doctest coverage requires nightly

Author: jonhoo

.github/workflows/coverage.yml

@@ -7,13 +7,13 @@ jobs:
   test:
     runs-on: ubuntu-latest
     container:
-      image: xd009642/tarpaulin
+      image: xd009642/tarpaulin:develop-nightly
       options: --security-opt seccomp=unconfined
     steps:
       - uses: actions/checkout@v2
       - name: Generate code coverage
         run: |
-          cargo tarpaulin --verbose --all-features --timeout 120 --run-types Doctests --run-types Tests --out Xml
+          cargo +nightly tarpaulin --verbose --all-features --workspace --timeout 120 --run-types Doctests --run-types Tests --out Xml
       - name: Upload to codecov.io
         uses: codecov/codecov-action@v2
         with: