[ci] also generate coverage report for doctests (#1081)

Per #1080.

.github/workflows/coverage.yml

@@ -1,6 +1,6 @@
 on:
   push:
-    branches: [ main, auto, canary ]
+    branches: [main, auto, canary]
   pull_request:
     branches:
       - main
@@ -18,18 +18,23 @@ jobs:
       CARGO_TERM_COLOR: always
     steps:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4
-      - uses: dtolnay/rust-toolchain@stable
+        # Nightly Rust is used for cargo llvm-cov --doc below.
+      - uses: dtolnay/rust-toolchain@nightly
         with:
           components: llvm-tools-preview
       - uses: Swatinem/rust-cache@a95ba195448af2da9b00fb742d14ffaaf3c21f43 # v2
-      
+
       - name: Install latest nextest release
         uses: taiki-e/install-action@nextest
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov
-      
+
       - name: Collect coverage data
-        run: cargo llvm-cov nextest --lcov --output-path lcov.info
+        # Generate separate reports for nextest and doctests, and combine them.
+        run: |
+          cargo llvm-cov --no-report nextest
+          cargo llvm-cov --no-report --doc
+          cargo llvm-cov report --doctests --lcov --output-path lcov.info
       - name: Upload coverage data to codecov
         uses: codecov/codecov-action@eaaf4bedf32dbdc6b720b63067d99c4d77d6047d # v3
         with: