use crate::cmp;use crate::fmt;use crate::mem;use crate::num::NonZeroUsize;use crate::ptr::NonNull; const fn size_align<T>() -> (usize, usize) {    (mem::size_of::<T>(), mem::align_of::<T>())} /// Layout of a block of memory.////// An instance of `Layout` describes a particular layout of memory./// You build a `Layout` up as an input to give to an allocator.////// All layouts have an associated size and a power-of-two alignment.////// (Note that layouts are *not* required to have non-zero size,/// even though `GlobalAlloc` requires that all memory requests/// be non-zero in size. A caller must either ensure that conditions/// like this are met, use specific allocators with looser/// requirements, or use the more lenient `AllocRef` interface.)#[stable(feature = "alloc_layout", since = "1.28.0")]#[derive(Copy, Clone, Debug, PartialEq, Eq)]#[lang = "alloc_layout"]pub struct Layout {    // size of the requested block of memory, measured in bytes.    size_: usize,     // alignment of the requested block of memory, measured in bytes.    // we ensure that this is always a power-of-two, because API's    // like `posix_memalign` require it and it is a reasonable    // constraint to impose on Layout constructors.    //    // (However, we do not analogously require `align >= sizeof(void*)`,    //  even though that is *also* a requirement of `posix_memalign`.)    align_: NonZeroUsize,} impl Layout {    /// Constructs a `Layout` from a given `size` and `align`,    /// or returns `LayoutErr` if any of the following conditions    /// are not met:    ///    /// * `align` must not be zero,    ///    /// * `align` must be a power of two,    ///    /// * `size`, when rounded up to the nearest multiple of `align`,    ///    must not overflow (i.e., the rounded value must be less than    ///    or equal to `usize::MAX`).    #[stable(feature = "alloc_layout", since = "1.28.0")]    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]    #[inline]    pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {        if !align.is_power_of_two() {            return Err(LayoutErr { private: () });        }         // (power-of-two implies align != 0.)         // Rounded up size is:        //   size_rounded_up = (size + align - 1) & !(align - 1);        //        // We know from above that align != 0. If adding (align - 1)        // does not overflow, then rounding up will be fine.        //        // Conversely, &-masking with !(align - 1) will subtract off        // only low-order-bits. Thus if overflow occurs with the sum,        // the &-mask cannot subtract enough to undo that overflow.        //        // Above implies that checking for summation overflow is both        // necessary and sufficient.        if size > usize::MAX - (align - 1) {            return Err(LayoutErr { private: () });        }         // SAFETY: the conditions for `from_size_align_unchecked` have been        // checked above.        unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }    }     /// Creates a layout, bypassing all checks.    ///    /// # Safety    ///    /// This function is unsafe as it does not verify the preconditions from    /// [`Layout::from_size_align`].    #[stable(feature = "alloc_layout", since = "1.28.0")]    #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]    #[inline]    pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {        // SAFETY: the caller must ensure that `align` is greater than zero.        Layout { size_: size, align_: unsafe { NonZeroUsize::new_unchecked(align) } }    }     /// The minimum size in bytes for a memory block of this layout.    #[stable(feature = "alloc_layout", since = "1.28.0")]    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]    #[inline]    pub const fn size(&self) -> usize {        self.size_    }     /// The minimum byte alignment for a memory block of this layout.    #[stable(feature = "alloc_layout", since = "1.28.0")]    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]    #[inline]    pub const fn align(&self) -> usize {        self.align_.get()    }     /// Constructs a `Layout` suitable for holding a value of type `T`.    #[stable(feature = "alloc_layout", since = "1.28.0")]    #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]    #[inline]    pub const fn new<T>() -> Self {        let (size, align) = size_align::<T>();        // SAFETY: the align is guaranteed by Rust to be a power of two and        // the size+align combo is guaranteed to fit in our address space. As a        // result use the unchecked constructor here to avoid inserting code        // that panics if it isn't optimized well enough.        unsafe { Layout::from_size_align_unchecked(size, align) }    }     /// Produces layout describing a record that could be used to    /// allocate backing structure for `T` (which could be a trait    /// or other unsized type like a slice).    #[stable(feature = "alloc_layout", since = "1.28.0")]    #[inline]    pub fn for_value<T: ?Sized>(t: &T) -> Self {        let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));        debug_assert!(Layout::from_size_align(size, align).is_ok());        // SAFETY: see rationale in `new` for why this is using the unsafe variant        unsafe { Layout::from_size_align_unchecked(size, align) }    }     /// Produces layout describing a record that could be used to    /// allocate backing structure for `T` (which could be a trait    /// or other unsized type like a slice).    ///    /// # Safety    ///    /// This function is only safe to call if the following conditions hold:    ///    /// - If `T` is `Sized`, this function is always safe to call.    /// - If the unsized tail of `T` is:    ///     - a [slice], then the length of the slice tail must be an intialized    ///       integer, and the size of the *entire value*    ///       (dynamic tail length + statically sized prefix) must fit in `isize`.    ///     - a [trait object], then the vtable part of the pointer must point    ///       to a valid vtable for the type `T` acquired by an unsizing coersion,    ///       and the size of the *entire value*    ///       (dynamic tail length + statically sized prefix) must fit in `isize`.    ///     - an (unstable) [extern type], then this function is always safe to    ///       call, but may panic or otherwise return the wrong value, as the    ///       extern type's layout is not known. This is the same behavior as    ///       [`Layout::for_value`] on a reference to an extern type tail.    ///     - otherwise, it is conservatively not allowed to call this function.    ///    /// [slice]: ../../std/primitive.slice.html    /// [trait object]: ../../book/ch17-02-trait-objects.html    /// [extern type]: ../../unstable-book/language-features/extern-types.html    #[unstable(feature = "layout_for_ptr", issue = "69835")]    pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {        // SAFETY: we pass along the prerequisites of these functions to the caller        let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };        debug_assert!(Layout::from_size_align(size, align).is_ok());        // SAFETY: see rationale in `new` for why this is using the unsafe variant        unsafe { Layout::from_size_align_unchecked(size, align) }    }     /// Creates a `NonNull` that is dangling, but well-aligned for this Layout.    ///    /// Note that the pointer value may potentially represent a valid pointer,    /// which means this must not be used as a "not yet initialized"    /// sentinel value. Types that lazily allocate must track initialization by    /// some other means.    #[unstable(feature = "alloc_layout_extra", issue = "55724")]    #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]    #[inline]    pub const fn dangling(&self) -> NonNull<u8> {        // SAFETY: align is guaranteed to be non-zero        unsafe { NonNull::new_unchecked(self.align() as *mut u8) }    }     /// Creates a layout describing the record that can hold a value    /// of the same layout as `self`, but that also is aligned to    /// alignment `align` (measured in bytes).    ///    /// If `self` already meets the prescribed alignment, then returns    /// `self`.    ///    /// Note that this method does not add any padding to the overall    /// size, regardless of whether the returned layout has a different    /// alignment. In other words, if `K` has size 16, `K.align_to(32)`    /// will *still* have size 16.    ///    /// Returns an error if the combination of `self.size()` and the given    /// `align` violates the conditions listed in [`Layout::from_size_align`].    #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]    #[inline]    pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {        Layout::from_size_align(self.size(), cmp::max(self.align(), align))    }     /// Returns the amount of padding we must insert after `self`    /// to ensure that the following address will satisfy `align`    /// (measured in bytes).    ///    /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`    /// returns 3, because that is the minimum number of bytes of    /// padding required to get a 4-aligned address (assuming that the    /// corresponding memory block starts at a 4-aligned address).    ///    /// The return value of this function has no meaning if `align` is    /// not a power-of-two.    ///    /// Note that the utility of the returned value requires `align`    /// to be less than or equal to the alignment of the starting    /// address for the whole allocated block of memory. One way to    /// satisfy this constraint is to ensure `align <= self.align()`.    #[unstable(feature = "alloc_layout_extra", issue = "55724")]    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]    #[inline]    pub const fn padding_needed_for(&self, align: usize) -> usize {        let len = self.size();         // Rounded up value is:        //   len_rounded_up = (len + align - 1) & !(align - 1);        // and then we return the padding difference: `len_rounded_up - len`.        //        // We use modular arithmetic throughout:        //        // 1. align is guaranteed to be > 0, so align - 1 is always        //    valid.        //        // 2. `len + align - 1` can overflow by at most `align - 1`,        //    so the &-mask with `!(align - 1)` will ensure that in the        //    case of overflow, `len_rounded_up` will itself be 0.        //    Thus the returned padding, when added to `len`, yields 0,        //    which trivially satisfies the alignment `align`.        //        // (Of course, attempts to allocate blocks of memory whose        // size and padding overflow in the above manner should cause        // the allocator to yield an error anyway.)         let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);        len_rounded_up.wrapping_sub(len)    }     /// Creates a layout by rounding the size of this layout up to a multiple    /// of the layout's alignment.    ///    /// This is equivalent to adding the result of `padding_needed_for`    /// to the layout's current size.    #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]    #[inline]    pub fn pad_to_align(&self) -> Layout {        let pad = self.padding_needed_for(self.align());        // This cannot overflow. Quoting from the invariant of Layout:        // > `size`, when rounded up to the nearest multiple of `align`,        // > must not overflow (i.e., the rounded value must be less than        // > `usize::MAX`)        let new_size = self.size() + pad;         Layout::from_size_align(new_size, self.align()).unwrap()    }     /// Creates a layout describing the record for `n` instances of    /// `self`, with a suitable amount of padding between each to    /// ensure that each instance is given its requested size and    /// alignment. On success, returns `(k, offs)` where `k` is the    /// layout of the array and `offs` is the distance between the start    /// of each element in the array.    ///    /// On arithmetic overflow, returns `LayoutErr`.    #[unstable(feature = "alloc_layout_extra", issue = "55724")]    #[inline]    pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {        // This cannot overflow. Quoting from the invariant of Layout:        // > `size`, when rounded up to the nearest multiple of `align`,        // > must not overflow (i.e., the rounded value must be less than        // > `usize::MAX`)        let padded_size = self.size() + self.padding_needed_for(self.align());        let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;         // SAFETY: self.align is already known to be valid and alloc_size has been        // padded already.        unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) }    }     /// Creates a layout describing the record for `self` followed by    /// `next`, including any necessary padding to ensure that `next`    /// will be properly aligned, but *no trailing padding*.    ///    /// In order to match C representation layout `repr(C)`, you should    /// call `pad_to_align` after extending the layout with all fields.    /// (There is no way to match the default Rust representation    /// layout `repr(Rust)`, as it is unspecified.)    ///    /// Note that the alignment of the resulting layout will be the maximum of    /// those of `self` and `next`, in order to ensure alignment of both parts.    ///    /// Returns `Ok((k, offset))`, where `k` is layout of the concatenated    /// record and `offset` is the relative location, in bytes, of the    /// start of the `next` embedded within the concatenated record    /// (assuming that the record itself starts at offset 0).    ///    /// On arithmetic overflow, returns `LayoutErr`.    ///    /// # Examples    ///    /// To calculate the layout of a `#[repr(C)]` structure and the offsets of    /// the fields from its fields' layouts:    ///    /// ```rust    /// # use std::alloc::{Layout, LayoutErr};    /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutErr> {    ///     let mut offsets = Vec::new();    ///     let mut layout = Layout::from_size_align(0, 1)?;    ///     for &field in fields {    ///         let (new_layout, offset) = layout.extend(field)?;    ///         layout = new_layout;    ///         offsets.push(offset);    ///     }    ///     // Remember to finalize with `pad_to_align`!    ///     Ok((layout.pad_to_align(), offsets))    /// }    /// # // test that it works    /// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }    /// # let s = Layout::new::<S>();    /// # let u16 = Layout::new::<u16>();    /// # let u32 = Layout::new::<u32>();    /// # let u64 = Layout::new::<u64>();    /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));    /// ```    #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]    #[inline]    pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {        let new_align = cmp::max(self.align(), next.align());        let pad = self.padding_needed_for(next.align());         let offset = self.size().checked_add(pad).ok_or(LayoutErr { private: () })?;        let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;         let layout = Layout::from_size_align(new_size, new_align)?;        Ok((layout, offset))    }     /// Creates a layout describing the record for `n` instances of    /// `self`, with no padding between each instance.    ///    /// Note that, unlike `repeat`, `repeat_packed` does not guarantee    /// that the repeated instances of `self` will be properly    /// aligned, even if a given instance of `self` is properly    /// aligned. In other words, if the layout returned by    /// `repeat_packed` is used to allocate an array, it is not    /// guaranteed that all elements in the array will be properly    /// aligned.    ///    /// On arithmetic overflow, returns `LayoutErr`.    #[unstable(feature = "alloc_layout_extra", issue = "55724")]    #[inline]    pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {        let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;        Layout::from_size_align(size, self.align())    }     /// Creates a layout describing the record for `self` followed by    /// `next` with no additional padding between the two. Since no    /// padding is inserted, the alignment of `next` is irrelevant,    /// and is not incorporated *at all* into the resulting layout.    ///    /// On arithmetic overflow, returns `LayoutErr`.    #[unstable(feature = "alloc_layout_extra", issue = "55724")]    #[inline]    pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {        let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;        Layout::from_size_align(new_size, self.align())    }     /// Creates a layout describing the record for a `[T; n]`.    ///    /// On arithmetic overflow, returns `LayoutErr`.    #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]    #[inline]    pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {        let (layout, offset) = Layout::new::<T>().repeat(n)?;        debug_assert_eq!(offset, mem::size_of::<T>());        Ok(layout.pad_to_align())    }} /// The parameters given to `Layout::from_size_align`/// or some other `Layout` constructor/// do not satisfy its documented constraints.#[stable(feature = "alloc_layout", since = "1.28.0")]#[derive(Clone, PartialEq, Eq, Debug)]pub struct LayoutErr {    private: (),} // (we need this for downstream impl of trait Error)#[stable(feature = "alloc_layout", since = "1.28.0")]impl fmt::Display for LayoutErr {    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {        f.write_str("invalid parameters to Layout::from_size_align")    }}