Function pmap_lanes 

pub fn pmap_lanes<L, C, T, U, E, F>(
    world: &C,
    lanes: &mut L,
    options: PmapOptions,
    root_items: Option<Vec<T>>,
    f: F,
) -> Result<Option<Vec<U>>, PmapError>
where
    L: Lanes,
    C: Communicator,
    T: Serialize + DeserializeOwned + Send,
    U: Serialize + DeserializeOwned + Send,
    E: Display + Send,
    F: Fn(T, &mut L::Lane<'_>) -> Result<U, E> + Sync,

Dynamically maps root-owned items across MPI ranks and caller-supplied lanes for each rank’s local batches.

Every rank must call this collective in the same order with identical options. Only options.root supplies items and receives ordered results. The call must originate on the MPI initialization/main thread, and MPI must provide MPI_THREAD_FUNNELED or stronger, because the coordinator keeps MPI progress on that thread while the lanes compute. Do not enter it from a worker of the executor’s own pool.

The coordinator is the protocol loop on the calling thread: it dispatches scheduler batches, keeps MPI progress, and applies the root’s immediate abort rules before anything is joined. Lanes execute the local batches with the requested LocalMode: Outer spreads a batch over the lanes, while Sequential and Inner hand the batch to one lane. When PmapOptions::prefetch is true, each remote domain may hold one additional batch so blocking task/result transfer overlaps adjacent batch execution; the root domain remains capacity one.

The per-item callback receives the lane resource of the lane that runs the item (&mut L::Lane<'_>), so a backend adapter can hand its own per-worker context, such as a tenferro child session, to the callback. pmap adapts the ordinary Fn(T) shape to RayonLanes’ unit lane.

A lanes executor that reports LanesError::SingleLane falls back explicitly to executing each batch on the coordinator thread through Lanes::run between MPI progress steps. This is the documented one-lane path; it never happens silently and never runs work twice.

Errors

Returns PmapErrorKind::Preflight when the collective preconditions fail (including entry from a Rayon worker), PmapErrorKind::User with a converged error key when a callback fails, PmapErrorKind::Wire or PmapErrorKind::Protocol for transport, protocol, or lane-executor failures, and PmapErrorKind::Reentrant when pmap is already active on this thread.