Add diff_engine build infrastructure and CI submodule checkout

- Add _diffengine Extension to setup/extensions.py (optional, only built
  if diff_engine_core submodule is initialized)
- Update setup.py to include diffengine in extensions list
- Update imports from dnlp_diff_engine to _diffengine
- Add submodules: recursive to all CI workflows that build CVXPY:
  - test_nlp_solvers.yml
  - test_optional_solvers.yml
  - test_backends.yml
  - cvxpygen.yml
  - docs.yml
  - gcsopt.yml
  - build.yml

Cherry-picked from submodule-setup branch.

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

Author: Transurgeon

.github/workflows/build.yml

@@ -54,6 +54,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Set Additional Envs
         run: |
           echo "PYTHON_SUBVERSION=$(echo $PYTHON_VERSION | cut -c 3-)" >> $GITHUB_ENV
@@ -114,6 +116,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}

.github/workflows/cvxpygen.yml

@@ -15,6 +15,8 @@ jobs:
         with:
           python-version: "3.12"
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Install pytest and cvxpygen
         run: |
           pip install . pytest hypothesis

.github/workflows/docs.yml

@@ -21,6 +21,8 @@ jobs:
 
         # Checkout master
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Install
         run: |
           python -m pip install -e .

.github/workflows/gcsopt.yml

@@ -15,6 +15,8 @@ jobs:
         with:
           python-version: "3.12"
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Install pytest and gcsopt
         run: |
           pip install . pytest hypothesis

.github/workflows/test_backends.yml

@@ -16,6 +16,8 @@ jobs:
         with:
           python-version: "3.12"
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Install cvxpy dependencies
         run: |
           pip install -e .

.github/workflows/test_nlp_solvers.yml

@@ -22,6 +22,8 @@ jobs:
         with:
           python-version: "3.12"
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Set up Conda
         uses: conda-incubator/setup-miniconda@v3
         with:

.github/workflows/test_optional_solvers.yml

@@ -20,6 +20,8 @@ jobs:
         os: [ ubuntu-22.04, macos-14 ]  # Windows excluded - cylp/CBC cannot be built
     steps:
       - uses: actions/checkout@v5
+        with:
+          submodules: recursive
       - name: Set Additional Envs
         run: |
           echo "PYTHON_SUBVERSION=$(echo $PYTHON_VERSION | cut -c 3-)" >> $GITHUB_ENV
@@ -31,7 +33,6 @@ jobs:
         with:
           python-version: 3.12
           enable-cache: true
-      - uses: actions/checkout@v5
       - name: Setup Julia 
         uses: julia-actions/setup-julia@v2
         with:

cvxpy/reductions/solvers/nlp_solvers/diff_engine/c_problem.py

@@ -22,11 +22,10 @@
 
 # Import the low-level C bindings
 try:
-    import dnlp_diff_engine as _diffengine
+    import _diffengine
 except ImportError as e:
     raise ImportError(
-        "dnlp-diff-engine is required for NLP solving. "
-        "Install with: pip install dnlp-diff-engine"
+        "NLP support requires diff-engine. Rebuild with: pip install -e ."
     ) from e
 
 from cvxpy.reductions.solvers.nlp_solvers.diff_engine.converters import (

cvxpy/reductions/solvers/nlp_solvers/diff_engine/converters.py

@@ -27,11 +27,10 @@
 
 # Import the low-level C bindings
 try:
-    import dnlp_diff_engine as _diffengine
+    import _diffengine
 except ImportError as e:
     raise ImportError(
-        "dnlp-diff-engine is required for NLP solving. "
-        "Install with: pip install dnlp-diff-engine"
+        "NLP support requires diff-engine. Rebuild with: pip install -e ."
     ) from e
 
 

setup.py

@@ -52,7 +52,12 @@ def finalize_options(self) -> None:
 
 setup_versioning.write_version_py()
 VERSION = setup_versioning.VERSION
-extensions = [setup_extensions.cvxcore, setup_extensions.sparsecholesky]
+extensions = [
+    setup_extensions.cvxcore,
+    setup_extensions.sparsecholesky,
+    setup_extensions.diffengine,  # None if submodule not initialized
+]
+extensions = [ext for ext in extensions if ext is not None]
 
 setup(
     name="cvxpy",