a.rs

pub fn voidfn() -> () {}

build.sh

rustc --crate-type=rlib a/a.rs

rustc \
  --emit=asm,llvm-ir -Cllvm-args='-print-isel-input=true -print-machineinstrs -fast-isel' \
  --crate-type=lib --extern a=liba.rlib \
  -Crelocation-model=static \
  main.rs \
  2>details

example1.ll

*** Final LLVM Code input to ISel ***

; Function Attrs: nonlazybind uwtable
define void @_ZN4main8example117h149bc290ecddb5b8E() unnamed_addr #0 {
start:
  call void @_ZN4main5takep17h851b97fbf6407c1bE(void ()* nonnull @_ZN1a6voidfn17h7d790e74ed3ff940E)
  br label %bb1

bb1:                                              ; preds = %start
  ret void
}

# After Instruction Selection:
# Machine code for function _ZN4main8example117h149bc290ecddb5b8E: IsSSA, TracksLiveness

bb.0.start:
  successors: %bb.1

  %0:gr64 = MOV64rm $noreg, 1, $noreg, target-flags(x86-gotpcrel) @_ZN1a6voidfn17h7d790e74ed3ff940E, $noreg
  ADJCALLSTACKDOWN64 0, 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp
  $rdi = COPY %0:gr64
  CALL64pcrel32 @_ZN4main5takep17h851b97fbf6407c1bE, <regmask $bh $bl $bp $bph $bpl $bx $ebp $ebx $hbp $hbx $rbp $rbx $r12 $r13 $r14 $r15 $r12b   $r13b $r14b $r15b $r12bh $r13bh $r14bh $r15bh $r12d $r13d $r14d $r15d $r12w $r13w $r14w $r15w $r12wh and 3 more...>, implicit $rsp, implicit      $ssp, implicit $rdi
  ADJCALLSTACKUP64 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp

bb.1.bb1:
; predecessors: %bb.0

  RETQ

# End machine code for function _ZN4main8example117h149bc290ecddb5b8E.

example2.ll

; Function Attrs: nonlazybind uwtable
define void @_ZN4main8example217h4f443cbc5e17bbddE() unnamed_addr #0 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*,    %"unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {
start:
  %personalityslot = alloca { i8*, i32 }, align 8
  %_v = alloca %Droppable, align 1
  invoke void @_ZN4main5takep17h851b97fbf6407c1bE(void ()* nonnull @_ZN1a6voidfn17h7d790e74ed3ff940E)
          to label %bb2 unwind label %cleanup

bb1:                                              ; preds = %bb3
  %0 = bitcast { i8*, i32 }* %personalityslot to i8**
  %1 = load i8*, i8** %0, align 8
  %2 = getelementptr inbounds { i8*, i32 }, { i8*, i32 }* %personalityslot, i32 0, i32 1
  call void @_Unwind_Resume(i8* %1)
  unreachable

bb2:                                              ; preds = %start
  call void @_ZN4core3ptr18real_drop_in_place17h6782efda6a962288E(%Droppable* nonnull align 1 %_v)
  br label %bb4

bb3:                                              ; preds = %cleanup
  call void @_ZN4core3ptr18real_drop_in_place17h6782efda6a962288E(%Droppable* nonnull align 1 %_v) #3
  br label %bb1

bb4:                                              ; preds = %bb2
  ret void
cleanup:                                          ; preds = %start
  %3 = landingpad { i8*, i32 }
          cleanup
  %4 = extractvalue { i8*, i32 } %3, 0
  %5 = extractvalue { i8*, i32 } %3, 1
  %6 = getelementptr inbounds { i8*, i32 }, { i8*, i32 }* %personalityslot, i32 0, i32 0
  store i8* %4, i8** %6, align 8
  %7 = getelementptr inbounds { i8*, i32 }, { i8*, i32 }* %personalityslot, i32 0, i32 1
  store i32 %5, i32* %7, align 8
  br label %bb3
}

# After Instruction Selection:
# Machine code for function _ZN4main8example217h4f443cbc5e17bbddE: IsSSA, TracksLiveness
Frame Objects:
  fi#0: size=16, align=8, at location [SP+8]
  fi#1: size=1, align=8, at location [SP+8]

bb.0.start:
  successors: %bb.2, %bb.5

  EH_LABEL <mcsymbol .Ltmp0>
  ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
  %0:gr64 = MOV64rm $rip, 1, $noreg, target-flags(x86-gotpcrel) @_ZN1a6voidfn17h7d790e74ed3ff940E, $noreg :: (load 8 from got)
  $rdi = COPY %0:gr64
  CALL64pcrel32 @_ZN4main5takep17h851b97fbf6407c1bE, <regmask $bh $bl $bp $bph $bpl $bx $ebp $ebx $hbp $hbx $rbp $rbx $r12 $r13 $r14 $r15 $r12b   $r13b $r14b $r15b $r12bh $r13bh $r14bh $r15bh $r12d $r13d $r14d $r15d $r12w $r13w $r14w $r15w $r12wh and 3 more...>, implicit $rsp, implicit      $ssp, implicit $rdi, implicit-def $rsp, implicit-def $ssp
  ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
  EH_LABEL <mcsymbol .Ltmp1>
  JMP_1 %bb.2

bb.1.bb1:
; predecessors: %bb.3

  %8:gr64 = MOV64rm %stack.0.personalityslot, 1, $noreg, 0, $noreg :: (dereferenceable load 8 from %ir.0)
  ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
  $rdi = COPY %8:gr64
  CALL64pcrel32 @_Unwind_Resume, <regmask $bh $bl $bp $bph $bpl $bx $ebp $ebx $hbp $hbx $rbp $rbx $r12 $r13 $r14 $r15 $r12b $r13b $r14b $r15b     $r12bh $r13bh $r14bh $r15bh $r12d $r13d $r14d $r15d $r12w $r13w $r14w $r15w $r12wh and 3 more...>, implicit $rsp, implicit $ssp, implicit $rdi,   implicit-def $rsp, implicit-def $ssp
  ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
  TRAP

bb.2.bb2:
; predecessors: %bb.0
  successors: %bb.4

  ADJCALLSTACKDOWN64 0, 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp
  %9:gr64 = LEA64r %stack.1._v, 1, $noreg, 0, $noreg
  $rdi = COPY %9:gr64
  CALL64pcrel32 @_ZN4core3ptr18real_drop_in_place17h6782efda6a962288E, <regmask $bh $bl $bp $bph $bpl $bx $ebp $ebx $hbp $hbx $rbp $rbx $r12 $r13 $r14 $r15 $r12b $r13b $r14b $r15b $r12bh $r13bh $r14bh $r15bh $r12d $r13d $r14d $r15d $r12w $r13w $r14w $r15w $r12wh and 3 more...>, implicit     $rsp, implicit $ssp, implicit $rdi
  ADJCALLSTACKUP64 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp
  JMP_1 %bb.4

bb.3.bb3:
; predecessors: %bb.5
  successors: %bb.1

  ADJCALLSTACKDOWN64 0, 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp
  %7:gr64 = LEA64r %stack.1._v, 1, $noreg, 0, $noreg
  $rdi = COPY %7:gr64
  CALL64pcrel32 @_ZN4core3ptr18real_drop_in_place17h6782efda6a962288E, <regmask $bh $bl $bp $bph $bpl $bx $ebp $ebx $hbp $hbx $rbp $rbx $r12 $r13 $r14 $r15 $r12b $r13b $r14b $r15b $r12bh $r13bh $r14bh $r15bh $r12d $r13d $r14d $r15d $r12w $r13w $r14w $r15w $r12wh and 3 more...>, implicit     $rsp, implicit $ssp, implicit $rdi
  ADJCALLSTACKUP64 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp
  JMP_1 %bb.1

bb.4.bb4:
; predecessors: %bb.2

  RETQ
bb.5.cleanup (landing-pad):
; predecessors: %bb.0
  successors: %bb.3
  liveins: $rax, $rdx
  EH_LABEL <mcsymbol .Ltmp2>
  %2:gr64 = COPY killed $rdx
  %1:gr64 = COPY killed $rax
  %6:gr32 = COPY %2.sub_32bit:gr64
  %5:gr64 = COPY %1:gr64
  MOV64mr %stack.0.personalityslot, 1, $noreg, 0, $noreg, killed %5:gr64 :: (store 8 into %ir.6)
  MOV32mr %stack.0.personalityslot, 1, $noreg, 8, $noreg, killed %6:gr32 :: (store 4 into %ir.7, align 8)
  JMP_1 %bb.3

# End machine code for function _ZN4main8example217h4f443cbc5e17bbddE.

main.rs

extern crate a;

pub fn takep(arg: fn() -> ()) -> () {}

pub fn example1() {
  takep(a::voidfn);
}

struct Droppable {}
impl Drop for Droppable {
    fn drop(&mut self) {}
}

pub fn example2() {
  let _v = Droppable{};
  takep(a::voidfn);
}