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Merged
rust-bors merged 2 commits into from
Feb 22, 2026
Merged

Just pass Layout directly to box_new_uninit #152737

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1b767bf
Just pass `Layout` directly to `box_new_uninit`
scottmcm Feb 17, 2026
4b03b97
What if we discourage mir-inlining of `box_new_uninit`?
scottmcm Feb 17, 2026
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24 changes: 7 additions & 17 deletions library/alloc/src/boxed.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -237,15 +237,14 @@ pub struct Box<
>(Unique<T>, A);

/// Monomorphic function for allocating an uninit `Box`.
///
/// # Safety
///
/// size and align need to be safe for `Layout::from_size_align_unchecked`.
#[inline]
// The is a separate function to avoid doing it in every generic version, but it
// looks small to the mir inliner (particularly in panic=abort) so leave it to
// the backend to decide whether pulling it in everywhere is worth doing.
#[rustc_no_mir_inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
#[cfg(not(no_global_oom_handling))]
unsafe fn box_new_uninit(size: usize, align: usize) -> *mut u8 {
let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
fn box_new_uninit(layout: Layout) -> *mut u8 {
match Global.allocate(layout) {
Ok(ptr) => ptr.as_mut_ptr(),
Err(_) => handle_alloc_error(layout),
Expand Down Expand Up @@ -284,10 +283,7 @@ impl<T> Box<T> {
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub fn new(x: T) -> Self {
// This is `Box::new_uninit` but inlined to avoid build time regressions.
// SAFETY: The size and align of a valid type `T` are always valid for `Layout`.
let ptr = unsafe {
box_new_uninit(<T as SizedTypeProperties>::SIZE, <T as SizedTypeProperties>::ALIGN)
} as *mut T;
let ptr = box_new_uninit(<T as SizedTypeProperties>::LAYOUT) as *mut T;
// Nothing below can panic so we do not have to worry about deallocating `ptr`.
// SAFETY: we just allocated the box to store `x`.
unsafe { core::intrinsics::write_via_move(ptr, x) };
Expand Down Expand Up @@ -317,14 +313,8 @@ impl<T> Box<T> {
// `Box::new`).

// SAFETY:
// - The size and align of a valid type `T` are always valid for `Layout`.
// - If `allocate` succeeds, the returned pointer exactly matches what `Box` needs.
unsafe {
mem::transmute(box_new_uninit(
<T as SizedTypeProperties>::SIZE,
<T as SizedTypeProperties>::ALIGN,
))
}
unsafe { mem::transmute(box_new_uninit(<T as SizedTypeProperties>::LAYOUT)) }
}

/// Constructs a new `Box` with uninitialized contents, with the memory
Expand Down
3 changes: 1 addition & 2 deletions tests/codegen-units/item-collection/opaque-return-impls.rs
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Expand Up @@ -45,8 +45,7 @@ pub fn foo2() -> Box<dyn TestTrait2> {
//~ MONO_ITEM fn foo2
//~ MONO_ITEM fn std::alloc::Global::alloc_impl_runtime
//~ MONO_ITEM fn std::boxed::Box::<TestStruct2>::new
//~ MONO_ITEM fn std::alloc::Layout::from_size_align_unchecked::precondition_check
//~ MONO_ITEM fn std::ptr::Alignment::new_unchecked::precondition_check
//~ MONO_ITEM fn std::boxed::box_new_uninit
//~ MONO_ITEM fn std::ptr::NonNull::<T>::new_unchecked::precondition_check

struct Counter {
Expand Down
122 changes: 14 additions & 108 deletions tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -12,8 +12,6 @@
let mut _10: std::boxed::Box<()>;
let mut _11: *const ();
let mut _14: usize;
let mut _15: usize;
let mut _24: usize;
scope 1 {
debug vp_ctx => _1;
let _5: *const ();
Expand All @@ -26,12 +24,12 @@
scope 4 {
debug _x => _8;
}
scope 20 (inlined foo) {
let mut _25: *const [()];
scope 9 (inlined foo) {
let mut _15: *const [()];
}
}
scope 18 (inlined slice_from_raw_parts::<()>) {
scope 19 (inlined std::ptr::from_raw_parts::<[()], ()>) {
scope 7 (inlined slice_from_raw_parts::<()>) {
scope 8 (inlined std::ptr::from_raw_parts::<[()], ()>) {
}
}
}
Expand All @@ -41,36 +39,6 @@
let mut _13: *mut u8;
scope 6 {
}
scope 7 (inlined boxed::box_new_uninit) {
let _16: std::alloc::Layout;
let mut _17: std::result::Result<std::ptr::NonNull<[u8]>, std::alloc::AllocError>;
let mut _18: isize;
let mut _20: !;
scope 8 {
let _19: std::ptr::NonNull<[u8]>;
scope 9 {
scope 13 (inlined NonNull::<[u8]>::as_mut_ptr) {
scope 14 (inlined NonNull::<[u8]>::as_non_null_ptr) {
scope 15 (inlined NonNull::<[u8]>::cast::<u8>) {
let mut _23: *mut [u8];
scope 16 (inlined NonNull::<[u8]>::as_ptr) {
}
}
}
scope 17 (inlined NonNull::<u8>::as_ptr) {
}
}
}
scope 11 (inlined <std::alloc::Global as Allocator>::allocate) {
scope 12 (inlined std::alloc::Global::alloc_impl) {
}
}
}
scope 10 (inlined #[track_caller] Layout::from_size_align_unchecked) {
let _21: ();
let mut _22: std::ptr::Alignment;
}
}
}

bb0: {
Expand All @@ -82,18 +50,7 @@
+ _4 = const ();
StorageLive(_12);
StorageLive(_13);
StorageLive(_14);
- _14 = const <() as std::mem::SizedTypeProperties>::SIZE;
+ _14 = const 0_usize;
StorageLive(_15);
- _15 = const <() as std::mem::SizedTypeProperties>::ALIGN;
+ _15 = const 1_usize;
StorageLive(_16);
StorageLive(_18);
StorageLive(_19);
StorageLive(_20);
StorageLive(_21);
switchInt(UbChecks) -> [0: bb6, otherwise: bb5];
_13 = boxed::box_new_uninit(const <() as std::mem::SizedTypeProperties>::LAYOUT) -> [return: bb2, unwind unreachable];
}

bb1: {
Expand All @@ -103,33 +60,8 @@
}

bb2: {
unreachable;
}

bb3: {
- _20 = handle_alloc_error(move _16) -> unwind unreachable;
+ _20 = handle_alloc_error(const Layout {{ size: 0_usize, align: std::ptr::Alignment {{ _inner_repr_trick: std::ptr::alignment::AlignmentEnum::_Align1Shl0 }} }}) -> unwind unreachable;
}

bb4: {
_19 = copy ((_17 as Ok).0: std::ptr::NonNull<[u8]>);
- StorageLive(_23);
+ nop;
_23 = copy _19 as *mut [u8] (Transmute);
_13 = copy _23 as *mut u8 (PtrToPtr);
- StorageDead(_23);
+ nop;
StorageDead(_17);
StorageDead(_21);
StorageDead(_20);
StorageDead(_19);
StorageDead(_18);
StorageDead(_16);
StorageDead(_15);
StorageDead(_14);
- _12 = copy _13 as *mut () (PtrToPtr);
_12 = copy _13 as *mut () (PtrToPtr);
- (*_12) = move _4;
+ _12 = copy _23 as *mut () (PtrToPtr);
+ (*_12) = const ();
_3 = copy _13 as std::boxed::Box<()> (Transmute);
StorageDead(_13);
Expand All @@ -147,21 +79,21 @@
+ nop;
StorageLive(_7);
_7 = copy _5;
StorageLive(_24);
_24 = const 1_usize;
- _6 = *const [()] from (copy _7, copy _24);
StorageLive(_14);
_14 = const 1_usize;
- _6 = *const [()] from (copy _7, copy _14);
+ _6 = *const [()] from (copy _5, const 1_usize);
StorageDead(_24);
StorageDead(_14);
StorageDead(_7);
StorageLive(_8);
StorageLive(_9);
_9 = copy _6;
StorageLive(_25);
- _25 = copy _9;
StorageLive(_15);
- _15 = copy _9;
- _8 = copy _9 as *mut () (PtrToPtr);
+ _25 = copy _6;
+ _15 = copy _6;
+ _8 = copy _5 as *mut () (PtrToPtr);
StorageDead(_25);
StorageDead(_15);
StorageDead(_9);
_0 = const ();
StorageDead(_8);
Expand All @@ -171,31 +103,5 @@
+ nop;
drop(_3) -> [return: bb1, unwind unreachable];
}

bb5: {
- _21 = Layout::from_size_align_unchecked::precondition_check(copy _14, copy _15) -> [return: bb6, unwind unreachable];
+ _21 = Layout::from_size_align_unchecked::precondition_check(const 0_usize, const 1_usize) -> [return: bb6, unwind unreachable];
}

bb6: {
StorageLive(_22);
- _22 = copy _15 as std::ptr::Alignment (Transmute);
- _16 = Layout { size: copy _14, align: move _22 };
+ _22 = const std::ptr::Alignment {{ _inner_repr_trick: std::ptr::alignment::AlignmentEnum::_Align1Shl0 }};
+ _16 = const Layout {{ size: 0_usize, align: std::ptr::Alignment {{ _inner_repr_trick: std::ptr::alignment::AlignmentEnum::_Align1Shl0 }} }};
StorageDead(_22);
StorageLive(_17);
- _17 = std::alloc::Global::alloc_impl_runtime(copy _16, const false) -> [return: bb7, unwind unreachable];
+ _17 = std::alloc::Global::alloc_impl_runtime(const Layout {{ size: 0_usize, align: std::ptr::Alignment {{ _inner_repr_trick: std::ptr::alignment::AlignmentEnum::_Align1Shl0 }} }}, const false) -> [return: bb7, unwind unreachable];
}

bb7: {
_18 = discriminant(_17);
switchInt(move _18) -> [0: bb4, 1: bb3, otherwise: bb2];
}
+ }
+
+ ALLOC0 (size: 8, align: 4) {
+ 01 00 00 00 00 00 00 00 │ ........
}

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