Add .abi3.so copy file, extension name helper

Also add more required config settings for stable ABI + unix (producing
a .abi3.so extension).

BUILD

@@ -45,6 +45,42 @@ config_setting(
     flag_values = {":py-limited-api": "unset"},
 )
 
+selects.config_setting_group(
+    name = "unix",
+    match_any = [
+        "@platforms//os:linux",
+        "@platforms//os:macos",
+    ],
+)
+
+# Config setting indicating that stable ABI extension build was requested.
+selects.config_setting_group(
+    name = "stable-abi",
+    match_any = [
+        ":cp312",
+        ":cp313",
+    ],
+)
+
+# A stable ABI build on Linux or Mac.
+# This requires a different extension name (.abi3.so instead of just .so).
+selects.config_setting_group(
+    name = "stable-abi-unix",
+    match_all = [
+        ":stable-abi",
+        ":unix",
+    ],
+)
+
+# An unlimited Python ABI build on Linux or Mac. Produces a regular .so file.
+selects.config_setting_group(
+    name = "unstable-abi-unix",
+    match_all = [
+        ":pyunlimitedapi",
+        ":unix",
+    ],
+)
+
 # Is the currently configured C++ compiler not MSVC?
 selects.config_setting_group(
     name = "nonmsvc",

build_defs.bzl

@@ -11,6 +11,7 @@ which can then be included e.g. as a `data` input in a ``native.py_library``.
 """
 
 load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
+load("@nanobind_bazel//:helpers.bzl", "extension_name")
 
 NANOBIND_COPTS = select({
     Label("@rules_cc//cc/compiler:msvc-cl"): [],
@@ -40,7 +41,6 @@ def nanobind_extension(
         features = [],
         deps = [],
         **kwargs):
-    # TODO: This might need a different suffix depending on SABI yes/no.
     native.cc_binary(
         name = name + ".so",
         srcs = srcs,
@@ -52,6 +52,12 @@ def nanobind_extension(
         **kwargs
     )
 
+    copy_file(
+        name = name + "_copy_so_to_abi3_so",
+        src = name + ".so",
+        out = name + ".abi3.so",
+    )
+
     copy_file(
         name = name + "_copy_so_to_pyd",
         src = name + ".so",
@@ -60,10 +66,7 @@ def nanobind_extension(
 
     native.alias(
         name = name,
-        actual = select({
-            "@platforms//os:windows": name + ".pyd",
-            "//conditions:default": name + ".so",
-        }),
+        actual = extension_name(name),
     )
 
 def nanobind_library(

helpers.bzl

@@ -13,10 +13,19 @@ def sizedefs():
         "@nanobind_bazel//:without_sizeopts": [],
     })
 
-# define the Python version hex if stable ABI builds are requested.
+# Define the Python version hex if stable ABI builds are requested.
 def pyversionhex():
     return select({
         "@nanobind_bazel//:cp312": ["Py_LIMITED_API=0x030C0000"],
         "@nanobind_bazel//:cp313": ["Py_LIMITED_API=0x030D0000"],
         "@nanobind_bazel//:pyunlimitedapi": [],
     })
+
+# Get the name for a built nanobind extension based on target platform
+# and stable ABI build yes/no.
+def extension_name(name):
+    return select({
+        "@platforms//os:windows": name + ".pyd",
+        "@nanobind_bazel//:stable-abi-unix": name + ".abi3.so",
+        "@nanobind_bazel//:unstable-abi-unix": name + ".so",
+    })