use crate::fmt;
use crate::iter::{FusedIterator, TrustedLen};
use crate::marker;

/// 创建一个不产生任何结果的迭代器。
///
/// # Examples
///
/// 基本用法：
///
/// ```
/// use std::iter;
///
/// // 这可能是 i32 上的迭代器，但是，事实并非如此。
/// let mut nope = iter::empty::<i32>();
///
/// assert_eq!(None, nope.next());
/// ```
#[stable(feature = "iter_empty", since = "1.2.0")]
#[rustc_const_stable(feature = "const_iter_empty", since = "1.32.0")]
pub const fn empty<T>() -> Empty<T> {
    Empty(marker::PhantomData)
}

/// 没有任何结果的迭代器。
///
/// 该 `struct` 由 [`empty()`] 函数创建。有关更多信息，请参见其文档。
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "iter_empty", since = "1.2.0")]
pub struct Empty<T>(marker::PhantomData<fn() -> T>);

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T> fmt::Debug for Empty<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Empty").finish()
    }
}

#[stable(feature = "iter_empty", since = "1.2.0")]
impl<T> Iterator for Empty<T> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        None
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, Some(0))
    }
}

#[stable(feature = "iter_empty", since = "1.2.0")]
impl<T> DoubleEndedIterator for Empty<T> {
    fn next_back(&mut self) -> Option<T> {
        None
    }
}

#[stable(feature = "iter_empty", since = "1.2.0")]
impl<T> ExactSizeIterator for Empty<T> {
    fn len(&self) -> usize {
        0
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for Empty<T> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for Empty<T> {}

// 而不是 #[derive]，因为这会在 T 上添加一个 Clone 绑定，这是不必要的。
//
#[stable(feature = "iter_empty", since = "1.2.0")]
impl<T> Clone for Empty<T> {
    fn clone(&self) -> Empty<T> {
        Empty(marker::PhantomData)
    }
}

// 不是 #[derive]，因为这会在 T 上添加一个默认界限，这是不必要的。
//
#[stable(feature = "iter_empty", since = "1.2.0")]
impl<T> Default for Empty<T> {
    fn default() -> Empty<T> {
        Empty(marker::PhantomData)
    }
}