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    #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
    #![doc(hidden)]

    use core::alloc::LayoutError;
    use core::cmp;
    use core::intrinsics;
    use core::mem::{self, ManuallyDrop, MaybeUninit};
    use core::ops::Drop;
    use core::ptr::{self, NonNull, Unique};
    use core::slice;

    use crate::alloc::{handle_alloc_error, Allocator, Global, Layout};
    use crate::boxed::Box;
    use crate::collections::TryReserveError::{self, *};

    #[cfg(test)]
    mod tests;

    enum AllocInit {
        /// The contents of the new memory are uninitialized.
        Uninitialized,
        /// The new memory is guaranteed to be zeroed.
        Zeroed,
    }

    /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
    /// a buffer of memory on the heap without having to worry about all the corner cases
    /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
    /// In particular:
    ///
    /// * Produces `Unique::dangling()` on zero-sized types.
    /// * Produces `Unique::dangling()` on zero-length allocations.
    /// * Avoids freeing `Unique::dangling()`.
    /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
    /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
    /// * Guards against overflowing your length.
    /// * Calls `handle_alloc_error` for fallible allocations.
    /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
    /// * Uses the excess returned from the allocator to use the largest available capacity.
    ///
    /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
    /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
    /// to handle the actual things *stored* inside of a `RawVec`.
    ///
    /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
    /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
    /// `Box<[T]>`, since `capacity()` won't yield the length.
    #[allow(missing_debug_implementations)]
    pub struct RawVec<T, A: Allocator = Global> {
        ptr: Unique<T>,
        cap: usize,
        alloc: A,
    }

    impl<T> RawVec<T, Global> {
        /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
        /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
        ///
        /// If you change `RawVec<T>::new` or dependencies, please take care to not
        /// introduce anything that would truly violate `min_const_fn`.
        ///
        /// NOTE: We could avoid this hack and check conformance with some
        /// `#[rustc_force_min_const_fn]` attribute which requires conformance
        /// with `min_const_fn` but does not necessarily allow calling it in
        /// `stable(...) const fn` / user code not enabling `foo` when
        /// `#[rustc_const_unstable(feature = "foo", issue = "01234")]` is present.
        pub const NEW: Self = Self::new();

        /// Creates the biggest possible `RawVec` (on the system heap)
        /// without allocating. If `T` has positive size, then this makes a
        /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
        /// `RawVec` with capacity `usize::MAX`. Useful for implementing
        /// delayed allocation.
        pub const fn new() -> Self {
            Self::new_in(Global)
        }

        /// Creates a `RawVec` (on the system heap) with exactly the
        /// capacity and alignment requirements for a `[T; capacity]`. This is
        /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
        /// zero-sized. Note that if `T` is zero-sized this means you will
        /// *not* get a `RawVec` with the requested capacity.
        ///
        /// # Panics
        ///
        /// Panics if the requested capacity exceeds `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        #[inline]
        pub fn with_capacity(capacity: usize) -> Self {
            Self::with_capacity_in(capacity, Global)
        }

        /// Like `with_capacity`, but guarantees the buffer is zeroed.
        #[inline]
        pub fn with_capacity_zeroed(capacity: usize) -> Self {
            Self::with_capacity_zeroed_in(capacity, Global)
        }

        /// Reconstitutes a `RawVec` from a pointer and capacity.
        ///
        /// # Safety
        ///
        /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
        /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
        /// systems). ZST vectors may have a capacity up to `usize::MAX`.
        /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
        #[inline]
        pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
            unsafe { Self::from_raw_parts_in(ptr, capacity, Global) }
        }
    }

    impl<T, A: Allocator> RawVec<T, A> {
        // Tiny Vecs are dumb. Skip to:
        // - 8 if the element size is 1, because any heap allocators is likely
        //   to round up a request of less than 8 bytes to at least 8 bytes.
        // - 4 if elements are moderate-sized (<= 1 KiB).
        // - 1 otherwise, to avoid wasting too much space for very short Vecs.
        const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
            8
        } else if mem::size_of::<T>() <= 1024 {
            4
        } else {
            1
        };

        /// Like `new`, but parameterized over the choice of allocator for
        /// the returned `RawVec`.
        #[rustc_allow_const_fn_unstable(const_fn)]
        pub const fn new_in(alloc: A) -> Self {
            // `cap: 0` means "unallocated". zero-sized types are ignored.
            Self { ptr: Unique::dangling(), cap: 0, alloc }
        }

        /// Like `with_capacity`, but parameterized over the choice of
        /// allocator for the returned `RawVec`.
        #[inline]
        pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
            Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
        }

        /// Like `with_capacity_zeroed`, but parameterized over the choice
        /// of allocator for the returned `RawVec`.
        #[inline]
        pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
            Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
        }

        /// Converts a `Box<[T]>` into a `RawVec<T>`.
        pub fn from_box(slice: Box<[T], A>) -> Self {
            unsafe {
                let (slice, alloc) = Box::into_raw_with_allocator(slice);
                RawVec::from_raw_parts_in(slice.as_mut_ptr(), slice.len(), alloc)
            }
        }

        /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
        ///
        /// Note that this will correctly reconstitute any `cap` changes
        /// that may have been performed. (See description of type for details.)
        ///
        /// # Safety
        ///
        /// * `len` must be greater than or equal to the most recently requested capacity, and
        /// * `len` must be less than or equal to `self.capacity()`.
        ///
        /// Note, that the requested capacity and `self.capacity()` could differ, as
        /// an allocator could overallocate and return a greater memory block than requested.
        pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
            // Sanity-check one half of the safety requirement (we cannot check the other half).
            debug_assert!(
                len <= self.capacity(),
                "`len` must be smaller than or equal to `self.capacity()`"
            );

            let me = ManuallyDrop::new(self);
            unsafe {
                let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
                Box::from_raw_in(slice, ptr::read(&me.alloc))
            }
        }

        fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
            if mem::size_of::<T>() == 0 {
                Self::new_in(alloc)
            } else {
                // We avoid `unwrap_or_else` here because it bloats the amount of
                // LLVM IR generated.
                let layout = match Layout::array::<T>(capacity) {
                    Ok(layout) => layout,
                    Err(_) => capacity_overflow(),
                };
                match alloc_guard(layout.size()) {
                    Ok(_) => {}
                    Err(_) => capacity_overflow(),
                }
                let result = match init {
                    AllocInit::Uninitialized => alloc.allocate(layout),
                    AllocInit::Zeroed => alloc.allocate_zeroed(layout),
                };
                let ptr = match result {
                    Ok(ptr) => ptr,
                    Err(_) => handle_alloc_error(layout),
                };

                Self {
                    ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
                    cap: Self::capacity_from_bytes(ptr.len()),
                    alloc,
                }
            }
        }

        /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
        ///
        /// # Safety
        ///
        /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
        /// `capacity`.
        /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
        /// systems). ZST vectors may have a capacity up to `usize::MAX`.
        /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
        /// guaranteed.
        #[inline]
        pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
            Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
        }

        /// Gets a raw pointer to the start of the allocation. Note that this is
        /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
        /// be careful.
        #[inline]
        pub fn ptr(&self) -> *mut T {
            self.ptr.as_ptr()
        }

        /// Gets the capacity of the allocation.
        ///
        /// This will always be `usize::MAX` if `T` is zero-sized.
        #[inline(always)]
        pub fn capacity(&self) -> usize {
            if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
        }

        /// Returns a shared reference to the allocator backing this `RawVec`.
        pub fn allocator(&self) -> &A {
            &self.alloc
        }

        fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
            if mem::size_of::<T>() == 0 || self.cap == 0 {
                None
            } else {
                // We have an allocated chunk of memory, so we can bypass runtime
                // checks to get our current layout.
                unsafe {
                    let align = mem::align_of::<T>();
                    let size = mem::size_of::<T>() * self.cap;
                    let layout = Layout::from_size_align_unchecked(size, align);
                    Some((self.ptr.cast().into(), layout))
                }
            }
        }

        /// Ensures that the buffer contains at least enough space to hold `len +
        /// additional` elements. If it doesn't already have enough capacity, will
        /// reallocate enough space plus comfortable slack space to get amortized
        /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
        /// itself to panic.
        ///
        /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
        /// the requested space. This is not really unsafe, but the unsafe
        /// code *you* write that relies on the behavior of this function may break.
        ///
        /// This is ideal for implementing a bulk-push operation like `extend`.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity exceeds `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(raw_vec_internals)]
        /// # extern crate alloc;
        /// # use std::ptr;
        /// # use alloc::raw_vec::RawVec;
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T: Clone> MyVec<T> {
        ///     pub fn push_all(&mut self, elems: &[T]) {
        ///         self.buf.reserve(self.len, elems.len());
        ///         // reserve would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         for x in elems {
        ///             unsafe {
        ///                 ptr::write(self.buf.ptr().add(self.len), x.clone());
        ///             }
        ///             self.len += 1;
        ///         }
        ///     }
        /// }
        /// # fn main() {
        /// #   let mut vector = MyVec { buf: RawVec::new(), len: 0 };
        /// #   vector.push_all(&[1, 3, 5, 7, 9]);
        /// # }
        /// ```
        pub fn reserve(&mut self, len: usize, additional: usize) {
            handle_reserve(self.try_reserve(len, additional));
        }

        /// The same as `reserve`, but returns on errors instead of panicking or aborting.
        pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
            if self.needs_to_grow(len, additional) {
                self.grow_amortized(len, additional)
            } else {
                Ok(())
            }
        }

        /// Ensures that the buffer contains at least enough space to hold `len +
        /// additional` elements. If it doesn't already, will reallocate the
        /// minimum possible amount of memory necessary. Generally this will be
        /// exactly the amount of memory necessary, but in principle the allocator
        /// is free to give back more than we asked for.
        ///
        /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
        /// the requested space. This is not really unsafe, but the unsafe code
        /// *you* write that relies on the behavior of this function may break.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity exceeds `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        pub fn reserve_exact(&mut self, len: usize, additional: usize) {
            handle_reserve(self.try_reserve_exact(len, additional));
        }

        /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
        pub fn try_reserve_exact(
            &mut self,
            len: usize,
            additional: usize,
        ) -> Result<(), TryReserveError> {
            if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
        }

        /// Shrinks the allocation down to the specified amount. If the given amount
        /// is 0, actually completely deallocates.
        ///
        /// # Panics
        ///
        /// Panics if the given amount is *larger* than the current capacity.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        pub fn shrink_to_fit(&mut self, amount: usize) {
            handle_reserve(self.shrink(amount));
        }
    }

    impl<T, A: Allocator> RawVec<T, A> {
        /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
        /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
        fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
            additional > self.capacity().wrapping_sub(len)
        }

        fn capacity_from_bytes(excess: usize) -> usize {
            debug_assert_ne!(mem::size_of::<T>(), 0);
            excess / mem::size_of::<T>()
        }

        fn set_ptr(&mut self, ptr: NonNull<[u8]>) {
            self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
            self.cap = Self::capacity_from_bytes(ptr.len());
        }

        // This method is usually instantiated many times. So we want it to be as
        // small as possible, to improve compile times. But we also want as much of
        // its contents to be statically computable as possible, to make the
        // generated code run faster. Therefore, this method is carefully written
        // so that all of the code that depends on `T` is within it, while as much
        // of the code that doesn't depend on `T` as possible is in functions that
        // are non-generic over `T`.
        fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
            // This is ensured by the calling contexts.
            debug_assert!(additional > 0);

            if mem::size_of::<T>() == 0 {
                // Since we return a capacity of `usize::MAX` when `elem_size` is
                // 0, getting to here necessarily means the `RawVec` is overfull.
                return Err(CapacityOverflow);
            }

            // Nothing we can really do about these checks, sadly.
            let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;

            // This guarantees exponential growth. The doubling cannot overflow
            // because `cap <= isize::MAX` and the type of `cap` is `usize`.
            let cap = cmp::max(self.cap * 2, required_cap);
            let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);

            let new_layout = Layout::array::<T>(cap);

            // `finish_grow` is non-generic over `T`.
            let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
            self.set_ptr(ptr);
            Ok(())
        }

        // The constraints on this method are much the same as those on
        // `grow_amortized`, but this method is usually instantiated less often so
        // it's less critical.
        fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
            if mem::size_of::<T>() == 0 {
                // Since we return a capacity of `usize::MAX` when the type size is
                // 0, getting to here necessarily means the `RawVec` is overfull.
                return Err(CapacityOverflow);
            }

            let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
            let new_layout = Layout::array::<T>(cap);

            // `finish_grow` is non-generic over `T`.
            let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
            self.set_ptr(ptr);
            Ok(())
        }

        fn shrink(&mut self, amount: usize) -> Result<(), TryReserveError> {
            assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");

            let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
            let new_size = amount * mem::size_of::<T>();

            let ptr = unsafe {
                let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
                self.alloc.shrink(ptr, layout, new_layout).map_err(|_| TryReserveError::AllocError {
                    layout: new_layout,
                    non_exhaustive: (),
                })?
            };
            self.set_ptr(ptr);
            Ok(())
        }
    }

    // This function is outside `RawVec` to minimize compile times. See the comment
    // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
    // significant, because the number of different `A` types seen in practice is
    // much smaller than the number of `T` types.)
    #[inline(never)]
    fn finish_grow<A>(
        new_layout: Result<Layout, LayoutError>,
        current_memory: Option<(NonNull<u8>, Layout)>,
        alloc: &mut A,
    ) -> Result<NonNull<[u8]>, TryReserveError>
    where
        A: Allocator,
    {
        // Check for the error here to minimize the size of `RawVec::grow_*`.
        let new_layout = new_layout.map_err(|_| CapacityOverflow)?;

        alloc_guard(new_layout.size())?;

        let memory = if let Some((ptr, old_layout)) = current_memory {
            debug_assert_eq!(old_layout.align(), new_layout.align());
            unsafe {
                // The allocator checks for alignment equality
                intrinsics::assume(old_layout.align() == new_layout.align());
                alloc.grow(ptr, old_layout, new_layout)
            }
        } else {
            alloc.allocate(new_layout)
        };

        memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })
    }

    unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
        /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
        fn drop(&mut self) {
            if let Some((ptr, layout)) = self.current_memory() {
                unsafe { self.alloc.deallocate(ptr, layout) }
            }
        }
    }

    // Central function for reserve error handling.
    #[inline]
    fn handle_reserve(result: Result<(), TryReserveError>) {
        match result {
            Err(CapacityOverflow) => capacity_overflow(),
            Err(AllocError { layout, .. }) => handle_alloc_error(layout),
            Ok(()) => { /* yay */ }
        }
    }

    // We need to guarantee the following:
    // * We don't ever allocate `> isize::MAX` byte-size objects.
    // * We don't overflow `usize::MAX` and actually allocate too little.
    //
    // On 64-bit we just need to check for overflow since trying to allocate
    // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
    // an extra guard for this in case we're running on a platform which can use
    // all 4GB in user-space, e.g., PAE or x32.

    #[inline]
    fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
        if usize::BITS < 64 && alloc_size > isize::MAX as usize {
            Err(CapacityOverflow)
        } else {
            Ok(())
        }
    }

    // One central function responsible for reporting capacity overflows. This'll
    // ensure that the code generation related to these panics is minimal as there's
    // only one location which panics rather than a bunch throughout the module.
    fn capacity_overflow() -> ! {
        panic!("capacity overflow");
    }