book-contracts.yml: delete the three rust-cache steps — cache-bin false was never the fix (PMAT-3539)

[noahgift]

Closes #3539. Ticket PMAT-3539. Part of the fleet-wide fix for paiml/infra#775; the infra-side guard is paiml/infra#778 (merged) and the call-site PRs go out alongside it so the nightly shared-cargo-home lane is never unpassable.

What was wrong

chapter-examples-compile, chapter-examples-run and book-integration-tests all run on [self-hosted, Linux, X64, clean-room] and carried Swatinem/rust-cache@v2 with cache-bin: "false". Each step's own comment explained the reasoning — the post step prunes ~/.cargo/bin, all 16 listeners share one $CARGO_HOME, and that input prevents it (infra#208, #320, #381).

It skips cleanBin and nothing else. The save step also runs cleanRegistry, unconditionally, deleting ${CARGO_HOME}/registry/src — the extracted crate sources every OTHER job on the box is compiling from at that moment.

Measured on intel 2026-09-19: eight rust-cache save steps in 100 minutes; every crate directory under the shared registry with a birth time pinned to the second one of them finished; an infra cargo kani build killed mid-crate with could not parse/generate dep info at .../serde_core-<hash>.d: No such file or directory.

This is aprender#2822's message, and the reaper is not the only cause of it

aprender#2822 attributes could not parse/generate dep info … No such file or directory to the ci-reaper. In the measured case the reaper had last run 27 minutes earlier, and the run after it logged swept=0. rust-cache is a second, independent cause of the same message.

The replacement comments say so at each site, because the practical consequence is a debugging rule: before naming the reaper, check the clock —

journalctl -u ci-reaper.service --since -2h
grep -l 'Post Run Swatinem/rust-cache' ~/data/actions-runner-*/_diag/Worker_*.log

— and check whether the failing thing compiles at all (a step that touches no crate sources cannot be hit by this mechanism).

Change

The three steps are deleted, not reconfigured: on a persistent runner ~/.cargo persists between jobs by itself, so the restore is a no-op and the save is the entire effect. Only a job-private CARGO_HOME would make the action safe — --target-dir, cargo install --root and a fresh $HOME do not, because cargo reads crate sources from $CARGO_HOME whatever the target dir is.

Measured

paiml/infra main's machines/clean-room/shared-home-guard/check.sh over the changed file prints no VIOLATION (it printed three before), and sweeping every other aprender workflow through the same guard produces no BLOCK anywhere else. grep -c 'uses: Swatinem/rust-cache' on book-contracts.yml: 3 → 0. No other file changes.

🤖 Generated with Claude Code

---

keep-open: #2822 is cited as evidence, not fixed here. This PR removes the last rust-cache site in aprender, which is ONE of that issue's causes — the same dep-info error also arrives from the ci-reaper's keep-N purge deleting a live build tree (infra, _reaper_unit_is_live) and from other repos' rust-cache saves still deleting the shared registry/src on the same host. #2822 closes when a run of aprender CI produces the error with none of those three present, which is a measurement nobody has taken yet.

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=3540 head=4b9e20d5376963f11c58720ead8f40205999ce41 verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "dbfb772b6566746cad86a27cdc2ba5210db53e74",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "FAIL",
   "findings": 3
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "39d5dc0c206eb9233b521842745128309dadd3e7",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 2
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "626c83dd447bdd5dfe06187fed49e0e8aa01dcef",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "NO-VERDICT",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "FAIL",
   "findings": 2
  },
  {
   "lane": 3,
   "verdict": "FAIL",
   "findings": 2
  }
 ]
}

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "92babaf36e486fcecd0f2a13d23aedb99e326eb4",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 4
  },
  {
   "lane": 2,
   "verdict": "NO-VERDICT",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "92babaf36e486fcecd0f2a13d23aedb99e326eb4",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "53478d5b8601a7b1b56849bb8775e1c66f395f26",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "FAIL",
   "findings": 1
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3539",
 "head": "21d77dee545386f022c2faa973861700c79b9be7",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}