Change:
<ins> - // MIR for `main` before ConstProp + // MIR for `main` after ConstProp fn main() -> () { let mut _0: (); let _1: A; let mut _2: std::boxed::Box<[bool]>; scope 1 { debug a => _1; } scope 2 (inlined <Box<[bool]> as Default>::default) { let _3: std::ptr::Unique<[bool]>; let mut _4: std::ptr::Unique<[bool; 0]>; scope 3 { debug ptr => _3; } scope 4 (inlined Unique::<[bool; 0]>::dangling) { let mut _5: std::ptr::NonNull<[bool; 0]>; scope 5 (inlined NonNull::<[bool; 0]>::dangling) { let mut _7: usize; scope 6 { let _6: *mut [bool; 0]; scope 7 { debug ptr => _6; scope 11 (inlined NonNull::<[bool; 0]>::new_unchecked) { debug ptr => _6; let mut _8: *const [bool; 0]; scope 12 { scope 13 (inlined NonNull::<T>::new_unchecked::runtime::<[bool; 0]>) { debug ptr => _6; scope 14 (inlined ptr::mut_ptr::<impl *mut [bool; 0]>::is_null) { debug self => _6; let mut _9: *mut u8; scope 15 { scope 16 (inlined ptr::mut_ptr::<impl *mut T>::is_null::runtime_impl) { debug ptr => _9; scope 17 (inlined ptr::mut_ptr::<impl *mut u8>::addr) { debug self => _9; scope 18 { scope 19 (inlined ptr::mut_ptr::<impl *mut u8>::cast::<()>) { debug self => _9; } } } } } } } } } } scope 8 (inlined align_of::<[bool; 0]>) { } scope 9 (inlined invalid_mut::<[bool; 0]>) { debug addr => _7; scope 10 { } } } } } } bb0: { StorageLive(_1); StorageLive(_2); StorageLive(_3); StorageLive(_4); StorageLive(_5); StorageLive(_6); StorageLive(_7); -         _7 = AlignOf([bool; 0]); -         _6 = _7 as *mut [bool; 0] (Transmute); +         _7 = const 1_usize; +         _6 = const {0x1 as *mut [bool; 0]}; StorageDead(_7); StorageLive(_8); StorageLive(_9); -         _8 = _6 as *const [bool; 0] (PointerCoercion(MutToConstPointer)); -         _5 = NonNull::<[bool; 0]> { pointer: _8 }; +         _8 = const {0x1 as *const [bool; 0]}; +         _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}; StorageDead(_9); StorageDead(_8); StorageDead(_6); -         _4 = Unique::<[bool; 0]> { pointer: move _5, _marker: const PhantomData::<[bool; 0]> }; +         _4 = const Unique::<[bool; 0]> {{ pointer: NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }}, _marker: PhantomData::<[bool; 0]> }}; StorageDead(_5); -         _3 = move _4 as std::ptr::Unique<[bool]> (PointerCoercion(Unsize)); +         _3 = const Unique::<[bool]> {{ pointer: NonNull::<[bool]> {{ pointer: Indirect { alloc_id: alloc7, offset: Size(0 bytes) }: *const [bool] }}, _marker: PhantomData::<[bool]> }}; StorageDead(_4); -         _2 = Box::<[bool]>(_3, const std::alloc::Global); +         _2 = const Box::<[bool]>(Unique::<[bool]> {{ pointer: NonNull::<[bool]> {{ pointer: Indirect { alloc_id: alloc10, offset: Size(0 bytes) }: *const [bool] }}, _marker: PhantomData::<[bool]> }}, std::alloc::Global); StorageDead(_3); -         _1 = A { foo: move _2 }; +         _1 = A { foo: const Box::<[bool]>(Unique::<[bool]> {{ pointer: NonNull::<[bool]> {{ pointer: Indirect { alloc_id: alloc11, offset: Size(0 bytes) }: *const [bool] }}, _marker: PhantomData::<[bool]> }}, std::alloc::Global) }; StorageDead(_2); _0 = const (); drop(_1) -> [return: bb1, unwind unreachable]; } bb1: { StorageDead(_1); return; } + } + + alloc11 (size: 8, align: 4) { +     01 00 00 00 00 00 00 00                         │ ........ + } + + alloc10 (size: 8, align: 4) { +     01 00 00 00 00 00 00 00                         │ ........ + } + + alloc7 (size: 8, align: 4) { +     01 00 00 00 00 00 00 00                         │ ........ } </ins>

Discussion:
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