CI: Optionally use sccache for compilation

This moves compiler caching for incremental builds away from GitLab
caching the whole target/ folder to caching each code unit in S3.
This aleviates the need to zip and unzip and just caches on the fly.

This feature is optional and gated behind the SCCACHE_BIN_URL env

.gitlab-ci.yml

@@ -26,16 +26,19 @@ variables:
   cache:
     paths:
       - cargohome
-      - target/
-    key: "build_cache--$TARGET--$CI_COMMIT_BRANCH--release"
+    key: "build_cache--$TARGET--$CI_COMMIT_BRANCH"
   variables:
     CARGO_PROFILE_RELEASE_LTO: "true"
     CARGO_PROFILE_RELEASE_CODEGEN_UNITS: "1"
+    CARGO_INCREMENTAL: "false" # https://matklad.github.io/2021/09/04/fast-rust-builds.html#ci-workflow
+    CARGO_HOME: $CI_PROJECT_DIR/cargohome
   before_script:
     - 'echo "Building for target $TARGET"'
-    - 'mkdir -p cargohome && CARGOHOME="cargohome"'
+    - "mkdir -p $CARGO_HOME"
     - "rustc --version && cargo --version && rustup show" # Print version info for debugging
     - "rustup target add $TARGET"
+    # If provided, bring in caching through sccache, which uses an external S3 endpoint to store compilation results:
+    - if [ -n "${SCCACHE_BIN_URL}" ]; then curl $SCCACHE_BIN_URL --output /sccache && chmod +x /sccache && export RUSTC_WRAPPER=/sccache; fi
   script:
     - time cargo build --target $TARGET --release
     - 'cp "target/$TARGET/release/conduit" "conduit-$TARGET"'
@@ -216,20 +219,20 @@ test:cargo:
   image: "rust:latest"
   tags: ["docker"]
   variables:
-    CARGO_HOME: "cargohome"
+    CARGO_HOME: "$CI_PROJECT_DIR/cargohome"
+    CARGO_INCREMENTAL: "false" # https://matklad.github.io/2021/09/04/fast-rust-builds.html#ci-workflow
   cache:
     paths:
-      - target
       - cargohome
-    key: test_cache
+    key: "test_cache--$CI_COMMIT_BRANCH"
   interruptible: true
   before_script:
-    - mkdir -p $CARGO_HOME && echo "using $CARGO_HOME to cache cargo deps"
+    - mkdir -p $CARGO_HOME
     - apt-get update -yqq
     - apt-get install -yqq --no-install-recommends build-essential libssl-dev pkg-config wget
     - rustup component add clippy rustfmt
-    - wget "https://faulty-storage.de/gitlab-report"
-    - chmod +x ./gitlab-report
+    # If provided, bring in caching through sccache, which uses an external S3 endpoint to store compilation results:
+    - if [ -n "${SCCACHE_BIN_URL}" ]; then curl $SCCACHE_BIN_URL --output /sccache && chmod +x /sccache && export RUSTC_WRAPPER=/sccache; fi
   script:
     - rustc --version && cargo --version # Print version info for debugging
     - cargo fmt --all -- --check