lance-graph-contract: make the aarch64 NEON path actually compile

[AdaWorldAPI]

Three compiler errors, all inside #[cfg(target_arch = "aarch64")] blocks in
mul.rs, and all present since the code was written. Nothing had ever built
this crate for aarch64, so nothing had ever parsed them.

How it surfaced

AdaWorldAPI/q2#146. That fork's test suite had been gated to
if: github.repository == 'quarto-dev/q2', so it had never run there. Un-gating
it put lance-graph-contract (a path-dep of q2 via [patch]) on a macOS
aarch64 runner for the first time:

error: cannot find macro `is_aarch64_feature_detected` in this scope
  --> crates/lance-graph-contract/src/mul.rs:983:24
error[E0435]: attempt to use a non-constant value in a constant
  --> crates/lance-graph-contract/src/mul.rs:1467:52
error[E0435]: attempt to use a non-constant value in a constant
  --> crates/lance-graph-contract/src/mul.rs:1468:52

The two defects

1 · is_aarch64_feature_detected! is not at the root of std. Its x86
sibling is, which is exactly why this is easy to get wrong — the x86 arm two
lines above compiles bare. The aarch64 macro lives under std::arch. Added the
import inside the aarch64 block.

2 · vshrq_n_u64 is an _n_ intrinsic. The shift is encoded into the
instruction, so it must be a constant — but extract_dim_pair took
shift: i32 as a runtime argument. All eight call sites already passed
literals (36, 4, 8, 12, 56), so this is an exact mechanical change to a
const SHIFT: i32 parameter. No behaviour change, no redesign, no call site
semantically altered.

Verification — reproduce first, then fix

rustup target add aarch64-unknown-linux-gnu

cargo check  -p lance-graph-contract --target aarch64-unknown-linux-gnu --all-targets
  → the same 3 errors, before the change      ← the failure reproduced locally

cargo clippy -p lance-graph-contract --target aarch64-unknown-linux-gnu --all-targets -- -D warnings
  → clean, after

cargo clippy -p lance-graph-contract --all-targets -- -D warnings      # native x86_64
  → clean, unchanged

cargo test   -p lance-graph-contract        → 21 passed, 0 failed
cargo fmt    -p lance-graph-contract -- --check  → clean

The change is entirely inside aarch64-gated blocks, so x86_64 codegen is
untouched by construction as well as by measurement.

The finding worth keeping

Not either bug — the class. #[cfg(target_arch = ...)] code is unparsed,
untypechecked, uncompiled text on every machine that is not that arch:
indistinguishable from a comment. Both defects would have been caught by
the compiler the first time anything built for aarch64, and nothing ever had.

For a zero-dep crate like this one the check costs one rustup target add and
a few seconds. Whether this repo's CI should carry an aarch64 check job is
the open question this leaves — it currently does not, and the defect reached
main and stayed there.

One more detail worth recording: the discovery came from a consumer's CI, on
a platform this repo does not test, during a run whose purpose was entirely
unrelated. Un-gating a suite that has never run is a measurement, and it
returns findings that are not about the change being made.

Board: .claude/board/EPIPHANIES.md gains
E-THE-AARCH64-PATH-HAD-NEVER-BEEN-COMPILED-1 (prepended, append-only).

🤖 Generated with Claude Code

https://claude.ai/code/session_01AGVLyRZNEKKBSfBDJfbY3V

Summary by CodeRabbit

• Bug Fixes

  • Fixed AArch64 compilation issues affecting the graph contract’s NEON-accelerated operations.
  • Improved compatibility for architecture-specific builds and consumer CI checks.
  • Preserved existing NEON evaluation behavior across supported platforms.

• Tests

  • Verified compilation and behavior on both AArch64 and x86_64 targets.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Essentials

Run ID: 5c88e32f-3d94-49eb-8450-4fe50c8d05fa

📥 Commits

Reviewing files that changed from the base of the PR and between ef72487 and 422f4f4.

📒 Files selected for processing (2)

• .claude/board/EPIPHANIES.md
• crates/lance-graph-contract/src/mul.rs

---

📝 Walkthrough

Walkthrough

The AArch64 NEON implementation now imports the required feature-detection macro and passes compile-time shift values to dimension extraction. Documentation records the compiler defects, fixes, and architecture verification.

Changes

AArch64 NEON compilation

Layer / File(s)	Summary
AArch64 compilation fixes crates/lance-graph-contract/src/mul.rs, .claude/board/EPIPHANIES.md	The capability probe imports is_aarch64_feature_detected. The NEON extraction helper uses a const-generic shift. All callers provide compile-time shift values. The documented finding records the defects and verification results.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: claude

Poem

A rabbit reads each line,
The patch grows clear beneath the moon,
Small changes hop in place,
Tests guard the garden path,
Reviews bloom before the dawn.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

---

_{Comment @coderabbitai help to get the list of available commands.}

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