simd-matrix: key native-resolving caches by the runner's target features - #345

Problem

On #344, realization/nightly × x86_64 failed with SIGILL inside a cached proc-macro, before any test ran:

target/debug/build/paste/…/libpaste-….so(+0x3595f)
error: rustc interrupted by SIGILL

The nightly and host-native rows build under the repo's target-cpu=native default, and with no global RUSTFLAGS that also applies to host artifacts such as proc-macros. The ubuntu-latest pool is heterogeneous: the workflow's own host-native comment records an AVX-512 runner and a non-AVX-512 runner in one run. Both rows cache under a key that depends only on the job (key: nightly, or the default for host-native). A proc-macro .so compiled for one runner's CPU is therefore restored on another, and rustc dies loading it.

Fix

Both rows now hash the target features rustc resolves for native on the runner and add the hash to the cache key (rust-cache's key is added to its automatic job key):

- name: native target features (cache key)
  id: cpu
  run: echo "features=$(rustc +nightly --print cfg -C target-cpu=native | grep target_feature | sha256sum | cut -c1-16)" >> "$GITHUB_OUTPUT"
- uses: Swatinem/rust-cache@v2
  with:
    key: nightly-${{ steps.cpu.outputs.features }}

The key is exactly what decides those artifacts. Runners with the same feature set share a cache, and different ones never do.

Scope

• Changed: only the nightly and host-native rows. They are the only rows whose artifacts depend on the runner's CPU.
• native: pins v3, so its artifacts are the same on every runner.
• native-v4: pins v4, so the same holds.
• neon and wasm: they cross-compile to other targets.
• ci.yaml: its workflow-global RUSTFLAGS: -D warnings replaces the config's target-cpu, so native never applies there.

Verification

• The YAML parses.
• The key command runs locally and yields a 16-hex-digit hash over 32 resolved features.
• The fix itself can only be confirmed by a run that lands on mixed runners. This PR's own nightly row is its first exercise.

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https://claude.ai/code/session_01EFw2WdKr1oxvaKCJC2ua2R

Generated by Claude Code