ci: cache Rust toolchain, registry and build directories

Every Rust job restores the stable toolchain, Cargo registry and its own
target directories from the runner cache. Keys combine the ISO week and
all Cargo.lock files, so dependencies are reused across pushes while
stale build output and old toolchains expire weekly. Release jobs keep
separate entries per plugin.

.gitea/actions/setup/action.yml

@@ -1,27 +1,69 @@
 name: Rust plugin toolchain
-description: Native checks and optional Windows GNU cross-compilation in Node's Debian container
+description: >-
+  Native checks and optional Windows GNU cross-compilation in Node's Debian
+  container. Restores the Rust toolchain, Cargo registry, and the caller's
+  build directories from the runner cache, and saves them after the job.
 inputs:
   windows:
     description: Install the Windows cross compiler
     default: "false"
+  cache-name:
+    description: Distinguishes build-directory caches of different jobs
+    required: true
+  target-paths:
+    description: Build directories to cache, one per line, relative to the workspace
+    default: target
 runs:
   using: composite
   steps:
+    - name: Cache keys
+      id: keys
+      shell: bash
+      run: |
+        # A new ISO week starts fresh caches, so stale build output cannot grow forever
+        # and the stable toolchain is refreshed regularly.
+        echo "week=$(date -u +%G-%V)" >> "$GITHUB_OUTPUT"
+    - name: Restore Rust toolchain and Cargo registry
+      uses: https://github.com/actions/cache@v4
+      with:
+        path: |
+          .cargo-ci/bin
+          .cargo-ci/registry/index
+          .cargo-ci/registry/cache
+          .cargo-ci/git/db
+          .rustup-ci
+        key: rust-${{ inputs.windows }}-${{ steps.keys.outputs.week }}-${{ hashFiles('Cargo.lock', 'plugins/*/plugin/Cargo.lock', 'tools/editor-check/Cargo.lock') }}
+        restore-keys: |
+          rust-${{ inputs.windows }}-${{ steps.keys.outputs.week }}-
+    - name: Restore build directories
+      uses: https://github.com/actions/cache@v4
+      with:
+        path: ${{ inputs.target-paths }}
+        # One entry per job, week, and dependency set: compiled dependencies are
+        # reused, and only this repository's crates rebuild on each push.
+        key: build-${{ inputs.cache-name }}-${{ steps.keys.outputs.week }}-${{ hashFiles('Cargo.lock', 'plugins/*/plugin/Cargo.lock', 'tools/editor-check/Cargo.lock') }}
+        restore-keys: |
+          build-${{ inputs.cache-name }}-${{ steps.keys.outputs.week }}-
     - name: Install build prerequisites
       shell: bash
       run: |
         apt-get update
         apt-get install -y --no-install-recommends python3 curl ca-certificates pkg-config libgl1-mesa-dev libx11-dev libxcursor-dev libx11-xcb-dev libxcb1-dev libxcb-dri2-0-dev libxcb-icccm4-dev libxkbcommon-x11-dev
+        if [ "${{ inputs.windows }}" = "true" ]; then
+          apt-get install -y --no-install-recommends gcc-mingw-w64-x86-64
+        fi
     - name: Install Rust
       shell: bash
       run: |
-        curl --proto '=https' --tlsv1.2 -fsSL https://sh.rustup.rs -o /tmp/rustup-init.sh
-        sh /tmp/rustup-init.sh -y --profile minimal --default-toolchain stable
+        if [ ! -x "$CARGO_HOME/bin/rustup" ]; then
+          curl --proto '=https' --tlsv1.2 -fsSL https://sh.rustup.rs -o /tmp/rustup-init.sh
+          sh /tmp/rustup-init.sh -y --profile minimal --default-toolchain none --no-modify-path
+        fi
         echo "$CARGO_HOME/bin" >> "$GITHUB_PATH"
-        "$CARGO_HOME/bin/rustup" component add rustfmt clippy
-    - name: Install Windows target
-      if: inputs.windows == 'true'
-      shell: bash
-      run: |
-        apt-get install -y --no-install-recommends gcc-mingw-w64-x86-64
-        rustup target add x86_64-pc-windows-gnu
+        # No-op when the restored toolchain is already current.
+        "$CARGO_HOME/bin/rustup" toolchain install stable --profile minimal -c rustfmt -c clippy
+        "$CARGO_HOME/bin/rustup" default stable
+        if [ "${{ inputs.windows }}" = "true" ]; then
+          "$CARGO_HOME/bin/rustup" target add x86_64-pc-windows-gnu
+        fi
+        "$CARGO_HOME/bin/rustc" --version

.gitea/workflows/check.yml

@@ -10,7 +10,10 @@ permissions:
 
 env:
   CARGO_HOME: ${{ gitea.workspace }}/.cargo-ci
+  RUSTUP_HOME: ${{ gitea.workspace }}/.rustup-ci
   CARGO_TERM_COLOR: always
+  # Incremental data is not reused between CI runs; it only inflates the cache.
+  CARGO_INCREMENTAL: "0"
 
 jobs:
   check:
@@ -26,6 +29,11 @@ jobs:
           token: ${{ secrets.GITEA_TOKEN }}
           persist-credentials: false
       - uses: ./.gitea/actions/setup
+        with:
+          cache-name: check-${{ matrix.plugin }}
+          target-paths: |
+            target
+            plugins/${{ matrix.plugin }}/plugin/target
       - run: python3 -m unittest discover -s scripts/tests -v
       - run: python3 scripts/check-plugin.py --plugin "${{ matrix.plugin }}"
 
@@ -38,6 +46,9 @@ jobs:
           token: ${{ secrets.GITEA_TOKEN }}
           persist-credentials: false
       - uses: ./.gitea/actions/setup
+        with:
+          cache-name: previews
+          target-paths: tools/editor-check/target
       - run: |
           cargo fmt --all -- --check
           cargo fmt --manifest-path tools/editor-check/Cargo.toml -- --check
@@ -65,6 +76,8 @@ jobs:
       - uses: ./.gitea/actions/setup
         with:
           windows: "true"
+          cache-name: windows-preview
+          target-paths: target/windows
       - run: |
           python3 scripts/build-plugin.py --plugin rivet --target x86_64-pc-windows-gnu
           python3 scripts/build-plugin.py --plugin patina --target x86_64-pc-windows-gnu

.gitea/workflows/release.yml

@@ -8,7 +8,9 @@ permissions:
 
 env:
   CARGO_HOME: ${{ gitea.workspace }}/.cargo-ci
+  RUSTUP_HOME: ${{ gitea.workspace }}/.rustup-ci
   CARGO_TERM_COLOR: always
+  CARGO_INCREMENTAL: "0"
   RELEASE_REF: ${{ gitea.ref }}
 
 jobs:
@@ -44,6 +46,10 @@ jobs:
       - uses: ./.gitea/actions/setup
         with:
           windows: "true"
+          # CARGO_TARGET_DIR holds both the native checks and the Windows build.
+          # Kept apart from branch builds; a release never reuses preview output.
+          cache-name: release-${{ needs.plan.outputs.plugin }}
+          target-paths: target/windows
       - run: |
           python3 -m unittest discover -s scripts/tests -v
           python3 scripts/check-plugin.py --plugin "$PLUGIN"

docs/releases.md

@@ -64,6 +64,24 @@ Node runtime, and a local composite action installs Rust and build dependencies.
 Windows binaries are cross-compiled with MinGW; no Windows runner or Linux release
 binary is required.
 
+### Build cache
+
+Every Rust job restores two caches through the runner's cache server
+(`actions/cache@v4`, run from the setup composite action):
+
+- **Toolchain:** rustup's stable toolchain, Cargo's binaries, and the registry and
+  git sources, keyed by ISO week, Windows target, and all three `Cargo.lock` files.
+- **Build directories:** each job's target directories (check per plugin,
+  previews, Windows preview, release per plugin), keyed the same way.
+
+A new week or a changed lockfile starts a fresh entry, so stale build output does
+not accumulate and the stable toolchain refreshes weekly. Within a week, pushes
+reuse compiled dependencies and rebuild only this repository's crates. Release
+jobs use their own entries and never reuse branch build output. A cache miss only
+costs time. The runner's cache server must be enabled (`cache.enabled` in
+`act_runner`'s config, the default) and must support post steps of actions
+inside composite actions (current `act_runner` releases do).
+
 The workflows use explicit action URLs and the v3-compatible artifact protocol.
 Only the publish job requests `contents: write`. It authenticates against the
 same Gitea instance using the automatic `secrets.GITEA_TOKEN`; no personal access