Feature f004 — Engine parity: both fidelity backends default-on and runtime-switchable, from the zoosky forks

Status: Superseded by yqr-m005 (2026-07-10) — yqr consolidated on a single engine (noyalib); the two-backend, runtime-switchable engine-parity story no longer holds. The --engine flag itself was removed in yqr-f011 (2026-07-28). (Was: Done.) Owner: yqr maintainers Last updated: 2026-07-08 Implements: yqr-a001 §4 (fidelity engine seam), consumes yqr-b002 2.2-2.7 Related: yqr-f002 (the seam + noyalib backend), yqr-f003 (backend A over the rust-yaml fork), yqr-b002 (the noyalib fixes this consumes), yqr-m002 §7.2 (the noyalib adapter)

1. Summary

Bring the two fidelity engines to parity and make --engine a genuine runtime switch:

  • Both backends ship in the default build. default = ["backend-noyalib", "backend-rust-yaml"] in Cargo.toml, so a single cargo build produces one binary that answers both --engine noyalib and --engine rust-yaml with no rebuild. --no-default-features still yields a minimal binary that carries neither backend (the classic re-serializing pipeline is unchanged, and --engine reports the backend as unavailable).
  • Engine sourcing. rust-yaml-rt tracks zoosky/rust-yaml feat/roundtrip-document (f003). noyalib originally tracked zoosky/noyalib feat/fidelity-span-fixes — an integration branch off the 0.0.13 release carrying the submitted upstream fixes for deficiencies 2.2-2.7 (yqr-b002) — so yqr could ship the fixed fidelity engine ahead of an upstream release. Once those fixes shipped in noyalib 0.0.14, yqr re-pinned to the crates.io release (noyalib = "0.0.14", 2026-07-10; see yqr-m004).
  • The noyalib adapter consumes the fixes. The guards that previously degraded to Synthetic now resolve to Found where the fork made the span faithful, and the redundant cross-check is removed (see §3).

No CLI surface changes: --engine <name> and its diagnostics are exactly as f002/f003 shipped them; the only observable change is that both names now work in the stock binary and the noyalib path emits more nodes verbatim.

2. Why default-on

f002/f003 gated each backend off by default to keep the stock build dependency-minimal. That made --engine a build-time choice: a binary built without a feature answered the flag with “not available in this build”. Parity and runtime switching require both backends compiled into one artifact, so the default feature set now enables both. Users who want the minimal build opt out with --no-default-features; the classic pipeline never depended on either backend and is untouched either way.

3. Adapter deltas consumed from the noyalib fork (b002 2.2-2.7)

The noyalib adapter (src/fidelity/noyalib.rs) re-verifies every candidate slice by re-parsing it against the typed value, so adopting the fixed engine can only upgrade Synthetic → Found, never emit a wrong node. The observable changes:

  • 2.3 keep-chomped block scalars — |+ / >+ spans now include the kept trailing blank lines, so the slice re-parses to the full value and is emitted verbatim (Found) instead of degrading.
  • 2.6 alias resolve-through — an alias reference resolves to the anchor value’s span, so *anc emits the anchor’s original bytes (Found).
  • 2.4 block-collection line-start — a nested block collection’s span starts at its first line’s indent; the adapter’s line-start extension becomes a fallback rather than the routine path.
  • 2.5 key-collision diagnostic — the fork’s loader raises Error::KeyCollision on the parse_stream path, so distinct string-colliding keys are refused at parse time. The adapter’s rust-yaml entry-count cross-check in open() (and its entry_counts_diverge helper) is removed as redundant.
  • 2.2 implicit-null → no span and 2.7 lone-CR line break — span_at returns no span for byte-less implicit nulls (still Synthetic), and a classic-Mac CR-only stream now scans and round-trips.

4. Acceptance criteria

  • default feature set enables both backend-noyalib and backend-rust-yaml; cargo build (no flags) produces a binary where both --engine noyalib and --engine rust-yaml work at runtime.
  • --no-default-features builds cleanly; --engine <name> reports the backend unavailable; the classic pipeline is unchanged.
  • noyalib is pinned to a source carrying b002 2.2-2.7 (at f004: zoosky/noyalib feat/fidelity-span-fixes; re-pinned to the crates.io noyalib = "0.0.14" release on 2026-07-10, see yqr-m004); rust-yaml-rt remains pinned to zoosky/rust-yaml feat/roundtrip-document.
  • The three adapter tests that flip with the fixed engine assert Found (keep-chomped, alias) or the line-start-inclusive bytes (duplicate collection), and the lone-CR case round-trips.
  • cargo fmt, cargo clippy -- -D warnings across the default, --no-default-features, backend-noyalib-only, and backend-rust-yaml-only profiles, cargo test (default and --no-default-features), the backend-noyalib fidelity harness, and cargo bench --no-run all pass.