You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: acks.rs
    path of file: ./repos/vector/lib/vector-buffers/src/topology
    
    the code of the file until where you have to start completion: use std::{collections::VecDeque, fmt};

use num_traits::{Bounded, CheckedAdd, CheckedSub, Unsigned, WrappingAdd, WrappingSub};

#[derive(Clone, Copy, Debug, PartialEq)]
enum PendingMarkerLength<N> {
    Known(N),
    Assumed(N),
    Unknown,
}

struct PendingMarker<N, D> {
    id: N,
    len: PendingMarkerLength<N>,
    data: Option<D>,
}

impl<N, D> fmt::Debug for PendingMarker<N, D>
where
    N: fmt::Debug,
    D: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("PendingMarker")
            .field("id", &self.id)
            .field("len", &self.len)
            .field("data", &self.data)
            .finish()
    }
}

/// The length of an eligible marker.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum EligibleMarkerLength<N> {
    /// The marker's length was declared upfront when added.
    Known(N),

    /// The marker's length was calculated based on imperfect information, and so while it should
    /// accurately represent a correct range that covers any gaps in the marker range, it may or may
    /// not represent one true marker, or possibly multiple markers.
    Assumed(N),
}

impl<N: Copy> EligibleMarkerLength<N> {
    fn len(&self) -> N {
        match self {
            EligibleMarkerLength::Known(len) | EligibleMarkerLength::Assumed(len) => *len,
        }
    }
}

/// A marker that has been fully acknowledged.
pub struct EligibleMarker<N, D> {
    pub id: N,
    pub len: EligibleMarkerLength<N>,
    pub data: Option<D>,
}

impl<N, D> PartialEq for EligibleMarker<N, D>
where
    N: PartialEq,
{
    fn eq(&self, other: &Self) -> bool {
        self.id == other.id && self.len == other.len
    }
}

impl<N, D> fmt::Debug for EligibleMarker<N, D>
where
    N: fmt::Debug,
    D: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("EligibleMarker")
            .field("id", &self.id)
            .field("len", &self.len)
            .field("data", &self.data)
            .finish()
    }
}

/// Error returned by `OrderedAcknowledgements::add_marker`.
///
/// In general, this error represents a breaking of ID monotonicity, or more likely, the loss of
/// records where entire records may have been skipped 
    
    pub fn get_next_eligible_marker(&mut self) -> Option<EligibleMarker<N, D>> {
        let effective_acked_marker_id = self.acked_marker_id.wrapping_add(&self.unclaimed_acks);

        trace!(
            acked_marker_id = %self.acked_marker_id,
            %effective_acked_marker_id,
            unclaimed_acks = %self.unclaimed_acks,
            pending_markers = self.pending_markers.len(),
            "Searching for eligible marker."
        );

        let maybe_eligible_marker =
            self.pending_markers
                .front()
                .and_then(|marker| match marker.len {
                    // If the acked marker ID is ahead of this marker ID, plus its length, it's been fully
                    // acknowledged and we can consume and yield the marker.  We have to double
                    // verify this by checking that there's enough unclaimed acks to support this
                    // length because otherwise we might fall victim to markers that simply generate
                    // a required acked marker ID that is equal to the actual acked marker ID when
                    // an amount of unclaimed acks exists that is not enough for this marker but is
                    // enough to align the effective/required IDs.
                    PendingMarkerLength::Known(len) => {
                        let required_acked_marker_id = marker.id.wrapping_add(&len);
                        if required_acked_marker_id <= effective_acked_marker_id
                            && self.unclaimed_acks >= len
                        {
                            Some((EligibleMarkerLength::Known(len), len))
                        } else {
                            None
                        }
                    }
                    // The marker has an assumed length, which means a marker was added after it,
                    // which implies that it is de facto eligible as unknown length markers do not
                    // consume acknowledgements and so are immediately eligible once an assumed
                    // length can be determined.
                    PendingMarkerLength::Assumed(len) => {
                        Some((EligibleMarkerLength::Assumed(len), N::min_value()))
                    }
                    // We don't yet know what the length is for this marker, so we're stuck waiting
                    // for another marker to be added before that can be determined.
                    PendingMarkerLength::Unknown => None,
                });

        // If we actually got an eligible marker, we need to actually remove it from the pending
        // marker queue and potentially adjust the amount of unclaimed acks we have.
        match maybe_eligible_marker {
            Some((len, acks_to_claim)) => {
                // If we actually got an eligible marker, we need to actually remove it from the pending
                // marker queue, potentially adjust the amount of unclaimed acks we have, and adjust
                // our acked marker ID.
                let PendingMarker { id, data, .. } = self
                    .pending_markers
                    .pop_front()
                    .unwrap_or_else(|| unreachable!("pending markers cannot be empty"));

                if acks_to_claim > N::min_value() {
                    self.unclaimed_acks = self
                        .unclaimed_acks
                        .checked_sub(&acks_to_claim)
                        .unwrap_or_else(|| {
                            unreachable!("should not be able to claim more acks than are unclaimed")
                        });
                }

                self.acked_marker_id = id.wrapping_add(&len.len());

                Some(EligibleMarker { id, len, data })
            }
            None => None,
        }
    }