ci: Faster coverage tests (#545)

Copies the idea from `hugr`. 

For the rust checks, the CI ran nightly tests followed by the same tests
with coverage instrumentation. This added unnecessary delay to the
coverage report.
This PR merges the two steps, so the rust codecov analysis should appear
~2:30 minutes faster.

For python, running with and without coverage takes almost the same
time. The PR merges the test runs, but still leaves the codecov upload
in a separate job (outside the critical path of the required checks).
This saves ~1:30.

Before (Coverage delay ~5mins):
<img width="690" alt="image"
src="https://github.com/user-attachments/assets/2139a314-258e-4212-92b4-f291f5a52e4e">
After (Coverage delay ~2mins):
<img width="691" alt="image"
src="https://github.com/user-attachments/assets/92c28af2-c5ab-4a10-981a-9ad94c9e80da">

.github/workflows/ci.yml

@@ -163,9 +163,9 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        # Pinned nightly version until this gets resolved:
-        # https://github.com/rust-lang/rust/issues/125474
-        rust: ['1.75', beta, 'nightly-2024-05-22']
+        # Stable is covered by `tests-stable-no-features` and `tests-stable-all-features`
+        # Nightly is covered by `tests-nightly-coverage`
+        rust: ['1.75', beta]
     name: tests (Rust ${{ matrix.rust }})
     steps:
       - uses: actions/checkout@v4
@@ -185,40 +185,18 @@ jobs:
       - name: Tests with all features
         run: cargo test --verbose --workspace --all-features
 
-  tests-py:
+  tests-nightly-coverage:
     needs: changes
-    if: ${{ needs.changes.outputs.python == 'true' }}
-    runs-on: ubuntu-latest
-    name: tests (Python)
-    steps:
-      - uses: actions/checkout@v4
-      - uses: mozilla-actions/sccache-action@v0.0.5
-      - name: Install stable toolchain
-        uses: dtolnay/rust-toolchain@stable
-      - name: Install poetry
-        run: pipx install poetry
-      - uses: actions/setup-python@v5
-        with:
-          python-version: '3.11'
-          cache: 'poetry'
-      - name: Build pyo3 bindings
-        run: |
-          poetry install
-          poetry run maturin develop
-      - name: Test pyo3 bindings
-        run: poetry run pytest
-
-  coverage-rs:
-    name: Check Rust coverage 🦀
-    needs: [changes, tests-rs-stable-no-features, tests-rs-stable-all-features, tests-rs-other, check-rs]
-    # Run only if there are changes in the relevant files and the check job passed or was skipped
-    if: always() && !failure() && !cancelled() && needs.changes.outputs.rust == 'true' && github.event_name != 'merge_group'
+    # Run only if there are changes in the relevant files
+    if: ${{ needs.changes.outputs.rust == 'true' && github.event_name != 'merge_group' }}
     runs-on: ubuntu-latest
+    name: tests (Rust nightly, coverage)
     steps:
       - uses: actions/checkout@v4
       - uses: mozilla-actions/sccache-action@v0.0.5
       - uses: dtolnay/rust-toolchain@master
         with:
+          # Nightly is required to count doctests coverage
           toolchain: 'nightly'
           components: llvm-tools-preview
       - name: Install cargo-llvm-cov
@@ -238,12 +216,11 @@ jobs:
           flags: rust
           token: ${{ secrets.CODECOV_TOKEN }}
 
-  coverage-py:
-    name: Check Python coverage 🐍
-    needs: [changes, tests-py, check-py]
-    # Run only if there are changes in the relevant files and the check job passed or was skipped
-    if: always() && !failure() && !cancelled() && needs.changes.outputs.python == 'true' && github.event_name != 'merge_group'
+  tests-py:
+    needs: changes
+    if: ${{ needs.changes.outputs.python == 'true' }}
     runs-on: ubuntu-latest
+    name: tests (Python)
     steps:
       - uses: actions/checkout@v4
       - uses: mozilla-actions/sccache-action@v0.0.5
@@ -261,6 +238,24 @@ jobs:
           poetry run maturin develop
       - name: Run python tests with coverage instrumentation
         run: poetry run pytest --cov=./ --cov-report=xml
+      - name: Upload coverage output artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: py-coverage
+          path: coverage.xml
+
+  coverage-py:
+    name: Upload Python coverage 🐍
+    needs: [changes, tests-py, check-py]
+    # Run only if there are changes in the relevant files and the check job passed or was skipped
+    if: always() && !failure() && !cancelled() && needs.changes.outputs.python == 'true' && github.event_name != 'merge_group'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/download-artifact@v3
+        with:
+          name: py-coverage
+          path: coverage.xml
       - name: Upload python coverage to codecov.io
         uses: codecov/codecov-action@v4
         with: