chore(ci): test workflow embetterments (tokio-rs#2176)

This branch makes the following changes to the CI test workflows:

- Consolidate all tests into a single workflow again. We had previously
  broken things out to allow restarting only some failed checks, but now
  GitHub Actions allows restarting individual jobs, which is much nicer,
  and we can combine everything into one workflow.
- Gate starting any tests/checks on an initial `cargo check` run. This
  should mean that if code doesn't compile, we don't spin up a huge
  number of test jobs that all end up failing, and delaying other PRs'
  CI runs.
- Use `cargo nextest` for running tests. This should make test runs a
  bit quicker, and also get us other nice features like retries for
  flaky tests.
- Switch to `taiki-e/install-action` for installing stuff like
  `cargo-hack`, `nextest`, and `wasm-pack`. This is a bit nicer than
  just `curl`ing stuff.
- Use a matrix for testing across toolchains/OSes, instead of having
  separate jobs. This reduces the complexity of the CI workflow a bit.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

.github/workflows/CI.yml

@@ -26,6 +26,9 @@ env:
   RUSTUP_MAX_RETRIES: 10
   # Don't emit giant backtraces in the CI logs.
   RUST_BACKTRACE: short
+  MSRV: 1.49.0
+  # TODO: remove this once tracing's MSRV is bumped.
+  APPENDER_MSRV: 1.53.0
 
 jobs:
   ### check jobs ###
@@ -85,6 +88,71 @@ jobs:
         token: ${{ secrets.GITHUB_TOKEN }}
         args: --all --examples --tests --benches -- -D warnings
 
+  style:
+    # Check style.
+    name: cargo fmt
+    needs: check
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@main
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: stable
+        components: rustfmt
+        profile: minimal
+        override: true
+    - name: rustfmt
+      uses: actions-rs/cargo@v1
+      with:
+        command: fmt
+        args: --all -- --check
+
+  warnings:
+    # Check for any warnings. This is informational and thus is allowed to fail.
+    runs-on: ubuntu-latest
+    needs: check
+    steps:
+    - uses: actions/checkout@main
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: stable
+        components: clippy
+        profile: minimal
+    - name: Clippy
+      uses: actions-rs/clippy-check@v1
+      with:
+        token: ${{ secrets.GITHUB_TOKEN }}
+        args: --all --examples --tests --benches -- -D warnings
+
+  minimal-versions:
+    # Check for minimal-versions errors where a dependency is too
+    # underconstrained to build on the minimal supported version of all
+    # dependencies in the dependency graph.
+    name: cargo check (-Zminimal-versions)
+    needs: check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          profile: minimal
+          override: true
+      - name: install cargo-hack
+        uses: taiki-e/install-action@cargo-hack
+      - name: "check --all-features -Z minimal-versions"
+        run: |
+          # Remove dev-dependencies from Cargo.toml to prevent the next `cargo update`
+          # from determining minimal versions based on dev-dependencies.
+          cargo hack --remove-dev-deps --workspace
+          # Update Cargo.lock to minimal version dependencies.
+          cargo update -Z minimal-versions
+          cargo hack check \
+            --package tracing \
+            --package tracing-core \
+            --package tracing-subscriber \
+            --all-features --ignore-private
+
   cargo-hack:
     needs: check
     name: cargo check (feature combinations)
@@ -143,6 +211,81 @@ jobs:
     name: "cargo check (+MSRV -Zminimal-versions)"
     needs: check
     runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: "install Rust ${{ env.MSRV }}"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ env.MSRV }}
+        profile: minimal
+    - name: "install Rust nightly"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        profile: minimal
+    - name: Select minimal versions
+      uses: actions-rs/cargo@v1
+      with:
+        command: update
+        args: -Z minimal-versions
+        toolchain: nightly
+    - name: Check
+      uses: actions-rs/cargo@v1
+      with:
+        command: check
+        # skip the following crates:
+        # - tracing-appender, as it has its own MSRV.
+        #   TODO(eliza): remove this when appender is on the same MSRV as
+        #   everything else
+        # - the examples, as they are not published & we don't care about
+        #   MSRV support for them.
+        # - tracing-futures, as it depends on ancient tokio versions.
+        #   TODO(eliza): remove this when the ancient tokio deps are dropped
+        args: >-
+          --workspace --all-features --locked
+          --exclude=tracing-appender
+          --exclude=tracing-examples
+          --exclude=tracing-futures
+        toolchain: ${{ env.MSRV }}
+
+  # TODO: remove this once tracing's MSRV is bumped.
+  check-msrv-appender:
+    # Run `cargo check` on our minimum supported Rust version (1.53.0).
+    name: "cargo check (tracing-appender MSRV)"
+    needs: check
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: "install Rust ${{ env.APPENDER_MSRV }}"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ env.APPENDER_MSRV }}
+        profile: minimal
+    - name: "install Rust nightly"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        profile: minimal
+    - name: Select minimal versions
+      uses: actions-rs/cargo@v1
+      with:
+        command: update
+        args: -Z minimal-versions
+        toolchain: nightly
+    - name: Check
+      uses: actions-rs/cargo@v1
+      with:
+        command: check
+        args: --all-features --locked -p tracing-appender
+        toolchain: ${{ env.APPENDER_MSRV }}
+
+  ### test jobs #############################################################
+
+  test:
+    # Test against stable Rust across macOS, Windows, and Linux, and against
+    # beta and nightly rust on Ubuntu.
+    name: "cargo test (${{ matrix.rust }} on ${{ matrix.os }})"
+    needs: check
     strategy:
       matrix:
         # cargo hack --feature-powerset will have a significant permutation