Refactor test and build jobs

Overall, we want to:

- Run as few jobs as possible
- Have the jobs finish as fast as possible
- Have the jobs redo as little work as possible

We change our CI jobs into:

1) A matrix job for all tests, split by crate.
2) A matrix job for testing certain feature combinations and MSRV.

We make this split because:

A test is no good unless run, thus the `test` job always activates
all features.
MSRV and exposed feature only concern the public API of a crate,
thus the `check` job doesn't run on all targets.

.github/workflows/ci.yml

@@ -11,61 +11,79 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  test-desktop:
-    name: Build and test
+  test: # Test all workspace crates
+    name: Test ${{ matrix.crate }}
     runs-on: ubuntu-latest
+    needs: gather_published_crates
     strategy:
+      fail-fast: false
       matrix:
-        args: [
-          "--no-default-features",
-          "--all-features",
-          "--benches --all-features",
-        ]
+        crate: ${{ fromJSON(needs.gather_published_crates.outputs.members) }}
+        include:
+          # Building the meta-crate for WASM is good enough.
+          - crate: libp2p
+            args: "--toolchain wasm32-unknown-unknown"
+          - crate: libp2p
+            features: "--toolchain wasm32-unknown-emscripten"
+          - crate: libp2p
+            features: "--toolchain wasm32-wasi"
+
     steps:
       - name: Install Protoc
         uses: arduino/setup-protoc@v1
 
       - uses: actions/checkout@v3
 
+      - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
+        with:
+          profile: minimal
+          toolchain: stable # Ensure tests always run on latest stable
+          override: true
+
       - uses: Swatinem/rust-cache@22c9328bcba27aa81a32b1bef27c7e3c78052531 # v2.0.1
         with:
-          key: ${{ matrix.args }}
+          key: ${{ matrix.crate }}-${{ matrix.args }}
 
-      - run: cargo test --workspace ${{ matrix.args }}
+      - run: cargo test --package ${{ matrix.crate }} --all-features ${{ matrix.args }}
 
-  test-wasm:
-    name: Build on WASM
+  check: # Check that various feature combinations compile whilst adhering to the stated MSRV.
     runs-on: ubuntu-latest
+    needs: gather_published_crates
     strategy:
+      fail-fast: false
       matrix:
-        toolchain: [
-          wasm32-unknown-emscripten,
-          wasm32-wasi
+        crate: ${{ fromJSON(needs.gather_published_crates.outputs.members) }}
+        args: [
+          "--all-features"
         ]
         include:
-          - toolchain: wasm32-unknown-unknown
-            args: "--features wasm-bindgen"
+          - crate: libp2p
+            args: "--features=all-transports --features tokio",
+          - crate: libp2p
+            features: "--features=all-transports --features async-std"
     steps:
       - name: Install Protoc
         uses: arduino/setup-protoc@v1
 
       - uses: actions/checkout@v3
 
-      - name: Install Rust ${{ matrix.toolchain }}
-        uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
+      - name: Get MSRV for ${{ matrix.crate }}
+        id: parse-msrv
+        run: |
+          RUST_VERSION=$(cargo metadata --format-version=1 --no-deps | jq -r '.packages[] | select(.name == "libp2p") | .rust_version')
+          echo "::set-output name=version::${RUST_VERSION}"
+
+      - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
         with:
-          toolchain: stable
-          target: ${{ matrix.toolchain }}
+          profile: minimal
+          toolchain: ${{ steps.parse-msrv.outputs.version }}
           override: true
 
       - uses: Swatinem/rust-cache@22c9328bcba27aa81a32b1bef27c7e3c78052531 # v2.0.1
-      with:
-        key: ${{ matrix.toolchain }}
+        with:
+          key: ${{ matrix.crate }}-${{ matrix.args }}
 
-      - name: Build on ${{ matrix.toolchain }}
-        # TODO: also run `cargo test`
-        # TODO: ideally we would build `--workspace`, but not all crates compile for WASM
-        run: cargo build --target=${{ matrix.toolchain }} ${{ matrix.args }}
+      - run: cargo check --package ${{ matrix.crate }} ${{ matrix.args }}
 
   check-rustdoc-links:
     name: Check rustdoc intra-doc links
@@ -115,34 +133,6 @@ jobs:
         with:
           command: custom-clippy # cargo alias to allow reuse of config locally
 
-  check-executor-features:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        executor: [
-          "--features tokio",
-          "--features async-std",
-          ""
-        ]
-    steps:
-    - name: Install Protoc
-      uses: arduino/setup-protoc@v1
-
-    - uses: actions/checkout@v3
-
-    - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
-      with:
-        profile: minimal
-        toolchain: stable
-        override: true
-
-    - uses: Swatinem/rust-cache@22c9328bcba27aa81a32b1bef27c7e3c78052531 # v2.0.1
-
-    - name: Run cargo check
-      uses: actions-rs/cargo@844f36862e911db73fe0815f00a4a2602c279505 # v1.0.3
-      with:
-        command: check --all-targets --features=all-transports ${{ matrix.executor }}
-
   integration-test:
     name: Integration tests
     runs-on: ubuntu-latest