Make pybind_extension() work on Windows.

Fixes #74.

MODULE.bazel

@@ -4,6 +4,7 @@ module(
     version = "2.11.1.bzl.2",
 )
 
+bazel_dep(name = "bazel_skylib", version = "1.5.0")
 bazel_dep(name = "platforms", version = "0.0.8")
 bazel_dep(name = "rules_cc", version = "0.0.9")
 bazel_dep(name = "rules_python", version = "0.31.0")

README.md

@@ -3,8 +3,8 @@
 Provided rules:
 
 -   `pybind_extension`: Builds a python extension, automatically adding the
-    required build flags and pybind11 dependencies. It defines a `*.so` target
-    which can be included as a `data` dependency of a `py_*` target.
+    required build flags and pybind11 dependencies. It defines a target which
+    can be included as a `data` dependency of a `py_*` target.
 -   `pybind_library`: Builds a C++ library, automatically adding the required
     build flags and pybind11 dependencies. This library can then be used as a
     dependency of a `pybind_extension`. The arguments match a `cc_library`.

build_defs.bzl

@@ -5,6 +5,8 @@
 
 """Build rules for pybind11."""
 
+load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
+
 def register_extension_info(**kwargs):
     pass
 
@@ -24,8 +26,15 @@ PYBIND_DEPS = [
 ]
 
 # Builds a Python extension module using pybind11.
-# This can be directly used in python with the import statement.
-# This adds rules for a .so binary file.
+# This can be directly used in Python with the import statement.
+# Assuming the name NAME, the following targets will be defined:
+#   1. NAME.so - the shared/dynamic library for the extension module
+#   2. NAME.pyd - a copy of NAME.so named for Python on Windows; see
+#                 https://github.com/pybind/pybind11_bazel/issues/74
+#   3. NAME - an alias pointing to either NAME.so or NAME.pyd as per
+#             the platform OS (not-Windows or Windows, respectively)
+# Generally, the user will "depend" on this extension module via the
+# data attribute of their py_* target; specifying NAME is preferred.
 def pybind_extension(
         name,
         copts = [],
@@ -55,6 +64,20 @@ def pybind_extension(
         **kwargs
     )
 
+    copy_file(
+        name = name + "_copy_so_to_pyd",
+        src = name + ".so",
+        out = name + ".pyd",
+    )
+
+    native.alias(
+        name = name,
+        actual = select({
+            "@platforms//os:windows": name + ".pyd",
+            "//conditions:default": name + ".so",
+        }),
+    )
+
 # Builds a pybind11 compatible library. This can be linked to a pybind_extension.
 def pybind_library(
         name,