12. Rust Parity Status¶

Filed: 2026-07-16. Closed: 2026-07-26.

This record began as the evidence ledger for capability-surface and observable- behavior differences between Rust and the four other flavors. The Rust 0.27.0 maintenance line has now resolved every actionable item originally recorded in §12.3 and §12.4. Rust remains a source-tree flavor that has not been published to crates.io, so the coordinated public-API corrections described here are pre-publication changes.

The closure was revalidated against the current Rust implementation, its conformance and regression tests, the canonical spec, and the four-flavor consensus on 2026-07-26. All five flavors cover all 403 library IDs. The focused Rust tests cited below now assert the canonical member shape and edge behavior rather than the earlier reduced Rust surface.

Three behavioral differences had already been corrected before this focused follow-up: FormVm no longer publishes an extra PropertyChanged("is_valid") onto the main hub; HierarchicalVm no longer emits spurious PropertyChanged("parent") messages on first children() materialization; and NotificationHub::resolve / dispose publish the pending snapshot before completing the corresponding waiter. The maintenance branch also completed baseline lifecycle capabilities and full forwarding-component delegation. It also completed inline-dispatch deadlock prevention, idempotent capability predicates, and atomic container admission.

12.1. Scope And Authority¶

spec/14-capabilities.md and the per-cluster ADRs (ADR-0010, ADR-0022, ADR-0023, ADR-0057) are the authoritative capability contract. The related behavioral authority comes from the source chapters cited by each item below, the accepted ADRs, and the four-flavor consensus where the spec deliberately leaves an edge unspecified.

No spec change was needed for this closure. Rust converged on the existing contract using idiomatic snake_case methods, VmxResult<T> for recoverable failures, and VMx-owned reactive facades. No third-party reactive runtime was introduced.

12.2. Canonical-Behavior Decision¶

The spec remains canonical. The completed work aligns Rust with the public surface and behavior already implemented by the other four flavors. The changes stay on the already-open Rust 0.27.0 / spec 3.23.0 source line; this record does not introduce a package or spec version change.

12.2.1. Message Sender Identity Is An Accepted Idiom¶

Rust's sender_id: usize plus sender_name: String message fields are the one intentional flavor-specific shape retained by this review. ADR-0120 records them as the ownership-safe Rust expression of the canonical sender-identity contract. The other four flavors retain runtime sender objects, while Rust messages remain owned values that do not retain or borrow a VM. Identity filtering behavior remains required across all five flavors.

12.3. Capability And Reusable-State Closure¶

12.3.1. Selection And Expansion Opt-In (CAP-020)¶

Resolved. Rust no longer blanket-implements the six opt-in selection and expansion traits on ComponentVm. The baseline Constructable, Destructable, and Reconstructable traits apply to every VmNode, while selection and expansion remain explicit opt-ins.

Evidence: src/capabilities.rs contains the lifecycle blanket implementations; the ComponentVm: Selectable compile-fail guard is attached to ComponentVm in src/components.rs. CAP-020 asserts the positive lifecycle bounds.

12.3.2. Filterable Predicate Shape (CAP-021)¶

Resolved. Filterable<T> now exposes the filter, set_filter, and can_filter accessors for an optional shared predicate. Implementors own the predicate storage; clearing with None removes the filter.

Evidence: CAP-021 exercises predicate replacement, evaluation, and clearing, and retains the compile-negative proof that core VMs do not opt in implicitly.

12.3.3. Pageable Complete Surface (CAP-022)¶

Resolved. Pageable now exposes page size, current page index, page count, paging-enabled state, and bounded first/previous/next/last navigation. PagedComposition implements the trait and clamps its current page after source or page-size changes.

Evidence: CAP-022 drives the trait through PagedComposition and covers clamping, pass-through mode, and navigation edge no-ops.

12.3.4. Expandable State Read (CAP-004)¶

Resolved. Expandable includes is_expanded, and expanded tree walking uses the expansion capability instead of a separate reduced tree hook.

Evidence: CAP-004 asserts the state read before and after expand; HIER-012 and the focused expandable-support tests verify lazy expanded-walk boundaries.

12.3.5. Searchable Term Mutation (CAP-008)¶

Resolved. Searchable exposes set_search_term, and SearchableState implements the complete capability.

Evidence: CAP-008 performs the mutation through the trait and verifies the filtered projection through the concrete reusable helper.

12.3.6. ExpandableState Construction And Disposal¶

Resolved. ExpandableState provides collapsed, explicitly initialized, and initially-expanded construction. It implements Expandable, Collapsible, and ExpansionTogglable; dispose completes the owned hub and makes later mutation inert.

Evidence: the expandable-support suite covers construction variants, capability dispatch, completion, idempotent disposal, and post-dispose no-op behavior.

12.3.7. SearchableState Post-Dispose Read¶

Resolved. A disposed SearchableState reads as an empty term, keeps later assignments inert, releases its source subscription, and does not own the source. Explicit search after disposal is likewise inert: it returns an empty projection without invoking the item provider or predicate.

Evidence: disposed_searchable_state_reads_an_empty_term and dispose_cancels_source_observation_without_owning_source cover the ownership contract. search_after_disposal_is_inert_without_calling_user_code uses side effects at both user-code boundaries to prove neither is invoked.

12.4. Behavioral Closure¶

12.4.1. Explicit Reparent Of A Detached Child¶

Resolved. reparent_child passes an explicit-reparent intent through the attachment path. A detached child therefore reports Reparented with index -1; ordinary add of a detached child remains Added with its inserted index.

Evidence: explicit_reparent_of_detached_child_reports_reparented verifies the four-flavor consensus without changing ADR-0105's ordinary-add rule.

12.4.2. Remove Of A Non-Member¶

Resolved. Hierarchy, group, and composite removal now return a successful no-op when the supplied child is not a member. No collection, selection, parent, or message state changes.

Evidence: removing_a_non_child_is_a_noop covers hierarchy state and messages. membership_uses_node_identity_when_partial_eq_is_value_based selects a real composite member before removing an equal-valued foreign node, then proves the member, current, and current flag remain stable; it also covers group membership.

12.4.3. Aggregate Slot Notification Ordering¶

Resolved. AggregateVm1 through AggregateVm6 publish every populated slot notification before constructing any child. The emitted notification set is therefore complete even when the first child construction fails.

Evidence: all_slot_notifications_precede_first_child_failure_for_arities_one_through_six records every populated-slot notification followed by only the failing first construction attempt for each arity. The companion success-path test records all slot notifications before any construction.

12.4.4. FormVm Direct Approval¶

Resolved. Direct approve() gates only on disposal and validity. The strict-mode dirty requirement remains solely on approve-command eligibility, so a strict, valid, clean form may still be persisted explicitly.

Evidence: the form suite separates strict-clean direct approval from command can_execute behavior and preserves invalid/disposed no-op coverage.

12.4.5. Base ComponentVm Type Surface¶

Resolved. Every canonical component VM family exposes view_model_type: component and readonly variants, groups, composites and their modeled/filtered/forwarding wrappers, aggregate arities one through six, hierarchies, forms, and async-resource VMs. Builders/options preserve selected component types, and forwarding wrappers delegate the wrapped family value. The earlier selection-gate, built-in-command, and full forwarding-component delegation work remains intact.

Evidence: CVM and builder tests compare default, overridden, and readonly type values. component_variants_expose_their_view_model_type additionally proves that the configured value delegates unchanged through two and three nested ForwardingComponentVm layers. every_component_vm_family_exposes_its_canonical_view_model_type enumerates the complete family surface with literal expected discriminators.

12.4.6. DerivedProperty Source Ownership And Recompute¶

Resolved. DerivedProperty now provides one- through five-source constructors over typed replaying ValueStream sources, owns exactly one subscription per supplied source, recomputes automatically when any source changes, suppresses equal outputs, and releases every owned subscription on disposal. Concurrent source emissions carry monotonically admitted snapshot revisions through one serialized result stream; a transform computed from an older snapshot cannot overwrite or publish after the newer latest-at-emission result.

Evidence: DPROP-002 through DPROP-005 mutate real sources; DPROP-012 executes the shared scenarios; the crate-private owns_exactly_one_subscription_per_source_until_disposal test directly counts the owned registrations before and after disposal without adding a public diagnostic API. Focused conformance tests cover distinct emission, value-stream completion, post-dispose isolation, and a deterministically gated older multi-source transform that resumes only after the newer result commits. The unit-level revisioned_commit_serializes_value_and_publication regression gates the old commit-before-notification window and verifies value and both publication order surfaces remain 1, 2.

12.4.7. Collections And Commands¶

Resolved. The complete residual set now matches the canonical behavior:

composite selection clears before observable removal publication;
ObservableList::remove_at returns VmxError::InvalidArgument for an out-of-range index;
FilteredCompositeVm defaults to SnapToFirst and implements PreserveIfVisible;
ConfirmationDecoratorCommand::dispose completes errors, and confirmed or post-dispose execution cannot emit afterward; and
RelayCommand and RelayCommandOf emit the optional final can_execute_changed notification before completing their hubs.

Evidence: the collection, filtered-composite, confirmation-decorator, relay- command, and composite suites contain focused assertions for each edge. confirmation_error_value_precedes_disposal_completion and relay_disposal_emits_one_final_can_execute_notification observe terminal callbacks directly and assert value-before-completion ordering.

12.4.8. Hot-Stream Completion And Notification Replay¶

Resolved. Rust now has a typed, replaying, completion-aware VMx-owned ValueStream<T>. MessageHub::subscribe_with_completion reports terminal completion, including immediately for late and null subscriptions. NotificationHub::pending_stream replays its current snapshot, publishes each committed update in order, and completes after the terminal empty snapshot. NullNotificationHub replays empty and completes immediately.

Evidence: the value-stream suite covers initial/late replay, completion, reentrancy, serialization, and panic isolation. It also drives two streams from two drainer threads through opposing callbacks, proving cross-stream enqueue does not create a wait cycle while the ordinary foreign-thread send/dispose tests retain synchronous completion. HUB and null-service tests cover normal, late, and immediate completion. NOTIF tests cover replay, publish-before-waiter ordering, concurrent terminal delivery, and null-hub behavior.

12.4.9. Executor-Neutral Pending Async Operations¶

Resolved. AsyncValue provides executor-neutral map and and_then continuations. make_confirm and ConfirmationDecoratorCommand no longer retain one native worker thread per unresolved decision. Confirmation execution returns the Future-compatible ConfirmationExecution handle while preserving blocking join() and non-blocking is_finished() conveniences.

First-wins resolution, resolver-thread continuation execution, panic isolation, post-dispose no-op behavior, synchronous completion, and preservation of the original Box<dyn Any + Send> panic payload through join() are covered. Resource-bound assertions use retained-continuation counts rather than timing sleeps.

The pre-publication API correction is intentional: explicitly typed JoinHandle<()> callers must migrate to ConfirmationExecution; inferred .join() callers retain the same call shape. No published crates.io artifact used the earlier return type.

spec/02-lifecycle.md §7 lists the parent's terminal disposal work as "…command teardown, and stream completion," but all five flavors complete the streams before tearing down the commands. The sentence remains an editorial ordering description: no conformance ID pins this intra-parent order.

ADR-0129 corrects the lifecycle wording to match the established five-flavor order. The correction adds no behavior or conformance ID. It is not a Rust behavior difference and does not reopen this ledger.

12.6. Disposition¶

Closed on the Rust 0.27.0 / spec 3.23.0 source line. Every actionable capability, reusable-state, structural, command, reactive, and async item recorded in §12.3 and §12.4 is resolved and backed by focused Rust tests. Rust is both catalog-complete and aligned with the canonical public concepts and observable behavior covered by this review.

The ADR-0120 sender representation in §12.2.1 remains the sole intentional flavor-specific shape in this ledger. The §12.5 editorial note applies to all five flavors and is not a parity backlog. Future differences still require spec/ADR review; this completed record is evidence of the 0.27.0 convergence, not permission to add undocumented divergence.