chore(fff-core): use as_chunks in flush_seen (clippy 1.98)

[gustav-fff]

No issue. Unblocks CI on #807 and every other open PR.

Root cause

cargo clippy --no-default-features --features zlob -- -D warnings fails on main:

error: using `chunks_exact` with a constant chunk size
   --> crates/fff-core/src/index/bigram_filter.rs:267:34
267 |         for (blk, block) in seen.chunks_exact(8).enumerate() {
    |                                  ^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<8>().0.iter()`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.98.0/index.html#chunks_exact_to_as_chunks
error: could not compile `fff-search` (lib) due to 1 previous error

chunks_exact_to_as_chunks is new in clippy 1.98. Stable rolled to 1.98.0 (88d9e12ae) on 2026-08-18; rust-toolchain.toml tracks channel = "stable" and the clippy job uses dtolnay/rust-toolchain@master with toolchain: stable, so every run since the roll fails. The line itself is from a487120 (#676, 2026-07-14) and was clean under 1.92.

Fix

One line in flush_seen: seen.chunks_exact(8) -> seen.as_chunks::<8>().0.iter(). SEEN_WORDS = 1024 is a multiple of 8, so the remainder slice is always empty and iteration is unchanged. Block type becomes &[u64; 8] instead of &[u64], which gives the vectorizer a static length for the OR-test fold — same or better codegen, no behavior change.

Steps to reproduce

git checkout main
rustup toolchain install 1.98.0 --component clippy --profile minimal
cargo +1.98.0 clippy --no-default-features --features zlob -- -D warnings

Expected: clean. Actual on main: the error above, exit 101. Under 1.92.0 it is clean, which is why nothing caught it earlier.

How verified

cargo +1.98.0 clippy --no-default-features --features zlob -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.51s

That is the full CI command and it is the only clippy job; no other 1.98 lint fires on the workspace.

cargo test -p fff-search --no-default-features --features zlob --lib
test result: ok. 159 passed; 0 failed
cargo fmt -- --check   # clean

Automated triage via Gustav. Honk-Honk 🪿

Summary by CodeRabbit

• Performance
  • Improved internal processing efficiency when flushing tracked bigrams, with no change to observable search behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 5b53b04d-b006-41a5-82a7-0c283c8ea39f

📥 Commits

Reviewing files that changed from the base of the PR and between d5b4abd and dedc0c8.

📒 Files selected for processing (1)

• crates/fff-core/src/index/bigram_filter.rs

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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📝 Walkthrough

Walkthrough

flush_seen now uses as_chunks::<8>() to scan the fixed-size bitmap. Bigram flushing remains unchanged.

Changes

Bigram filter bitmap scan

Layer / File(s)	Summary
Compile-time bitmap chunking crates/fff-core/src/index/bigram_filter.rs	flush_seen replaces chunks_exact(8) with as_chunks::<8>(). Existing bitmap scanning and bigram flushing remain unchanged.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to dedc0

This localized lint-compatibility change preserves behavior and has passed the stated checks; no actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the flush_seen change and its Clippy-related purpose.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch triage-bot/clippy-as-chunks

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