[almightychang]

Closes #1211

Summary

Validate the selected llc by version plus the rustc -vV commit instead of its file hash. Keep SHA-256 as provenance. Explicit --llc comparison probes remain unchecked.

repository source identity                 per-evidence provenance
┌──────────────────────────────┐           ┌──────────────────────┐
│ upstream.lock                │           │ evidence/catalog     │
│ rustc commit (enforced)      │──────────▶│ llc SHA-256 (printed)│
└──────────────┬───────────────┘           └──────────────────────┘
               │
               ▼
      llc version + commit
          selected probe

Chose upstream.lock because one pinned Rust toolchain probes all records, and the lock's [llvm] provenance line already named this commit in prose. A per-evidence field would duplicate the commit 1025 times, back-fill existing evidence from that prose line, and churn evidence, catalog.json, and generated stamps.

The commit check sits in validate_backend_identity next to the version check, as the issue comment specified, so the listed tests exercise one function; the same expected commit is passed for every record.

Changes

• Add required [rust_toolchain] commit_hash = "e457a7b0d326d67b4322ef0d11bd715cfaeda48f".
• Parse lock and rustc -vV commits as exactly 40 lowercase hex characters.
• Check version and commit for selected probes; print the rustc commit next to the llc SHA-256, which is now provenance only.
• Fail loudly for missing or malformed toolchain metadata.
• Update the Justfile, workflow, and book upstream.lock row.
• Add no Nix CI lane, per fix(nix): advance the rust-overlay pin to the toolchain's nightly, and gate the flake in CI #1210.

Testing

Before, origin/main 596a6353d failed under Nix:

error: rust-toolchain llc SHA-256 mismatch: selected evidence records 2e5bc688625ca7d98bca9b5f0f0d14525d2eb59e8a9842964e480df3ffd1f921, found ca7af83e237af640fe337faff2eff49055988921933186ab5535b38addeb7b49; use an explicit `--llc` only for a comparison probe

After, the first selected-backend lines show the same source identity with different provenance bytes:

rustup: selected evidence backend LLVM version 23.1.0-rust-1.100.0-nightly (rustc commit e457a7b0d326d67b4322ef0d11bd715cfaeda48f; llc SHA-256 2e5bc688625ca7d98bca9b5f0f0d14525d2eb59e8a9842964e480df3ffd1f921) lowered abs_bf16x2 to `abs.bf16x2 <register>, <register>;` for selected evidence target sm_80 +ptx70
nix:   selected evidence backend LLVM version 23.1.0-rust-1.100.0-nightly (rustc commit e457a7b0d326d67b4322ef0d11bd715cfaeda48f; llc SHA-256 ca7af83e237af640fe337faff2eff49055988921933186ab5535b38addeb7b49) lowered abs_bf16x2 to `abs.bf16x2 <register>, <register>;` for selected evidence target sm_80 +ptx70

The full gate passed under rustup and nix develop:

$ cargo run -p cuda-intrinsics-gen -- probe --all --skip-terminal --per-target
probed 2374 target configurations across all 1025 generated intrinsic routes

Negative controls rejected a zeroed commit and a missing table. An explicit --llc still passed with the zeroed lock in comparison mode.

$ cargo fmt --all -- --check
pass
$ cargo clippy -p cuda-intrinsics-gen --all-targets -- -D warnings
pass
$ cargo test -p cuda-intrinsics-gen
314 passed, 0 failed
$ cargo run -p cuda-intrinsics-gen -- check
generated intrinsic outputs are current
$ sha256sum intrinsics/catalog.json
20bc41c2ab93828b7f97b6ee90efccfc118317a590803216a3a045b1c2879889  intrinsics/catalog.json

Tests cover different bytes with the same identity, commit and version mismatches, comparison mode, missing and malformed lock metadata, parse_rustc_verbose, and preflight intrinsic context.

The catalog and evidence files did not change, so the catalog stamp script was not needed. aarch64 was not tested because no machine was available.

• just check passes, run as one command inside nix develop (the shell the issue is about): exit 0 in 4m26s, all recipes including check-intrinsics and doc-check
• cargo oxide run <example> passes — not applicable to this host-side generator change

Checklist

• All commits signed off (git commit -s)
• SPDX headers on new source files — no new files

[nihalpasham]

Reviewed and validated:

• probe identity now comes from rustc -vV commit + llc version, not the binary's bytes that nixpkgs rewrites
• the old SHA-256 stays as printed provenance only
• closes just check cannot pass inside the recommended nix develop shell: the probe pins llc by SHA-256, nixpkgs rewrites it #1211, fork CI green

Merging.