//! 固定长度数组的辅助函数和类型。
//!
//! *[See also the array primitive type](array).*

#![stable(feature = "core_array", since = "1.36.0")]

use crate::borrow::{Borrow, BorrowMut};
use crate::cmp::Ordering;
use crate::convert::{Infallible, TryFrom};
use crate::fmt;
use crate::hash::{self, Hash};
use crate::iter::TrustedLen;
use crate::mem::{self, MaybeUninit};
use crate::ops::{
    ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
};
use crate::slice::{Iter, IterMut};

mod equality;
mod iter;

#[stable(feature = "array_value_iter", since = "1.51.0")]
pub use iter::IntoIter;

/// 创建一个数组 `[T; N]`，其中每个数组元素 `T` 都由 `cb` 调用返回。
///
/// # Arguments
///
/// * `cb`: 回调，其中传入的参数是当前数组索引。
///
/// # Example
///
/// ```rust
/// #![feature(array_from_fn)]
///
/// let array = core::array::from_fn(|i| i);
/// assert_eq!(array, [0, 1, 2, 3, 4]);
/// ```
#[inline]
#[unstable(feature = "array_from_fn", issue = "89379")]
pub fn from_fn<F, T, const N: usize>(mut cb: F) -> [T; N]
where
    F: FnMut(usize) -> T,
{
    let mut idx = 0;
    [(); N].map(|_| {
        let res = cb(idx);
        idx += 1;
        res
    })
}

/// 创建一个数组 `[T; N]`，其中每个易出错的数组元素 `T` 由 `cb` 调用返回。
/// 与 [`from_fn`] 的元素创建不能失败不同，如果任何元素创建失败，此版本将返回错误。
///
///
/// 这个函数的返回类型取决于闭包的返回类型。
/// 如果您从闭包返回 `Result<T, E>`，您会得到一个 `Result<[T； N]; E>`。
/// 如果您从闭包中返回 `Option<T>`，您会得到一个 `Option<[T； N]>`.
///
/// # Arguments
///
/// * `cb`: 回调，其中传入的参数是当前数组索引。
///
/// # Example
///
/// ```rust
/// #![feature(array_from_fn)]
/// # // 显然，这些文档测试仍在 2018 版上
/// # use std::convert::TryInto;
///
/// let array: Result<[u8; 5], _> = std::array::try_from_fn(|i| i.try_into());
/// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
///
/// let array: Result<[i8; 200], _> = std::array::try_from_fn(|i| i.try_into());
/// assert!(array.is_err());
///
/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_add(100));
/// assert_eq!(array, Some([100, 101, 102, 103]));
///
/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_sub(100));
/// assert_eq!(array, None);
/// ```
#[inline]
#[unstable(feature = "array_from_fn", issue = "89379")]
pub fn try_from_fn<F, R, const N: usize>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
where
    F: FnMut(usize) -> R,
    R: Try,
    R::Residual: Residual<[R::Output; N]>,
{
    // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
    //
    unsafe { try_collect_into_array_unchecked(&mut (0..N).map(cb)) }
}

/// 将 quotes 转换为 `T`，将 quotes 转换为长度为 1 的数组 (不进行复制)。
#[stable(feature = "array_from_ref", since = "1.53.0")]
#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
pub const fn from_ref<T>(s: &T) -> &[T; 1] {
    // SAFETY: 将 `&T` 转换为 `&[T; 1]` 是声音。
    unsafe { &*(s as *const T).cast::<[T; 1]>() }
}

/// 将变量引用转换为 `T`，将变量引用转换为长度为 1 的数组 (不进行复制)。
#[stable(feature = "array_from_ref", since = "1.53.0")]
#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
    // SAFETY: 将 `&mut T` 转换为 `&mut [T; 1]` 是声音。
    unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
}

/// 从切片到数组的转换失败时返回的错误类型。
#[stable(feature = "try_from", since = "1.34.0")]
#[derive(Debug, Copy, Clone)]
pub struct TryFromSliceError(());

#[stable(feature = "core_array", since = "1.36.0")]
impl fmt::Display for TryFromSliceError {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self.__description(), f)
    }
}

impl TryFromSliceError {
    #[unstable(
        feature = "array_error_internals",
        reason = "available through Error trait and this method should not \
                     be exposed publicly",
        issue = "none"
    )]
    #[inline]
    #[doc(hidden)]
    pub fn __description(&self) -> &str {
        "could not convert slice to array"
    }
}

#[stable(feature = "try_from_slice_error", since = "1.36.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl const From<Infallible> for TryFromSliceError {
    fn from(x: Infallible) -> TryFromSliceError {
        match x {}
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, const N: usize> AsRef<[T]> for [T; N] {
    #[inline]
    fn as_ref(&self) -> &[T] {
        &self[..]
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, const N: usize> AsMut<[T]> for [T; N] {
    #[inline]
    fn as_mut(&mut self) -> &mut [T] {
        &mut self[..]
    }
}

#[stable(feature = "array_borrow", since = "1.4.0")]
#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
impl<T, const N: usize> const Borrow<[T]> for [T; N] {
    fn borrow(&self) -> &[T] {
        self
    }
}

#[stable(feature = "array_borrow", since = "1.4.0")]
#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
impl<T, const N: usize> const BorrowMut<[T]> for [T; N] {
    fn borrow_mut(&mut self) -> &mut [T] {
        self
    }
}

#[stable(feature = "try_from", since = "1.34.0")]
impl<T, const N: usize> TryFrom<&[T]> for [T; N]
where
    T: Copy,
{
    type Error = TryFromSliceError;

    fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
        <&Self>::try_from(slice).map(|r| *r)
    }
}

#[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
where
    T: Copy,
{
    type Error = TryFromSliceError;

    fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
        <Self>::try_from(&*slice)
    }
}

#[stable(feature = "try_from", since = "1.34.0")]
impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
    type Error = TryFromSliceError;

    fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
        if slice.len() == N {
            let ptr = slice.as_ptr() as *const [T; N];
            // SAFETY: 好的，因为我们只是检查了长度是否合适
            unsafe { Ok(&*ptr) }
        } else {
            Err(TryFromSliceError(()))
        }
    }
}

#[stable(feature = "try_from", since = "1.34.0")]
impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
    type Error = TryFromSliceError;

    fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
        if slice.len() == N {
            let ptr = slice.as_mut_ptr() as *mut [T; N];
            // SAFETY: 好的，因为我们只是检查了长度是否合适
            unsafe { Ok(&mut *ptr) }
        } else {
            Err(TryFromSliceError(()))
        }
    }
}

/// 数组的哈希值与对应的 X 像素的哈希值相同，符合实现的要求。
///
///
/// ```
/// #![feature(build_hasher_simple_hash_one)]
/// use std::hash::BuildHasher;
///
/// let b = std::collections::hash_map::RandomState::new();
/// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
/// assert_eq!(b.hash_one(a), b.hash_one(s));
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Hash, const N: usize> Hash for [T; N] {
    fn hash<H: hash::Hasher>(&self, state: &mut H) {
        Hash::hash(&self[..], state)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&&self[..], f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
    type Item = &'a mut T;
    type IntoIter = IterMut<'a, T>;

    fn into_iter(self) -> IterMut<'a, T> {
        self.iter_mut()
    }
}

#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
impl<T, I, const N: usize> Index<I> for [T; N]
where
    [T]: Index<I>,
{
    type Output = <[T] as Index<I>>::Output;

    #[inline]
    fn index(&self, index: I) -> &Self::Output {
        Index::index(self as &[T], index)
    }
}

#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
impl<T, I, const N: usize> IndexMut<I> for [T; N]
where
    [T]: IndexMut<I>,
{
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut Self::Output {
        IndexMut::index_mut(self as &mut [T], index)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
    #[inline]
    fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
        PartialOrd::partial_cmp(&&self[..], &&other[..])
    }
    #[inline]
    fn lt(&self, other: &[T; N]) -> bool {
        PartialOrd::lt(&&self[..], &&other[..])
    }
    #[inline]
    fn le(&self, other: &[T; N]) -> bool {
        PartialOrd::le(&&self[..], &&other[..])
    }
    #[inline]
    fn ge(&self, other: &[T; N]) -> bool {
        PartialOrd::ge(&&self[..], &&other[..])
    }
    #[inline]
    fn gt(&self, other: &[T; N]) -> bool {
        PartialOrd::gt(&&self[..], &&other[..])
    }
}

/// 实现数组 [按字典顺序](Ord#lexicographical-comparison) 的比较。
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Ord, const N: usize> Ord for [T; N] {
    #[inline]
    fn cmp(&self, other: &[T; N]) -> Ordering {
        Ord::cmp(&&self[..], &&other[..])
    }
}

#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
impl<T: Copy, const N: usize> Copy for [T; N] {}

#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
impl<T: Clone, const N: usize> Clone for [T; N] {
    #[inline]
    fn clone(&self) -> Self {
        SpecArrayClone::clone(self)
    }

    #[inline]
    fn clone_from(&mut self, other: &Self) {
        self.clone_from_slice(other);
    }
}

trait SpecArrayClone: Clone {
    fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
}

impl<T: Clone> SpecArrayClone for T {
    #[inline]
    default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
        // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
        //
        unsafe { collect_into_array_unchecked(&mut array.iter().cloned()) }
    }
}

impl<T: Copy> SpecArrayClone for T {
    #[inline]
    fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
        *array
    }
}

// 不能使用 const 泛型来完成 Default impls，因为 `[T; 0]` 不需要实现 Default，并且尚不支持针对不同数字使用不同的 impl 块。
//
//

macro_rules! array_impl_default {
    {$n:expr, $t:ident $($ts:ident)*} => {
        #[stable(since = "1.4.0", feature = "array_default")]
        impl<T> Default for [T; $n] where T: Default {
            fn default() -> [T; $n] {
                [$t::default(), $($ts::default()),*]
            }
        }
        array_impl_default!{($n - 1), $($ts)*}
    };
    {$n:expr,} => {
        #[stable(since = "1.4.0", feature = "array_default")]
        #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
        impl<T> const Default for [T; $n] {
            fn default() -> [T; $n] { [] }
        }
    };
}

array_impl_default! {32, T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T}

#[lang = "array"]
impl<T, const N: usize> [T; N] {
    /// 返回大小与 `self` 相同的数组，并将函数 `f` 按顺序应用于每个元素。
    ///
    /// 如果您不一定需要新的固定大小的数组，请考虑改用 [`Iterator::map`]。
    ///
    /// # 关于性能和栈使用的注意事项
    ///
    /// 不幸的是，这种方法的使用目前并不总是像它们可能的那样优化。
    /// 这主要涉及大数组，因为小数组上的映射似乎优化得很好。
    /// 另请注意，在调试模式下 (即
    /// 没有任何优化)，这种方法可以使用大量的栈空间 (数组大小的几倍或更多)。
    ///
    /// 因此，在性能关键的代码中，尽量避免在大型数组上使用此方法或检查发出的代码。
    /// 还要尽量避免链接 maps (例如
    /// `arr.map(...).map(...)`).
    ///
    /// 在许多情况下，您可以通过在阵列上调用 `.iter()` 或 `.into_iter()` 来代替使用 [`Iterator::map`]。
    /// 只有当您真的需要一个与结果大小相同的新数组时，才需要 `[T; N]::map`
    /// Rust 的惰性迭代器往往会得到很好的优化。
    ///
    /// # Examples
    ///
    /// ```
    /// let x = [1, 2, 3];
    /// let y = x.map(|v| v + 1);
    /// assert_eq!(y, [2, 3, 4]);
    ///
    /// let x = [1, 2, 3];
    /// let mut temp = 0;
    /// let y = x.map(|v| { temp += 1; v * temp });
    /// assert_eq!(y, [1, 4, 9]);
    ///
    /// let x = ["Ferris", "Bueller's", "Day", "Off"];
    /// let y = x.map(|v| v.len());
    /// assert_eq!(y, [6, 9, 3, 3]);
    /// ```
    ///
    ///
    ///
    ///
    ///
    ///
    #[stable(feature = "array_map", since = "1.55.0")]
    pub fn map<F, U>(self, f: F) -> [U; N]
    where
        F: FnMut(T) -> U,
    {
        // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
        //
        unsafe { collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
    }

    /// 一个容易出错的函数 `f` 应用于数组 `self` 上的每个元素，以返回与 `self` 或遇到的第一个错误大小相同的数组。
    ///
    ///
    /// 这个函数的返回类型取决于闭包的返回类型。
    /// 如果您从闭包返回 `Result<T, E>`，您会得到一个 `Result<[T； N]; E>`。
    /// 如果您从闭包中返回 `Option<T>`，您会得到一个 `Option<[T； N]>`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(array_try_map)]
    /// let a = ["1", "2", "3"];
    /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
    /// assert_eq!(b, [2, 3, 4]);
    ///
    /// let a = ["1", "2a", "3"];
    /// let b = a.try_map(|v| v.parse::<u32>());
    /// assert!(b.is_err());
    ///
    /// use std::num::NonZeroU32;
    /// let z = [1, 2, 0, 3, 4];
    /// assert_eq!(z.try_map(NonZeroU32::new), None);
    /// let a = [1, 2, 3];
    /// let b = a.try_map(NonZeroU32::new);
    /// let c = b.map(|x| x.map(NonZeroU32::get));
    /// assert_eq!(c, Some(a));
    /// ```
    #[unstable(feature = "array_try_map", issue = "79711")]
    pub fn try_map<F, R>(self, f: F) -> ChangeOutputType<R, [R::Output; N]>
    where
        F: FnMut(T) -> R,
        R: Try,
        R::Residual: Residual<[R::Output; N]>,
    {
        // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
        //
        unsafe { try_collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
    }

    /// 将两个阵列压缩为成对的单个阵列。
    ///
    /// `zip()` 返回一个新数组，其中每个元素都是一个元组，其中第一个元素来自第一个数组，第二个元素来自第二个数组。
    ///
    /// 换句话说，它将两个数组压缩在一起，成为一个数组。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(array_zip)]
    /// let x = [1, 2, 3];
    /// let y = [4, 5, 6];
    /// let z = x.zip(y);
    /// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
    /// ```
    ///
    #[unstable(feature = "array_zip", issue = "80094")]
    pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
        let mut iter = IntoIterator::into_iter(self).zip(rhs);

        // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
        //
        unsafe { collect_into_array_unchecked(&mut iter) }
    }

    /// 返回包含整个数组的切片。等效于 `&s[..]`。
    #[stable(feature = "array_as_slice", since = "1.57.0")]
    pub const fn as_slice(&self) -> &[T] {
        self
    }

    /// 返回包含整个数组的可变切片。
    /// 等效于 `&mut s[..]`。
    #[stable(feature = "array_as_slice", since = "1.57.0")]
    pub fn as_mut_slice(&mut self) -> &mut [T] {
        self
    }

    /// 借用每个元素，并返回一个引用数组，其大小与 `self` 相同。
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(array_methods)]
    ///
    /// let floats = [3.1, 2.7, -1.0];
    /// let float_refs: [&f64; 3] = floats.each_ref();
    /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
    /// ```
    ///
    /// 如果与其他方法 (例如 [`map`](#method.map)) 结合使用，则此方法特别有用。
    /// 这样，如果原始数组的元素不是 [`Copy`]，则可以避免移动原始数组。
    ///
    /// ```
    /// #![feature(array_methods)]
    ///
    /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
    /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
    /// assert_eq!(is_ascii, [true, false, true]);
    ///
    /// // 我们仍然可以访问原始数组：它尚未移动。
    /// assert_eq!(strings.len(), 3);
    /// ```
    ///
    ///
    ///
    #[unstable(feature = "array_methods", issue = "76118")]
    pub fn each_ref(&self) -> [&T; N] {
        // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
        //
        unsafe { collect_into_array_unchecked(&mut self.iter()) }
    }

    /// 借用每个元素，并返回与 `self` 相同大小的可变引用数组。
    ///
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(array_methods)]
    ///
    /// let mut floats = [3.1, 2.7, -1.0];
    /// let float_refs: [&mut f64; 3] = floats.each_mut();
    /// *float_refs[0] = 0.0;
    /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
    /// assert_eq!(floats, [0.0, 2.7, -1.0]);
    /// ```
    ///
    #[unstable(feature = "array_methods", issue = "76118")]
    pub fn each_mut(&mut self) -> [&mut T; N] {
        // SAFETY: 我们肯定知道该迭代器将产生完全 `N` 项。
        //
        unsafe { collect_into_array_unchecked(&mut self.iter_mut()) }
    }

    /// 在一个索引处将一个数组的引用一分为二。
    ///
    /// 第一个将包含来自 `[0, M)` 的所有索引 (不包括索引 `M` 本身)，第二个将包含来自 `[M, N)` 的所有索引 (不包括索引 `N` 本身)。
    ///
    ///
    /// # Panics
    ///
    /// 如果 `M > N`，就会出现 panics。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let v = [1, 2, 3, 4, 5, 6];
    ///
    /// {
    ///    let (left, right) = v.split_array_ref::<0>();
    ///    assert_eq!(left, &[]);
    ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.split_array_ref::<2>();
    ///     assert_eq!(left, &[1, 2]);
    ///     assert_eq!(right, &[3, 4, 5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.split_array_ref::<6>();
    ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
    ///     assert_eq!(right, &[]);
    /// }
    /// ```
    ///
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
        (&self[..]).split_array_ref::<M>()
    }

    /// 在一个索引处将一个可变数组的引用一分为二。
    ///
    /// 第一个将包含来自 `[0, M)` 的所有索引 (不包括索引 `M` 本身)，第二个将包含来自 `[M, N)` 的所有索引 (不包括索引 `N` 本身)。
    ///
    ///
    /// # Panics
    ///
    /// 如果 `M > N`，就会出现 panics。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let mut v = [1, 0, 3, 0, 5, 6];
    /// let (left, right) = v.split_array_mut::<2>();
    /// assert_eq!(left, &mut [1, 0][..]);
    /// assert_eq!(right, &mut [3, 0, 5, 6]);
    /// left[1] = 2;
    /// right[1] = 4;
    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
    /// ```
    ///
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
        (&mut self[..]).split_array_mut::<M>()
    }

    /// 将一个数组引用从末尾的一个索引处一分为二。
    ///
    /// 第一个将包含来自 `[0, N - M)` 的所有索引 (不包括索引 `N - M` 本身)，第二个将包含来自 `[N - M, N)` 的所有索引 (不包括索引 `N` 本身)。
    ///
    ///
    /// # Panics
    ///
    /// 如果 `M > N`，就会出现 panics。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let v = [1, 2, 3, 4, 5, 6];
    ///
    /// {
    ///    let (left, right) = v.rsplit_array_ref::<0>();
    ///    assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
    ///    assert_eq!(right, &[]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.rsplit_array_ref::<2>();
    ///     assert_eq!(left, &[1, 2, 3, 4]);
    ///     assert_eq!(right, &[5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.rsplit_array_ref::<6>();
    ///     assert_eq!(left, &[]);
    ///     assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
    /// }
    /// ```
    ///
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
        (&self[..]).rsplit_array_ref::<M>()
    }

    /// 将一个附属数组引用从末尾的一个索引处一分为二。
    ///
    /// 第一个将包含来自 `[0, N - M)` 的所有索引 (不包括索引 `N - M` 本身)，第二个将包含来自 `[N - M, N)` 的所有索引 (不包括索引 `N` 本身)。
    ///
    ///
    /// # Panics
    ///
    /// 如果 `M > N`，就会出现 panics。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_array)]
    ///
    /// let mut v = [1, 0, 3, 0, 5, 6];
    /// let (left, right) = v.rsplit_array_mut::<4>();
    /// assert_eq!(left, &mut [1, 0]);
    /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
    /// left[1] = 2;
    /// right[1] = 4;
    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
    /// ```
    ///
    #[unstable(
        feature = "split_array",
        reason = "return type should have array as 2nd element",
        issue = "90091"
    )]
    #[inline]
    pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
        (&mut self[..]).rsplit_array_mut::<M>()
    }
}

/// 从 `iter` 中提取 `N` 项，并将它们作为数组返回。
/// 如果迭代器产生的结果少于 `N` 项，则此函数显示未定义的行为。
///
///
/// 有关详细信息，请参见 [`try_collect_into_array`]。
///
/// # Safety
///
/// 取决于调用者，以确保 `iter` 至少产生 `N` 项。
/// 违反此条件会导致未定义的行为。
unsafe fn try_collect_into_array_unchecked<I, T, R, const N: usize>(iter: &mut I) -> R::TryType
where
    // Note: `TrustedLen` 在这里有点实验。
    // 这只是一个内联函数，因此如果发现此绑定不是一个好主意，请随时删除。
    //
    // 在这种情况下，请记住还要移除下面的下限 `debug_assert!`!
    I: Iterator + TrustedLen,
    I::Item: Try<Output = T, Residual = R>,
    R: Residual<[T; N]>,
{
    debug_assert!(N <= iter.size_hint().1.unwrap_or(usize::MAX));
    debug_assert!(N <= iter.size_hint().0);

    // SAFETY: 函数契约涵盖的范围。
    unsafe { try_collect_into_array(iter).unwrap_unchecked() }
}

// `try_collect_into_array_unchecked` 的可靠版本。
unsafe fn collect_into_array_unchecked<I, const N: usize>(iter: &mut I) -> [I::Item; N]
where
    I: Iterator + TrustedLen,
{
    let mut map = iter.map(NeverShortCircuit);

    // SAFETY: 相同的安全注意事项
    // 迭代器长度适用于 `try_collect_into_array_unchecked` 和 `collect_into_array_unchecked`
    //
    match unsafe { try_collect_into_array_unchecked(&mut map) } {
        NeverShortCircuit(array) => array,
    }
}

/// 从 `iter` 中提取 `N` 项，并将它们作为数组返回。如果迭代器生成的项少于 `N`，则返回 `None`，并且丢弃所有已经生成的项。
///
/// 由于迭代器是作为变量引用传递的，并且此函数最多 `N` 次调用 `next`，因此以后仍然可以使用迭代器来检索剩余项。
///
///
/// 如果 `iter.next()` 出现 panic，则丢弃迭代器已经生成的所有项。
///
///
///
///
fn try_collect_into_array<I, T, R, const N: usize>(iter: &mut I) -> Option<R::TryType>
where
    I: Iterator,
    I::Item: Try<Output = T, Residual = R>,
    R: Residual<[T; N]>,
{
    if N == 0 {
        // SAFETY: 空数组始终有人居住，并且没有有效性不变量。
        return unsafe { Some(Try::from_output(mem::zeroed())) };
    }

    struct Guard<'a, T, const N: usize> {
        array_mut: &'a mut [MaybeUninit<T>; N],
        initialized: usize,
    }

    impl<T, const N: usize> Drop for Guard<'_, T, N> {
        fn drop(&mut self) {
            debug_assert!(self.initialized <= N);

            // SAFETY: 这个切片将只包含初始化的对象。
            unsafe {
                crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
                    &mut self.array_mut.get_unchecked_mut(..self.initialized),
                ));
            }
        }
    }

    let mut array = MaybeUninit::uninit_array::<N>();
    let mut guard = Guard { array_mut: &mut array, initialized: 0 };

    while let Some(item_rslt) = iter.next() {
        let item = match item_rslt.branch() {
            ControlFlow::Break(r) => {
                return Some(FromResidual::from_residual(r));
            }
            ControlFlow::Continue(elem) => elem,
        };

        // SAFETY: `guard.initialized` 从 0 开始，在循环中加 1，一旦到达 N (`array.len()`)，循环就中止。
        //
        //
        unsafe {
            guard.array_mut.get_unchecked_mut(guard.initialized).write(item);
        }
        guard.initialized += 1;

        // 检查整个数组是否已初始化。
        if guard.initialized == N {
            mem::forget(guard);

            // SAFETY: 以上条件断言所有元素都已初始化。
            //
            let out = unsafe { MaybeUninit::array_assume_init(array) };
            return Some(Try::from_output(out));
        }
    }

    // 仅当 `guard.initialized` 到达 `N` 之前迭代器已耗尽时，才可以达到此目的。
    //
    // 还要注意的是，这里将 `guard` 放在了这里，丢弃了所有已经初始化的元素。
    None
}