perf(cache): make the result cache pay only where it helps

.cargo/config.toml

@@ -0,0 +1,14 @@
+# Build acceleration via sccache.
+#
+# `rustc-wrapper` routes every rustc invocation through sccache's shared compile
+# cache, so clean builds and branch switches reuse previously-compiled artifacts
+# (the pinned oxc / ruff git dependencies dominate build time and cache well).
+#
+# Requires `sccache` on PATH:
+#   cargo install sccache --locked   # or: brew install sccache
+#
+# CI provisions it via mozilla-actions/sccache-action. If you do not have
+# sccache installed, either install it (one command above) or override locally
+# with `CARGO_BUILD_RUSTC_WRAPPER=""` / `RUSTC_WRAPPER=""`.
+[build]
+rustc-wrapper = "sccache"

.github/workflows/ci.yaml

@@ -17,6 +17,12 @@ permissions:
 env:
   CARGO_TERM_COLOR: always
   RUST_BACKTRACE: 1
+  # sccache: .cargo/config.toml sets rustc-wrapper = "sccache", so every
+  # compiling job must have sccache on PATH. The action below installs it and
+  # these vars point it at the GitHub Actions cache backend. Incremental
+  # compilation is disabled because sccache cannot cache incremental units.
+  SCCACHE_GHA_ENABLED: "true"
+  CARGO_INCREMENTAL: 0
 
 jobs:
   fmt:
@@ -29,6 +35,7 @@ jobs:
         with:
           components: rustfmt
       - uses: Swatinem/rust-cache@v2
+      - uses: mozilla-actions/sccache-action@v0.0.9
       - run: cargo fmt --all --check
 
   clippy:
@@ -45,6 +52,7 @@ jobs:
         with:
           components: clippy
       - uses: Swatinem/rust-cache@v2
+      - uses: mozilla-actions/sccache-action@v0.0.9
       - run: cargo clippy --workspace --exclude conformance --all-targets -- -D warnings
 
   test:
@@ -59,6 +67,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
+      - uses: mozilla-actions/sccache-action@v0.0.9
       - run: cargo test --workspace --exclude conformance
 
   deny:

.github/workflows/publish.yaml

@@ -184,6 +184,12 @@ jobs:
     needs: [meta, create_release]
     if: needs.meta.outputs.release_assets_exist != 'true'
     runs-on: ${{ matrix.os }}
+    # .cargo/config.toml sets rustc-wrapper = "sccache"; native builds get
+    # sccache from the action below. The Alpine container build neutralizes the
+    # wrapper instead (see that step), since rust:alpine has no sccache.
+    env:
+      SCCACHE_GHA_ENABLED: "true"
+      CARGO_INCREMENTAL: 0
     strategy:
       fail-fast: false
       matrix:
@@ -230,6 +236,10 @@ jobs:
         with:
           key: ${{ matrix.triple }}
 
+      - name: Setup sccache
+        if: ${{ !matrix.alpine }}
+        uses: mozilla-actions/sccache-action@v0.0.9
+
       # ruby-prism-sys runs bindgen at build time and needs libclang + libc
       # headers; native Linux runners get them from apt.
       - name: Install clang for bindgen (Linux)
@@ -256,9 +266,13 @@ jobs:
           # dynamically linked (-crt-static off) to dlopen libclang. Building
           # host-native applies the flag everywhere. Then place the binary at the
           # triple path the shared packaging step expects.
+          # RUSTC_WRAPPER="" disables the sccache wrapper that .cargo/config.toml
+          # sets repo-wide; rust:alpine has no sccache, and the env var overrides
+          # the config value.
           docker run --rm -v "$PWD":/work -w /work \
             -e CARGO_TARGET_DIR=/work/target \
             -e LIBCLANG_PATH=/usr/lib \
+            -e RUSTC_WRAPPER="" \
             -e RUSTFLAGS="-C target-feature=-crt-static" \
             rust:alpine sh -c '
               set -e