Draft: select native compilers from the C++ toolchain for sdist wheel builds

docs/uv.md

@@ -20,10 +20,14 @@ configurations, such as going from a Darwin macbook to a Linux container image
 using only the normal Bazel `platforms` machinery.
 
 **Correct source builds** - Because uv performs package source builds as a
-normal part of your build, it's able to use hermetic or even source built Python
-toolchains in addition to Bazel-defined dependencies and C compilers. Future
-support for sysroots is planned. Due to its phasing, `pip.parse` is stuck doing
-all this non-hermetically.
+normal part of your build, it can use hermetic or even source-built Python
+toolchains in addition to Bazel-defined dependencies. Native builds select the
+C and C++ tools from Bazel's configured compile actions, but do not reuse their
+action flags or environment because PEP 517 backends own the compile and link
+commands. Tools that require target, sysroot, resource, wrapper, or environment
+configuration must provide a backend-compatible command through the package
+`env`; generic C++ toolchain projection is not supported. Due to its phasing,
+`pip.parse` is stuck doing all this non-hermetically.
 
 **Editable requirements** - Uv provides an `uv.override_requirement()` tag which
 allows locked requirements to be replaced with 1stparty Bazel `py_library`

e2e/cases/uv-sdist-native-build/BUILD.bazel

@@ -1,26 +1,29 @@
 # Regression test: python-geohash (sdist-only, C++ extension) must be buildable
-# when native_build_toolchain_type is registered via @rules_py_tools//:all.
+# with native_build_toolchain_type registered via @rules_py_tools//:all.
 #
 # The pep517_native_whl rule requires the native_build_toolchain_type to resolve,
-# asserting that the exec and target platforms match. This test verifies that the
-# per-platform toolchain entries generated into @rules_py_tools are reachable.
+# asserting that the exec and target platforms match. The source patch also
+# introduces a C++ standard-library dependency, so linking fails if the helper
+# invokes the configured compiler in C rather than C++ driver mode.
 
 load("@aspect_rules_py//py:defs.bzl", "py_test")
 
+exports_files(["require_cxx_driver.patch"])
+
 py_test(
     name = "test",
     srcs = ["test_geohash.py"],
+    dep_group = "uv-sdist-native-build",
     main = "test_geohash.py",
-    target_compatible_with = ["@platforms//os:linux"],
     deps = ["@pypi_uv_sdist_native_build//python_geohash"],
 )
 
 py_test(
     name = "venv_test",
     srcs = ["test_geohash.py"],
+    dep_group = "uv-sdist-native-build",
     expose_venv = True,
     isolated = False,
     main = "test_geohash.py",
-    target_compatible_with = ["@platforms//os:linux"],
     deps = ["@pypi_uv_sdist_native_build//python_geohash"],
 )

e2e/cases/uv-sdist-native-build/pyproject.toml

@@ -3,5 +3,7 @@ name = "uv-sdist-native-build"
 version = "0.0.0"
 requires-python = ">=3.11"
 dependencies = [
+    "build==1.4.0",
     "python-geohash",
+    "setuptools==75.8.2",
 ]

e2e/cases/uv-sdist-native-build/require_cxx_driver.patch

@@ -0,0 +1,30 @@
+diff --git a/setup.py b/setup.py
+--- a/setup.py
++++ b/setup.py
+@@ -9,0 +10 @@ c1=Extension('_geohash',
++c1.sources.append('src/native_probe.c')
+diff --git a/src/geohash.cpp b/src/geohash.cpp
+--- a/src/geohash.cpp
++++ b/src/geohash.cpp
+@@ -19,0 +20,4 @@
++#include <string>
++
++extern "C" int native_c_probe(void);
++
+@@ -725,0 +731,6 @@
++
++static PyObject *py_native_probe(PyObject *, PyObject *) {
++	std::string suffix = "++";
++	return PyLong_FromLong(native_c_probe() + suffix.size());
++}
++
+@@ -732,0 +745 @@
++	{"native_probe", py_native_probe, METH_NOARGS, "exercise C and C++ compilation and linkage"},
+diff --git a/src/native_probe.c b/src/native_probe.c
+new file mode 100644
+--- /dev/null
++++ b/src/native_probe.c
+@@ -0,0 +1,3 @@
++int native_c_probe(void) {
++    return 40;
++}

e2e/cases/uv-sdist-native-build/setup.MODULE.bazel

@@ -22,6 +22,8 @@ uv.project(
 uv.override_package(
     name = "python-geohash",
     lock = "//cases/uv-sdist-native-build:uv.lock",
+    pre_build_patch_strip = 1,
+    pre_build_patches = ["//cases/uv-sdist-native-build:require_cxx_driver.patch"],
     resource_set = "mem_4g",
 )
 

e2e/cases/uv-sdist-native-build/test_geohash.py

@@ -1,9 +1,11 @@
 """Verify that python-geohash (built from sdist via pep517_native_whl) is importable."""
 
 import geohash
+import _geohash
 
 
 def test_encode_decode():
+    assert _geohash.native_probe() == 42
     lat, lon = 37.7749, -122.4194
     encoded = geohash.encode(lat, lon)
     assert encoded, "geohash.encode should return a non-empty string"

uv/private/extension/defs.bzl

@@ -407,9 +407,9 @@ def _parse_projects(module_ctx, hub_specs):
                         pre_build_patches = [str(p) for p in pkg_override.pre_build_patches]
                         pre_build_patch_strip = pkg_override.pre_build_patch_strip
 
-                    # `toolchains` / `env` on `uv.override_package` augment
-                    # the defaults baked into sdist_build's BUILD template —
-                    # they don't replace them. Empty == no augmentation.
+                    # `toolchains` on `uv.override_package` extends the
+                    # defaults baked into sdist_build's BUILD template. `env`
+                    # is merged over those defaults, replacing named values.
                     extra_toolchains = []
                     extra_env = {}
                     resource_set = "default"
@@ -782,7 +782,7 @@ _override_package_tag = tag_class(
         ),
         "env": attr.string_dict(
             default = {},
-            doc = "Extra environment variables merged into the build action's `env` dict. Values may reference $(VAR) make-variables sourced from the default CC toolchain or any extra `toolchains` listed above.",
+            doc = "Environment variables merged over the native-build defaults. Values may reference $(VAR) make-variables sourced from the default CC toolchain or any extra `toolchains` listed above.",
         ),
 
         # Pre-build patches: applied to extracted sdist source before wheel build.