test(e2e): cover nanobind_extension against the PBS py_cc toolchain (#1299)

Adds an e2e case that builds a real `nanobind_extension` in a graph
whose only Python toolchains come from `python_interpreters` (the
rules_python `python.toolchain()` drop-in), exercising the
`@rules_python//python/cc:current_py_cc_headers` path that nanobind and
pybind11 use to resolve Python headers.

`pbs-cc-toolchain` already covers the bare `cc_binary` +
`current_py_cc_headers`/`current_py_cc_libs` path; this adds the
**nanobind** consumer, which is how most real-world native extensions
reach `current_py_cc_headers`. It guards the py_cc toolchain
registration (issue #1095) against regression.

Two targets:
- `example_ext_build_test` — a `build_test` over the
`nanobind_extension`; fails at analysis if `python_interpreters` does
not register `@rules_python//python/cc:toolchain_type`.
- `example_ext_test` — compiles, links, loads and calls the extension
under a PBS runtime (py 3.13).

**End-user visible:** no (test-only). **Breaking:** no.

**Test plan:** `bazel test //nanobind-py-cc/...` in `e2e/cases` passes
locally (build_test + runtime import/call; Linux x86_64, llvm
toolchain).

Context: came out of testing the py_cc gap for a real nanobind consumer
(discussed in #1131); @jbedard suggested opening this.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ctcjab

e2e/cases/MODULE.bazel

@@ -9,6 +9,7 @@ bazel_dep(name = "tar.bzl", version = "0.10.1")
 bazel_dep(name = "rules_python", version = "1.9.0")
 bazel_dep(name = "platforms", version = "1.0.0")
 bazel_dep(name = "llvm", version = "0.8.3")
+bazel_dep(name = "nanobind_bazel", version = "2.12.0")
 bazel_dep(name = "rules_oci", version = "2.2.7")
 bazel_dep(name = "rules_shell", version = "0.4.1")
 

e2e/cases/MODULE.bazel.lock

@@ -0,0 +1,28 @@
+load("@aspect_rules_py//py:defs.bzl", "py_test")
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+load("@nanobind_bazel//:build_defs.bzl", "nanobind_extension")
+
+# Regression coverage for https://github.com/aspect-build/rules_py/issues/1095:
+# a nanobind_extension resolves Python headers through
+# @rules_python//python/cc:current_py_cc_headers, so it only analyzes if
+# python_interpreters registers the py_cc toolchain over the PBS interpreter.
+
+nanobind_extension(
+    name = "example_ext",
+    srcs = ["example_ext.cpp"],
+)
+
+build_test(
+    name = "example_ext_build_test",
+    targets = [":example_ext"],
+)
+
+py_test(
+    name = "example_ext_test",
+    size = "small",
+    srcs = ["example_ext_test.py"],
+    imports = ["."],  # so `import example_ext` finds the .so
+    main = "example_ext_test.py",
+    python_version = "3.13",
+    deps = [":example_ext"],
+)

e2e/cases/nanobind-py-cc/BUILD.bazel

@@ -0,0 +1,8 @@
+#include <nanobind/nanobind.h>
+#include <nanobind/stl/string.h>
+
+// A minimal nanobind extension. See BUILD.bazel for what it guards (issue #1095).
+NB_MODULE(example_ext, m) {
+    m.def("add", [](int a, int b) { return a + b; });
+    m.def("greet", [](const std::string &name) { return "Hello, " + name + "!"; });
+}

e2e/cases/nanobind-py-cc/example_ext.cpp

@@ -0,0 +1,6 @@
+import example_ext
+
+assert example_ext.add(2, 3) == 5, example_ext.add(2, 3)
+assert example_ext.greet("World") == "Hello, World!", example_ext.greet("World")
+
+print("nanobind extension imported and called successfully")