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    /// An iterator that always continues to yield `None` when exhausted.
    ///
    /// Calling next on a fused iterator that has returned `None` once is guaranteed
    /// to return [`None`] again. This trait should be implemented by all iterators
    /// that behave this way because it allows optimizing [`Iterator::fuse()`].
    ///
    /// Note: In general, you should not use `FusedIterator` in generic bounds if
    /// you need a fused iterator. Instead, you should just call [`Iterator::fuse()`]
    /// on the iterator. If the iterator is already fused, the additional [`Fuse`]
    /// wrapper will be a no-op with no performance penalty.
    ///
    /// [`Fuse`]: crate::iter::Fuse
    #[stable(feature = "fused", since = "1.26.0")]
    #[rustc_unsafe_specialization_marker]
    pub trait FusedIterator: Iterator {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<I: FusedIterator + ?Sized> FusedIterator for &mut I {}

    /// An iterator that reports an accurate length using size_hint.
    ///
    /// The iterator reports a size hint where it is either exact
    /// (lower bound is equal to upper bound), or the upper bound is [`None`].
    /// The upper bound must only be [`None`] if the actual iterator length is
    /// larger than [`usize::MAX`]. In that case, the lower bound must be
    /// [`usize::MAX`], resulting in a [`Iterator::size_hint()`] of
    /// `(usize::MAX, None)`.
    ///
    /// The iterator must produce exactly the number of elements it reported
    /// or diverge before reaching the end.
    ///
    /// # Safety
    ///
    /// This trait must only be implemented when the contract is upheld. Consumers
    /// of this trait must inspect [`Iterator::size_hint()`]’s upper bound.
    #[unstable(feature = "trusted_len", issue = "37572")]
    #[rustc_unsafe_specialization_marker]
    pub unsafe trait TrustedLen: Iterator {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<I: TrustedLen + ?Sized> TrustedLen for &mut I {}

    /// An iterator that when yielding an item will have taken at least one element
    /// from its underlying [`SourceIter`].
    ///
    /// Calling any method that advances the iterator, e.g.  [`next()`] or [`try_fold()`],
    /// guarantees that for each step at least one value of the iterator's underlying source
    /// has been moved out and the result of the iterator chain could be inserted
    /// in its place, assuming structural constraints of the source allow such an insertion.
    /// In other words this trait indicates that an iterator pipeline can be collected in place.
    ///
    /// [`SourceIter`]: crate::iter::SourceIter
    /// [`next()`]: Iterator::next
    /// [`try_fold()`]: Iterator::try_fold
    #[unstable(issue = "none", feature = "inplace_iteration")]
    pub unsafe trait InPlaceIterable: Iterator {}