feat: Add Push::size_hint, VecPush terminal operator, use in dfir codegen [ci-bench]

[MingweiSamuel]

Added size_hint(self: Pin<&mut Self>, hint: (usize, Option<usize>)) to the Push trait as the push-side analog of Pull::size_hint. This allows producers to announce how many items they're about to send, enabling downstream operators and sinks to pre-allocate.

New terminal operator:

• VecPush<Buf>: pushes items into a Vec, uses size_hint to call Vec::reserve(hint.0) for pre-allocation. Gated on alloc feature.
• Constructor: push::vec_push(buf) creates a VecPush from a &mut Vec<T>.

BREAKING CHANGE: Updates the Handoff trait to allow pushing to &mut Vec<T> directly instead of one-by-one, and allows pre-allocating.

[github-actions]

📊 Benchmark Results

✅ Benchmark completed! You can download the results from the links below.

Run History:

• Run #3540 - ✅ Complete (Download Artifact)
• Run #3535 - ✅ Complete (Download Artifact)
• Run #3534 - ❌ Cancelled
• Run #3527 - ✅ Complete (Download Artifact)
• Run #3526 - ✅ Complete (Download Artifact)

_{Last updated: 2026-03-24T18:48:16.229Z}

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[MingweiSamuel]

Shows some small performance improvements. A few performance decreases within noise.

[Copilot]

Pull request overview

Adds push-side size hints and a new Vec-collecting terminal push operator to enable downstream pre-allocation, and updates DFIR codegen/runtime plumbing to take advantage of the new bulk-push path.

Changes:

• Add Push::size_hint and propagate it through push combinators/sinks, plus new alloc-gated VecPush/push::vec_push.
• Update DFIR runtime Handoff / SendCtx APIs to expose a “borrow mutable inner for giving” path used by codegen.
• Update macros/tests and various pulls to account for the new size-hint requirements and type changes.

Reviewed changes

Copilot reviewed 33 out of 33 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
dfir_rs/tests/compile-fail-stable/surface_demuxenum_wrongfields_2.stderr	Update expected diagnostics due to new VecPush type appearing in trait impl lists.
dfir_rs/tests/compile-fail-stable/surface_demuxenum_wrongfields_1.stderr	Same: refresh compile-fail expectations after push API expansion.
dfir_rs/tests/compile-fail-stable/surface_demuxenum_wrongenum.stderr	Same: refresh stderr golden output for changed trait impl candidates.
dfir_rs/src/util/demux_enum.rs	Extend DemuxEnumPush with a size_hint hook for derived demux pushes.
dfir_rs/src/scheduled/port.rs	Add SendCtx::borrow_mut_give() to support bulk writes into handoffs.
dfir_rs/src/scheduled/handoff/vector.rs	Implement borrow_mut_give for VecHandoff by borrowing the input buffer mutably.
dfir_rs/src/scheduled/handoff/tee.rs	Add borrow_mut_give stub for TeeingHandoff (currently todo!()).
dfir_rs/src/scheduled/handoff/mod.rs	Add borrow_mut_give requirement to the Handoff trait.
dfir_rs/src/compiled/push/demux_enum.rs	Forward Push::size_hint into DemuxEnumPush::size_hint.
dfir_rs/src/compiled/pull/lattice_bimorphism.rs	Implement required Pull::size_hint (returns (0, None) due to UDF dependence).
dfir_pipes/src/push/vec_push.rs	New alloc-gated VecPush terminal operator collecting into Vec and reserving via size_hint.
dfir_pipes/src/push/unzip.rs	Propagate size_hint to both downstream pushes.
dfir_pipes/src/push/test_utils.rs	Record size_hint calls in TestPush history for testing/validation.
dfir_pipes/src/push/sink.rs	Add no-op size_hint implementation for Sink.
dfir_pipes/src/push/resolve_futures.rs	Forward size_hint downstream (1 future → 1 output).
dfir_pipes/src/push/persist.rs	Reserve internal buffer and forward size_hint downstream.
dfir_pipes/src/push/mod.rs	Add Push::size_hint; export alloc-gated VecPush + push::vec_push constructor.
dfir_pipes/src/push/map.rs	Forward size_hint unchanged.
dfir_pipes/src/push/inspect.rs	Forward size_hint unchanged.
dfir_pipes/src/push/for_each.rs	Add no-op size_hint (terminal side-effecting push).
dfir_pipes/src/push/flatten.rs	Conservative size_hint propagation (forces (0, None)).
dfir_pipes/src/push/flat_map.rs	Conservative size_hint propagation (forces (0, None)).
dfir_pipes/src/push/filter_map.rs	Adjust size_hint lower bound to 0 and forward upper bound.
dfir_pipes/src/push/filter.rs	Adjust size_hint lower bound to 0 and forward upper bound.
dfir_pipes/src/push/fanout.rs	Propagate size_hint to both downstream pushes.
dfir_pipes/src/push/demux_var.rs	Add size_hint to variadic demux and propagate recursively.
dfir_pipes/src/pull/symmetric_hash_join.rs	Refactor new-tick path to pull::iter and add size_hint impls returning (0, None) (TODO for estimates).
dfir_pipes/src/pull/poll_fn.rs	Implement required Pull::size_hint (UDF-dependent, returns (0, None)).
dfir_pipes/src/pull/mod.rs	Make Pull::size_hint required (remove default impl) to prevent accidental omission.
dfir_pipes/src/pull/from_fn.rs	Implement required Pull::size_hint (UDF-dependent, returns (0, None)).
dfir_pipes/src/pull/cross_singleton.rs	Implement size_hint with singleton-state-aware lower-bound adjustment.
dfir_macro/src/lib.rs	Update demux enum derive output to use Pin::as_mut and generate size_hint plumbing.
dfir_lang/src/graph/meta_graph.rs	Switch send-port codegen from per-item give(Some(v)) to borrowing the inner vec and using vec_push.

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[MingweiSamuel]

Looks good