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ZalatharZalathar
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Rollup merge of #144566 - scottmcm:align-of-slice, r=oli-obk
Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` I spotted this while working on the inliner (#144561). In particular, if [`Layout::for_value`](https://doc.rust-lang.org/std/alloc/struct.Layout.html#method.for_value) inlines, then it can be pretty easy to end up with an `align_of_val::<[T]>` today (demo: <https://rust.godbolt.org/z/Tesnscj4a>) where we can save at least a block, if not more, by using the version that's an rvalue and not a call.
2 parents f837028 + 8ef9233 commit 826c462

8 files changed

+516
-0
lines changed

‎compiler/rustc_mir_transform/src/instsimplify.rs

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@@ -55,6 +55,7 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
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let terminator = block.terminator.as_mut().unwrap();
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ctx.simplify_primitive_clone(terminator, &mut block.statements);
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ctx.simplify_align_of_slice_val(terminator, &mut block.statements);
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ctx.simplify_intrinsic_assert(terminator);
5960
ctx.simplify_nounwind_call(terminator);
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simplify_duplicate_switch_targets(terminator);
@@ -252,6 +253,36 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
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terminator.kind = TerminatorKind::Goto { target: *destination_block };
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}
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256+
// Convert `align_of_val::<[T]>(ptr)` to `align_of::<T>()`, since the
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// alignment of a slice doesn't actually depend on metadata at all
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// and the element type is always `Sized`.
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//
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// This is here so it can run after inlining, where it's more useful.
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// (LowerIntrinsics is done in cleanup, before the optimization passes.)
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fn simplify_align_of_slice_val(
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&self,
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terminator: &mut Terminator<'tcx>,
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statements: &mut Vec<Statement<'tcx>>,
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) {
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if let TerminatorKind::Call {
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func, args, destination, target: Some(destination_block), ..
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} = &terminator.kind
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&& args.len() == 1
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&& let Some((fn_def_id, generics)) = func.const_fn_def()
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&& self.tcx.is_intrinsic(fn_def_id, sym::align_of_val)
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&& let ty::Slice(elem_ty) = *generics.type_at(0).kind()
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{
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statements.push(Statement::new(
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terminator.source_info,
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StatementKind::Assign(Box::new((
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*destination,
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Rvalue::NullaryOp(NullOp::AlignOf, elem_ty),
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))),
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));
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terminator.kind = TerminatorKind::Goto { target: *destination_block };
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}
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}
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fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
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let TerminatorKind::Call { ref func, ref mut unwind, .. } = terminator.kind else {
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return;

‎tests/mir-opt/instsimplify/align_of_slice.of_val_slice.InstSimplify-after-simplifycfg.diff

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@@ -0,0 +1,22 @@
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- // MIR for `of_val_slice` before InstSimplify-after-simplifycfg
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+ // MIR for `of_val_slice` after InstSimplify-after-simplifycfg
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fn of_val_slice(_1: &[T]) -> usize {
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debug slice => _1;
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let mut _0: usize;
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let mut _2: *const [T];
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bb0: {
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StorageLive(_2);
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_2 = &raw const (*_1);
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- _0 = std::intrinsics::align_of_val::<[T]>(move _2) -> [return: bb1, unwind unreachable];
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+ _0 = AlignOf(T);
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+ goto -> bb1;
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}
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bb1: {
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StorageDead(_2);
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return;
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}
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}
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‎tests/mir-opt/instsimplify/align_of_slice.rs

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@@ -0,0 +1,12 @@
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//@ test-mir-pass: InstSimplify-after-simplifycfg
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//@ needs-unwind
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#![crate_type = "lib"]
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#![feature(core_intrinsics)]
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// EMIT_MIR align_of_slice.of_val_slice.InstSimplify-after-simplifycfg.diff
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pub fn of_val_slice<T>(slice: &[T]) -> usize {
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// CHECK-LABEL: fn of_val_slice(_1: &[T])
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// CHECK: _0 = AlignOf(T);
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unsafe { core::intrinsics::align_of_val(slice) }
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}

‎tests/mir-opt/pre-codegen/drop_boxed_slice.generic_in_place.PreCodegen.after.32bit.panic-abort.mir

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@@ -0,0 +1,108 @@
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// MIR for `generic_in_place` after PreCodegen
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fn generic_in_place(_1: *mut Box<[T]>) -> () {
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debug ptr => _1;
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let mut _0: ();
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scope 1 (inlined drop_in_place::<Box<[T]>> - shim(Some(Box<[T]>))) {
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scope 2 (inlined <Box<[T]> as Drop>::drop) {
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let _2: std::ptr::NonNull<[T]>;
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let mut _3: *mut [T];
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let mut _4: *const [T];
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let _12: ();
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scope 3 {
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let _8: std::ptr::alignment::AlignmentEnum;
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scope 4 {
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scope 12 (inlined Layout::size) {
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}
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scope 13 (inlined Unique::<[T]>::cast::<u8>) {
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scope 14 (inlined NonNull::<[T]>::cast::<u8>) {
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scope 15 (inlined NonNull::<[T]>::as_ptr) {
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}
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}
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}
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scope 16 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
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scope 17 (inlined Unique::<u8>::as_non_null_ptr) {
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}
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}
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scope 18 (inlined <std::alloc::Global as Allocator>::deallocate) {
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let mut _9: *mut u8;
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scope 19 (inlined Layout::size) {
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}
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scope 20 (inlined NonNull::<u8>::as_ptr) {
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}
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scope 21 (inlined std::alloc::dealloc) {
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let mut _11: usize;
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scope 22 (inlined Layout::size) {
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}
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scope 23 (inlined Layout::align) {
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scope 24 (inlined std::ptr::Alignment::as_usize) {
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let mut _10: u32;
40+
}
41+
}
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}
43+
}
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}
45+
scope 5 (inlined Unique::<[T]>::as_ptr) {
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scope 6 (inlined NonNull::<[T]>::as_ptr) {
47+
}
48+
}
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scope 7 (inlined Layout::for_value_raw::<[T]>) {
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let mut _5: usize;
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let mut _6: usize;
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scope 8 {
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scope 11 (inlined #[track_caller] Layout::from_size_align_unchecked) {
54+
let mut _7: std::ptr::Alignment;
55+
}
56+
}
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scope 9 (inlined size_of_val_raw::<[T]>) {
58+
}
59+
scope 10 (inlined align_of_val_raw::<[T]>) {
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}
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}
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}
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}
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}
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bb0: {
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StorageLive(_2);
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_2 = copy (((*_1).0: std::ptr::Unique<[T]>).0: std::ptr::NonNull<[T]>);
69+
StorageLive(_4);
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_3 = copy _2 as *mut [T] (Transmute);
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_4 = copy _2 as *const [T] (Transmute);
72+
StorageLive(_6);
73+
_5 = std::intrinsics::size_of_val::<[T]>(move _4) -> [return: bb1, unwind unreachable];
74+
}
75+
76+
bb1: {
77+
_6 = AlignOf(T);
78+
StorageLive(_7);
79+
_7 = copy _6 as std::ptr::Alignment (Transmute);
80+
_8 = move (_7.0: std::ptr::alignment::AlignmentEnum);
81+
StorageDead(_7);
82+
StorageDead(_6);
83+
StorageDead(_4);
84+
switchInt(copy _5) -> [0: bb4, otherwise: bb2];
85+
}
86+
87+
bb2: {
88+
StorageLive(_9);
89+
_9 = copy _3 as *mut u8 (PtrToPtr);
90+
StorageLive(_11);
91+
StorageLive(_10);
92+
_10 = discriminant(_8);
93+
_11 = move _10 as usize (IntToInt);
94+
StorageDead(_10);
95+
_12 = alloc::alloc::__rust_dealloc(move _9, move _5, move _11) -> [return: bb3, unwind unreachable];
96+
}
97+
98+
bb3: {
99+
StorageDead(_11);
100+
StorageDead(_9);
101+
goto -> bb4;
102+
}
103+
104+
bb4: {
105+
StorageDead(_2);
106+
return;
107+
}
108+
}

‎tests/mir-opt/pre-codegen/drop_boxed_slice.generic_in_place.PreCodegen.after.32bit.panic-unwind.mir

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,108 @@
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// MIR for `generic_in_place` after PreCodegen
2+
3+
fn generic_in_place(_1: *mut Box<[T]>) -> () {
4+
debug ptr => _1;
5+
let mut _0: ();
6+
scope 1 (inlined drop_in_place::<Box<[T]>> - shim(Some(Box<[T]>))) {
7+
scope 2 (inlined <Box<[T]> as Drop>::drop) {
8+
let _2: std::ptr::NonNull<[T]>;
9+
let mut _3: *mut [T];
10+
let mut _4: *const [T];
11+
let _12: ();
12+
scope 3 {
13+
let _8: std::ptr::alignment::AlignmentEnum;
14+
scope 4 {
15+
scope 12 (inlined Layout::size) {
16+
}
17+
scope 13 (inlined Unique::<[T]>::cast::<u8>) {
18+
scope 14 (inlined NonNull::<[T]>::cast::<u8>) {
19+
scope 15 (inlined NonNull::<[T]>::as_ptr) {
20+
}
21+
}
22+
}
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scope 16 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
24+
scope 17 (inlined Unique::<u8>::as_non_null_ptr) {
25+
}
26+
}
27+
scope 18 (inlined <std::alloc::Global as Allocator>::deallocate) {
28+
let mut _9: *mut u8;
29+
scope 19 (inlined Layout::size) {
30+
}
31+
scope 20 (inlined NonNull::<u8>::as_ptr) {
32+
}
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scope 21 (inlined std::alloc::dealloc) {
34+
let mut _11: usize;
35+
scope 22 (inlined Layout::size) {
36+
}
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scope 23 (inlined Layout::align) {
38+
scope 24 (inlined std::ptr::Alignment::as_usize) {
39+
let mut _10: u32;
40+
}
41+
}
42+
}
43+
}
44+
}
45+
scope 5 (inlined Unique::<[T]>::as_ptr) {
46+
scope 6 (inlined NonNull::<[T]>::as_ptr) {
47+
}
48+
}
49+
scope 7 (inlined Layout::for_value_raw::<[T]>) {
50+
let mut _5: usize;
51+
let mut _6: usize;
52+
scope 8 {
53+
scope 11 (inlined #[track_caller] Layout::from_size_align_unchecked) {
54+
let mut _7: std::ptr::Alignment;
55+
}
56+
}
57+
scope 9 (inlined size_of_val_raw::<[T]>) {
58+
}
59+
scope 10 (inlined align_of_val_raw::<[T]>) {
60+
}
61+
}
62+
}
63+
}
64+
}
65+
66+
bb0: {
67+
StorageLive(_2);
68+
_2 = copy (((*_1).0: std::ptr::Unique<[T]>).0: std::ptr::NonNull<[T]>);
69+
StorageLive(_4);
70+
_3 = copy _2 as *mut [T] (Transmute);
71+
_4 = copy _2 as *const [T] (Transmute);
72+
StorageLive(_6);
73+
_5 = std::intrinsics::size_of_val::<[T]>(move _4) -> [return: bb1, unwind unreachable];
74+
}
75+
76+
bb1: {
77+
_6 = AlignOf(T);
78+
StorageLive(_7);
79+
_7 = copy _6 as std::ptr::Alignment (Transmute);
80+
_8 = move (_7.0: std::ptr::alignment::AlignmentEnum);
81+
StorageDead(_7);
82+
StorageDead(_6);
83+
StorageDead(_4);
84+
switchInt(copy _5) -> [0: bb4, otherwise: bb2];
85+
}
86+
87+
bb2: {
88+
StorageLive(_9);
89+
_9 = copy _3 as *mut u8 (PtrToPtr);
90+
StorageLive(_11);
91+
StorageLive(_10);
92+
_10 = discriminant(_8);
93+
_11 = move _10 as usize (IntToInt);
94+
StorageDead(_10);
95+
_12 = alloc::alloc::__rust_dealloc(move _9, move _5, move _11) -> [return: bb3, unwind unreachable];
96+
}
97+
98+
bb3: {
99+
StorageDead(_11);
100+
StorageDead(_9);
101+
goto -> bb4;
102+
}
103+
104+
bb4: {
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StorageDead(_2);
106+
return;
107+
}
108+
}

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