Fix CI: extension-module feature and wheel installation

- Move pyo3/extension-module to optional feature (not needed for tests)
- Update pyproject.toml to use the new feature name
- Use pip --find-links for wheel installation (glob wasn't expanding)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: igerber

.github/workflows/rust-test.yml

@@ -81,7 +81,7 @@ jobs:
         run: |
           pip install maturin
           maturin build --release
-          pip install target/wheels/*.whl
+          pip install --find-links=target/wheels diff-diff
 
       - name: Verify Rust backend is available
         run: |

pyproject.toml

@@ -57,7 +57,7 @@ Issues = "https://github.com/igerber/diff-diff/issues"
 
 [tool.maturin]
 # Build the Rust extension module
-features = ["pyo3/extension-module"]
+features = ["extension-module"]
 # Python source is in the root directory
 python-source = "."
 # Module name for the compiled extension

rust/Cargo.toml

@@ -10,8 +10,13 @@ name = "diff_diff_rust"
 # cdylib for Python extension, rlib for running tests
 crate-type = ["cdylib", "rlib"]
 
+[features]
+default = []
+# extension-module is only needed for cdylib builds, not for cargo test
+extension-module = ["pyo3/extension-module"]
+
 [dependencies]
-pyo3 = { version = "0.20", features = ["extension-module"] }
+pyo3 = "0.20"
 numpy = "0.20"
 ndarray = { version = "0.15", features = ["rayon"] }
 rand = "0.8"