 = !{}
!1 = !{i32 0, i32 2}
!2 = !{i32 0, i32 1114112}
!3 = !{i8 0, i8 2}

opt -O3 output

While deleting: token %cleanuppad2
Use still stuck around after Def is destroyed:  cleanupret from %cleanuppad2 unwind label %bb18
Use still stuck around after Def is destroyed:  cleanupret from %cleanuppad2 unwind label %bb18
opt: /home/ariel/Rust/rust/src/llvm/lib/IR/Value.cpp:82: virtual llvm::Value::~Value(): Assertion `use_empty() && "Uses remain when a value is destroyed!"' failed.
0  opt             0x0000000001826fe5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  opt             0x0000000001824956 llvm::sys::RunSignalHandlers() + 54
2  opt             0x0000000001824b6a
3  libpthread.so.0 0x00007fb6b5eddd30
4  libc.so.6       0x00007fb6b50c4478 gsignal + 56
5  libc.so.6       0x00007fb6b50c58fa abort + 362
6  libc.so.6       0x00007fb6b50bd3a7
7  libc.so.6       0x00007fb6b50bd452
8  opt             0x00000000017c33fb
9  opt             0x00000000015caaa7
10 opt             0x0000000001659b2c llvm::BasicBlock::~BasicBlock() + 140
11 opt             0x0000000001659dbb llvm::BasicBlock::eraseFromParent() + 139
12 opt             0x00000000005c1594
13 opt             0x00000000013c1d76
14 opt             0x00000000013c2d31 llvm::SimplifyCFG(llvm::BasicBlock*, llvm::TargetTransformInfo const&, unsigned int, llvm::AssumptionCache*) + 65
15 opt             0x00000000013c0f02
16 opt             0x00000000013c2d31 llvm::SimplifyCFG(llvm::BasicBlock*, llvm::TargetTransformInfo const&, unsigned int, llvm::AssumptionCache*) + 65
17 opt             0x00000000005c4e2c
18 opt             0x00000000013c1dcd
19 opt             0x00000000013c2d31 llvm::SimplifyCFG(llvm::BasicBlock*, llvm::TargetTransformInfo const&, unsigned int, llvm::AssumptionCache*) + 65
20 opt             0x00000000011d6ff9
21 opt             0x0000000001796ca3 llvm::FPPassManager::runOnFunction(llvm::Function&) + 643
22 opt             0x0000000001796db9 llvm::legacy::FunctionPassManagerImpl::run(llvm::Function&) + 137
23 opt             0x0000000001796f04 llvm::legacy::FunctionPassManager::run(llvm::Function&) + 36
24 opt             0x00000000005d5d09 main + 7321
25 libc.so.6       0x00007fb6b50b1610 __libc_start_main + 240
26 opt             0x0000000000615129 _start + 41
Stack dump:
0.      Program arguments: opt example.ll -S -O3 
1.      Running pass 'Simplify the CFG' on function '@_ZN4core4iter8iterator8Iterator4fold17h0471ccca27326883E'
Aborted

I’m having a hard time reproducing the issue.

Both opt -O3 and opt -simplifycfg work fine here and produce

; ModuleID = '<stdin>'
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"

%"2.std::str::Chars" = type { %"2.std::slice::Iter<u8>" }
%"2.std::slice::Iter<u8>" = type { i8*, i8*, %"2.std::marker::PhantomData<&'static u8>" }
%"2.std::marker::PhantomData<&'static u8>" = type {}

declare i32 @__CxxFrameHandler3(...) unnamed_addr

; Function Attrs: inlinehint norecurse nounwind readnone uwtable
define i64 @_ZN4core4iter8iterator8Iterator4fold17h0471ccca27326883E(%"2.std::str::Chars"* noalias nocapture readnone dereferenceable(16), i64) unnamed_addr #0 personality i32 (...)* @__CxxFrameHandler3 {
entry-block:
  ret i64 %1
}

attributes #0 = { inlinehint norecurse nounwind readnone uwtable }

and

target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"

%"2.std::str::Chars" = type { %"2.std::slice::Iter<u8>" }
%"2.std::slice::Iter<u8>" = type { i8*, i8*, %"2.std::marker::PhantomData<&'static u8>" }
%"2.std::marker::PhantomData<&'static u8>" = type {}
%"2.std::option::Option<char>" = type { i32, [0 x i32], [1 x i32] }

; Function Attrs: argmemonly nounwind
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1) #0

declare i32 @__CxxFrameHandler3(...) unnamed_addr

; Function Attrs: inlinehint uwtable
define internal i64 @something() unnamed_addr #1 personality i32 (...)* @__CxxFrameHandler3 {
entry-block:
  ret i64 0
}

; Function Attrs: inlinehint uwtable
define i64 @_ZN4core4iter8iterator8Iterator4fold17h0471ccca27326883E(%"2.std::str::Chars"* noalias nocapture dereferenceable(16), i64) unnamed_addr #1 personality i32 (...)* @__CxxFrameHandler3 {
entry-block:
  %arg1 = alloca i64
  %self = alloca %"2.std::str::Chars"
  %init = alloca i64
  %accum = alloca i64
  %_result = alloca {}
  %iter = alloca %"2.std::str::Chars"
  %x = alloca i32
  %temp1 = alloca {}
  %temp3 = alloca %"2.std::str::Chars"
  %temp4 = alloca %"2.std::str::Chars"
  %temp5 = alloca %"2.std::option::Option<char>"
  %temp7 = alloca %"2.std::str::Chars"*
  %temp10 = alloca { i64, i32 }
  %temp15 = alloca i8
  %temp16 = alloca i8
  store i64 %1, i64* %arg1
  store i8 0, i8* %temp16
  store i8 0, i8* %temp15
  store i8 1, i8* %temp15
  %2 = bitcast %"2.std::str::Chars"* %0 to i8*
  %3 = bitcast %"2.std::str::Chars"* %self to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 16, i32 8, i1 false)
  %4 = load i64, i64* %arg1
  store i64 %4, i64* %init
  %5 = load i64, i64* %init
  store i8 1, i8* %temp16
  store i64 %5, i64* %accum
  store i8 0, i8* %temp15
  %6 = bitcast %"2.std::str::Chars"* %self to i8*
  %7 = bitcast %"2.std::str::Chars"* %temp4 to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %7, i8* %6, i64 16, i32 8, i1 false)
  %8 = bitcast %"2.std::str::Chars"* %temp3 to i8*
  %9 = bitcast %"2.std::str::Chars"* %iter to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %9, i8* %8, i64 16, i32 8, i1 false)
  br label %bb4

bb1:                                              ; preds = %bb16, %bb15
  cleanupret from %cleanuppad9 unwind to caller

bb2:                                              ; preds = %bb18, %bb17
  cleanupret from %cleanuppad7 unwind label %bb16

bb4:                                              ; preds = %bb10, %entry-block
  store %"2.std::str::Chars"* %iter, %"2.std::str::Chars"** %temp7
  %10 = load %"2.std::str::Chars"*, %"2.std::str::Chars"** %temp7, !nonnull !0
  %11 = invoke i64 @something()
          to label %bb7 unwind label %bb18

bb7:                                              ; preds = %bb4
  %12 = bitcast %"2.std::option::Option<char>"* %temp5 to i64*
  store i64 %11, i64* %12, align 4
  %13 = getelementptr inbounds %"2.std::option::Option<char>", %"2.std::option::Option<char>"* %temp5, i32 0, i32 0
  %14 = load i32, i32* %13, !range !1
  %switch = icmp ult i32 %14, 1
  br i1 %switch, label %bb12, label %bb9

bb9:                                              ; preds = %bb7
  %15 = bitcast %"2.std::option::Option<char>"* %temp5 to { i32, i32 }*
  %16 = getelementptr inbounds { i32, i32 }, { i32, i32 }* %15, i32 0, i32 1
  %17 = load i32, i32* %16, !range !2
  store i32 %17, i32* %x
  store i8 0, i8* %temp16
  %18 = load i64, i64* %accum
  %19 = load i32, i32* %x, !range !2
  %20 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 0
  store i64 %18, i64* %20
  %21 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 1
  store i32 %19, i32* %21
  %22 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 0
  %23 = load i64, i64* %22
  %24 = getelementptr inbounds { i64, i32 }, { i64, i32 }* %temp10, i32 0, i32 1
  %25 = load i32, i32* %24
  %26 = invoke i64 @something()
          to label %bb10 unwind label %bb18

bb10:                                             ; preds = %bb9
  store i8 1, i8* %temp16
  store i64 %26, i64* %accum
  br label %bb4

bb12:                                             ; preds = %bb7
  store i8 0, i8* %temp16
  %27 = load i64, i64* %accum
  ret i64 %27

bb15:                                             ; preds = %bb16
  store i8 0, i8* %temp15
  br label %bb1

bb16:                                             ; preds = %bb2
  %cleanuppad9 = cleanuppad within none []
  %28 = load i8, i8* %temp15, !range !3
  %29 = trunc i8 %28 to i1
  br i1 %29, label %bb15, label %bb1

bb17:                                             ; preds = %bb18
  store i8 0, i8* %temp16
  br label %bb2

bb18:                                             ; preds = %bb4, %bb9
  %cleanuppad7 = cleanuppad within none []
  %30 = load i8, i8* %temp16, !range !3
  %31 = trunc i8 %30 to i1
  br i1 %31, label %bb17, label %bb2
}

; Function Attrs: nounwind
declare void @llvm.assume(i1) #2

attributes #0 = { argmemonly nounwind }
attributes #1 = { inlinehint uwtable }
attributes #2 = { nounwind }

!0 = !{}
!1 = !{i32 0, i32 2}
!2 = !{i32 0, i32 1114112}
!3 = !{i8 0, i8 2}

respectively, where

LLVM (http://llvm.org/):
  LLVM version 3.8.0
  Optimized build.
  Built May  7 2016 (15:37:50).
  Default target: x86_64-unknown-linux-gnu
  Host CPU: haswell