CI: main and merge-queue runs share the work with the self-hosted runner

Clippy, the doc tests and test shards 1-2 of pushes to main and merge-queue
groups run on the self-hosted runner, alongside GitHub's runners for
everything else. Pull requests, the nightly run and Run workflow stay on
GitHub's runners. Clippy and the doc tests keep warm build directories
outside the checkout (scripts/ci/warm-target.sh).

The test archive is still built on GitHub's runners: the self-hosted
machine has a newer glibc, so binaries built there don't start on
ubuntu-24.04.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.github/workflows/ci.yml

@@ -8,6 +8,13 @@
 # Tests are built once and run in 4 shards. A few heavy tests (see .config/nextest.toml) are
 # skipped on pull requests and run on pushes to main and nightly.
 # Cross-compilation is opt-in through Run workflow; release builds remain in release.yml.
+# Pushes to main and merge-queue groups use both pools: Clippy, the doc tests and test shards
+# 1-2 run on the self-hosted runner (`self-hosted-fast`, warm build directories via
+# scripts/ci/warm-target.sh), everything else on GitHub's runners. The test archive is built on
+# GitHub's runners: the self-hosted machine has a newer glibc, so its binaries wouldn't start
+# on ubuntu-24.04, while the archive runs there unchanged (same /home/runner/work checkout
+# path). Pull requests, the nightly run and Run workflow use GitHub's runners only; the
+# self-hosted runner refuses any job that isn't a push to main or a merge-queue group.
 name: CI
 
 on:
@@ -134,14 +141,19 @@ jobs:
     name: Native Linux Clippy
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    runs-on: ${{ (github.event_name == 'merge_group' || (github.event_name == 'push' && github.ref == 'refs/heads/main')) && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Warm build directory (self-hosted runner)
+        if: runner.environment == 'self-hosted'
+        run: scripts/ci/warm-target.sh clippy
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy
-      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+      # The self-hosted runner keeps its build directory and Cargo home instead.
+      - if: runner.environment == 'github-hosted'
+        uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
         with:
           key: native-clippy
       - name: Clippy
@@ -200,7 +212,7 @@ jobs:
   tests:
     name: Native Linux tests (${{ matrix.shard }}/4)
     needs: [changes, build-tests]
-    runs-on: ubuntu-24.04
+    runs-on: ${{ ((github.event_name == 'merge_group' || (github.event_name == 'push' && github.ref == 'refs/heads/main')) && matrix.shard <= 2) && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 60
     strategy:
       fail-fast: false
@@ -356,7 +368,7 @@ jobs:
     name: Doc tests
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    runs-on: ${{ (github.event_name == 'merge_group' || (github.event_name == 'push' && github.ref == 'refs/heads/main')) && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 45
     env:
       CARGO_INCREMENTAL: "0"
@@ -365,8 +377,13 @@ jobs:
       SCCACHE_IGNORE_SERVER_IO_ERROR: "1"
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Warm build directory (self-hosted runner)
+        if: runner.environment == 'self-hosted'
+        run: scripts/ci/warm-target.sh doc-tests
       - uses: dtolnay/rust-toolchain@stable
-      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+      # The self-hosted runner keeps its build directory and Cargo home instead.
+      - if: runner.environment == 'github-hosted'
+        uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
         with:
           key: doc-tests
       - name: Set up compiler cache

scripts/ci/warm-target.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Self-hosted runner only: keep a job's Cargo build directory between runs, outside the
+# checkout (actions/checkout's clean would delete it), and point CARGO_TARGET_DIR at it for the
+# rest of the job. Usage: warm-target.sh NAME
+# Each self-hosted runner has its own /home/runner, so two never share a directory.
+set -euo pipefail
+name=${1:?usage: warm-target.sh NAME}
+dir=/home/runner/work/cargo-target/$name
+# Start over when it has grown past 40 GB: old dependency versions pile up.
+if [ -d "$dir" ] && [ "$(du -sm "$dir" | cut -f1)" -gt 40000 ]; then
+  echo "warm-target: $dir is over 40 GB, starting a fresh build"
+  rm -rf "$dir"
+fi
+mkdir -p "$dir"
+echo "CARGO_TARGET_DIR=$dir" >> "$GITHUB_ENV"
+echo "warm-target: $dir ($(du -sh "$dir" | cut -f1))"