CI: build faster and stop running out of disk and cache (#1075)

- Debug/test builds use line-tables-only debug info: the workspace test
  build is 1.9x faster (509 s -> 271 s, measured on Windows) and its
  target dir 31 GB -> 13 GB, which also fixes the Linux job's craft-fonts
  build running out of disk.
- The corpus job's --release build drops thin LTO and the single codegen
  unit: 2.7x faster to build (744 s -> 273 s); it was 23.5 of 37 minutes.
- Only main saves rust-cache entries (PRs restore main's), with
  cache-on-failure, so per-PR copies no longer evict main's caches from
  the 10 GB budget.
- The Linux job deletes unused image toolchains in the background.

Co-authored-by: Claude Opus 5.5 <[email protected]>

.github/workflows/ci.yml

@@ -17,6 +17,10 @@ on:
 env:
   CARGO_TERM_COLOR: always
   RUST_BACKTRACE: 1
+  # Debug and test builds keep line tables (file:line in panic backtraces) but drop the rest of the
+  # debug info: faster codegen and links (Windows PDBs most of all), and a target dir small enough
+  # for the Linux job's second, craft-fonts build (it ran out of disk with full debug info).
+  CARGO_PROFILE_DEV_DEBUG: line-tables-only
 
 # A newer push to a PR branch cancels its older run. Runs on main are never cancelled: a merge
 # that lands while main's run is in progress waits for it, so every main state that gets a run is
@@ -66,9 +70,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # Unused toolchains in the image (Android SDK, .NET, GHC, CodeQL) are deleted in the
+      # background: the space is free before the craft-fonts build needs it, and nothing waits.
       - name: Install Linux system dependencies (egui/winit/rfd)
         if: runner.os == 'Linux'
         run: |
+          nohup sudo rm -rf /usr/local/lib/android /usr/share/dotnet /opt/ghc /usr/local/.ghcup \
+            /opt/hostedtoolcache/CodeQL </dev/null >/dev/null 2>&1 &
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends \
             libxkbcommon-dev libwayland-dev libx11-dev libxrandr-dev libxi-dev \
@@ -78,7 +86,13 @@ jobs:
         with:
           components: clippy, rustfmt
 
+      # Only main saves caches; PRs restore main's. Every PR used to save its own set (up to
+      # 3.8 GB), usable by that PR only, so the repo's 10 GB cache budget kept evicting main's.
+      # A failing test still saves: the cache holds compiled dependencies, not test results.
       - uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.ref == 'refs/heads/main' }}
+          cache-on-failure: true
 
       - name: Format
         if: runner.os == 'Linux'
@@ -133,12 +147,21 @@ jobs:
     # pin or manifest does, so normally this restores in seconds (fork PRs read the base cache).
     name: corpus tests
     runs-on: ubuntu-latest
+    env:
+      # test-corpus builds with --release, but here without the release profile's thin LTO and
+      # single codegen unit: they ran LTO over the dependency graph for each of the ~95 corpus test
+      # binaries, so building took 23.5 of the job's 37 minutes while the tests ran for 1.5.
+      # Turning them off builds 2.7x faster; the code under test is the same.
+      CARGO_PROFILE_RELEASE_LTO: "false"
+      CARGO_PROFILE_RELEASE_CODEGEN_UNITS: "16"
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
         with:
           key: corpus
+          save-if: ${{ github.ref == 'refs/heads/main' }}
+          cache-on-failure: true
       - name: Restore test corpora
         uses: actions/cache@v4
         with:
@@ -160,6 +183,8 @@ jobs:
       - uses: Swatinem/rust-cache@v2
         with:
           key: wasm32
+          save-if: ${{ github.ref == 'refs/heads/main' }}
+          cache-on-failure: true
       - name: Check wasm-safe crates
         run: cargo xtask wasm