[CI] Check required-features of examples in parallel

.github/workflows/ci.yml

@@ -64,8 +64,6 @@ jobs:
       uses: actions/checkout@v4
 
     # Set up the Rust toolchain.
-    #
-    # Disable the cache since it's not relevant for formatting.
     - name: Set up Rust
       uses: actions-rust-lang/setup-rust-toolchain@v1
       with:
@@ -100,6 +98,30 @@ jobs:
         msrv=`cargo metadata --no-deps --format-version 1 | jq -r '.packages[]|select(.name=="domain")|.rust_version'`
         echo "msrv=$msrv" >> "$GITHUB_OUTPUT"
 
+  # Determine Examples
+  # ------------------
+  #
+  # Checks need to be run for every example, and they are generated into a
+  # strategy matrix so that they can run in parallel.
+  determine-examples:
+    name: Determine Examples
+    runs-on: ubuntu-latest
+    outputs:
+      examples: ${{ steps.determine-examples.outputs.examples }}
+    steps:
+
+    # Load the repository.
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    # Determine the examples.
+    - name: Determine examples
+      id: determine-examples
+      run: |
+        examples=`cargo metadata --no-deps --format-version 1 \
+          | jq -c '[.packages[].targets[]|select(.kind|any(.=="example"))|{example:.name,features:(.["required-features"]|join(","))}]'`
+        echo "examples=$examples" >> "$GITHUB_OUTPUT"
+
   # Check Feature Flags
   # -------------------
   #
@@ -109,6 +131,15 @@ jobs:
   check-feature-flags:
     name: Check feature flags
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        features:
+          - ""
+          - "unstable-crypto-sign"
+          - "ring"
+          - "openssl"
+          - "unstable-crypto-sign,ring"
+          - "unstable-crypto-sign,openssl"
     steps:
 
     # Load the repository.
@@ -135,24 +166,52 @@ jobs:
         key: ${{ runner.os }}-${{ steps.setup-rust.outputs.cachekey }}
 
     # Do the actual feature flag checks.
-    #
-    # NOTE: This does not benefit from the 'target' folder cached by a 'cargo
-    # check --all-features --all-targets' execution.  Due to the minimal
-    # dependency set, it still runs fairly quickly.
-    - name: Check empty feature set
-      run: cargo check --all-targets --no-default-features
+    - name: Check features "${{ matrix.features }}"
+      run: cargo check --all-targets --no-default-features --features=${{ matrix.features }}
 
-    # Check the required feature flags for every example.
-    - name: Check required features of examples
-      run: |
-        # Scrape crate metadata and construct the right 'check' commands.
-        # Cargo deosn't have an option to select the right features for us.
-        # See: https://github.com/rust-lang/cargo/issues/4663
-        cargo metadata --no-deps --format-version 1 \
-        | jq -r '.packages[].targets[]|select(.kind|any(.=="example"))|{name,features:(.["required-features"]|join(","))}|"\(.name) \(.features)"' \
-        | while read -r name features; do
-          cargo check --example=$name --no-default-features --features=$features
-        done
+  # Check Examples
+  # --------------
+  #
+  # While other jobs are already checking that examples compile with the full
+  # feature set, this will check that the 'required-features' specified for
+  # every example is also correct.
+  check-examples:
+    name: Check examples
+    runs-on: ubuntu-latest
+    needs: determine-examples
+    strategy:
+      matrix:
+        example: []
+        features: []
+        include: ${{ fromJSON(needs.determine-examples.outputs.examples) }}
+    steps:
+
+    # Load the repository.
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    # Set up the Rust toolchain.
+    - name: Set up Rust
+      id: setup-rust
+      uses: actions-rust-lang/setup-rust-toolchain@v1
+      with:
+        toolchain: stable
+        cache: false
+
+    # Restore a cache of dependencies and 'target'.
+    - name: Restore a dependency cache
+      id: cache-restore
+      uses: actions/cache/restore@v4
+      with:
+        path: |
+          ~/.cargo
+          target/
+        # Cache by OS and Rust version.
+        key: ${{ runner.os }}-${{ steps.setup-rust.outputs.cachekey }}
+
+    # Check examples against their minimal feature flags.
+    - name: Check example "${{ matrix.example }}"
+      run: cargo check --example=${{ matrix.example }} --no-default-features --features=${{ matrix.features }}
 
   # Check Minimal Versions
   # ----------------------