refactor(uv): warn instead of silently skipping pyproject parsing without tomllib (#1247)

detect_native silently returned empty [build-system] requires on
pre-3.11 interpreters, misdetecting modern sdists. Warn on stderr when
pyproject.toml is present but tomllib is not, keep the rest of the tool
(native detection, setup.cfg/setup.py parsing) working, and pin the unit
test to the 3.13 toolchain the fetch-time inspection actually uses.

### Changes are visible to end-users: no

### Test plan

- Covered by existing test cases
- New test cases added

Author: jbedard

uv/private/sdist_configure/BUILD.bazel

@@ -17,6 +17,9 @@ py_test(
         "detect_native_test.py",
     ],
     main = "detect_native_test.py",
+    # detect_native.py requires Python >= 3.11 (tomllib); at fetch time it
+    # runs under the PBS interpreter provisioned for sdist inspection.
+    python_version = "3.13",
 )
 
 bzl_library(

uv/private/sdist_configure/detect_native.py

@@ -9,11 +9,11 @@
 
 See //uv/private/sdist_configure:defs.bzl for the full interface contract.
 
-Requires Python >= 3.11 (for tomllib).
+Requires Python >= 3.11 (tomllib) to parse pyproject.toml; on older
+interpreters [build-system] metadata is skipped with a warning.
 """
 
 import configparser
-import importlib
 import os
 import importlib.abc
 import importlib.machinery
@@ -112,6 +112,12 @@ def _parse_pyproject_build_system(content):
     - backend_path: the backend-path list, or None
     """
     if tomllib is None:
+        print(
+            "WARNING: pyproject.toml present but tomllib is unavailable "
+            "(Python < 3.11); [build-system] build requires will be missed. "
+            "Run the sdist configure tool with Python >= 3.11.",
+            file=sys.stderr,
+        )
         return [], None, None
     data = tomllib.loads(content)
     build_system = data.get("build-system", {})

uv/private/sdist_configure/detect_native_test.py

@@ -1,7 +1,6 @@
 """Tests for detect_native.py sdist configure tool."""
 
 import io
-import json
 import os
 import sys
 import tarfile
@@ -10,26 +9,6 @@
 
 from uv.private.sdist_configure.detect_native import detect
 
-try:
-    import tomllib
-    HAS_TOMLLIB = True
-except ModuleNotFoundError:
-    HAS_TOMLLIB = False
-
-
-class _Skip(Exception):
-    """Raised to skip a test."""
-
-
-def requires_tomllib(fn):
-    """Skip test if tomllib is not available (Python < 3.11)."""
-    def wrapper():
-        if not HAS_TOMLLIB:
-            raise _Skip("requires Python >= 3.11 (tomllib)")
-        return fn()
-    wrapper.__name__ = fn.__name__
-    return wrapper
-
 
 def _make_tar_gz(members):
     """Create a .tar.gz archive in a temp file from a dict of {name: content}.
@@ -142,7 +121,6 @@ def test_rust():
 # --- pyproject.toml parsing ---
 
 
-@requires_tomllib
 def test_pyproject_build_requires():
     pyproject = """\
 [build-system]
@@ -160,6 +138,25 @@ def test_pyproject_build_requires():
     assert "cython" in result["build_requires"]
 
 
+def test_pyproject_degrades_without_tomllib():
+    """On a pre-3.11 interpreter, pyproject parsing is skipped with a warning
+    while the rest of the tool (native detection etc.) keeps working."""
+    from uv.private.sdist_configure import detect_native
+    archive = _make_tar_gz({
+        "pkg-1.0/": None,
+        "pkg-1.0/pyproject.toml": '[build-system]\nrequires = ["cython"]\n',
+        "pkg-1.0/pkg/fast.pyx": "# cython source",
+    })
+    saved = detect_native.tomllib
+    detect_native.tomllib = None
+    try:
+        result = detect(archive, {})
+    finally:
+        detect_native.tomllib = saved
+    assert result["build_requires"] == []
+    assert result["is_native"] is True
+
+
 # --- setup.cfg parsing ---
 
 
@@ -195,7 +192,6 @@ def test_legacy_setup_py_infers_setuptools():
     assert "wheel" in result["build_requires"]
 
 
-@requires_tomllib
 def test_pyproject_does_not_infer_setuptools():
     """A package with pyproject.toml should NOT get implicit setuptools."""
     archive = _make_tar_gz({
@@ -215,7 +211,6 @@ def test_pyproject_does_not_infer_setuptools():
 # --- extra_deps resolution ---
 
 
-@requires_tomllib
 def test_extra_deps_resolved_from_available():
     """Declared build deps that exist in available_deps become extra_deps."""
     pyproject = """\
@@ -240,7 +235,6 @@ def test_extra_deps_resolved_from_available():
     assert "cython" in result["extra_deps"]
 
 
-@requires_tomllib
 def test_extra_deps_excludes_already_provided():
     """Deps already in the explicit deps list are not reported as extra."""
     pyproject = """\
@@ -266,7 +260,6 @@ def test_extra_deps_excludes_already_provided():
     assert "cython" in result["extra_deps"]
 
 
-@requires_tomllib
 def test_extra_deps_unresolvable_not_included():
     """Deps not in available_deps are silently omitted from extra_deps."""
     pyproject = """\
@@ -324,7 +317,6 @@ def test_zip_archive():
     assert "pkg-1.0/pkg/ext.c" in result["native_files"]
 
 
-@requires_tomllib
 def test_zip_archive_with_pyproject():
     archive = _make_zip({
         "pkg-1.0/pkg/__init__.py": "",
@@ -342,7 +334,6 @@ def test_zip_archive_with_pyproject():
 # --- Config files at root (no top-level directory) ---
 
 
-@requires_tomllib
 def test_flat_archive():
     """Some archives don't have a top-level directory prefix."""
     archive = _make_tar_gz({
@@ -757,22 +748,18 @@ def test_setup_cfg_and_setup_py_merged():
 
 if __name__ == "__main__":
     failures = []
-    skipped = []
     test_fns = [v for k, v in sorted(globals().items()) if k.startswith("test_") and callable(v)]
     for fn in test_fns:
         try:
             fn()
             print(f"  PASS  {fn.__name__}")
-        except _Skip as e:
-            print(f"  SKIP  {fn.__name__}: {e}")
-            skipped.append(fn.__name__)
         except Exception as e:
             print(f"  FAIL  {fn.__name__}: {e}")
             failures.append(fn.__name__)
 
     total = len(test_fns)
-    passed = total - len(failures) - len(skipped)
-    print(f"\n{passed} passed, {len(skipped)} skipped, {len(failures)} failed (of {total})")
+    passed = total - len(failures)
+    print(f"\n{passed} passed, {len(failures)} failed (of {total})")
     if failures:
         print(f"Failures: {', '.join(failures)}")
         sys.exit(1)