Enable building abi3 shared objects

This depends on PyO3/pyo3#1125

Author: alex

setuptools_rust/build.py

@@ -285,7 +285,14 @@ def build_extension(self, ext):
 
                 ext.install_script(ext_path)
             else:
-                ext_path = build_ext.get_ext_fullpath(target_fname)
+                # Technically it's supposed to contain a
+                # `setuptools.Extension`, but in practice the only attribute it
+                # checks is `ext.py_limited_api`.
+                build_ext.ext_map[ext.name] = ext
+                try:
+                    ext_path = build_ext.get_ext_fullpath(target_fname)
+                finally:
+                    del build_ext.ext_map[ext.name]
 
             try:
                 os.makedirs(os.path.dirname(ext_path))

setuptools_rust/extension.py

@@ -47,6 +47,10 @@ class RustExtension:
       optional : bool
         if it is true, a build failure in the extension will not abort the
         build process, but instead simply not install the failing extension.
+      py_limited_api : bool
+        Same as `py_limited_api` on `setuptools.Extension`. Note that if you
+        set this to True, your extension must pass the appropriate feature
+        flags to pyo3 (ensuring that `unstable-apis` feature is disabled).
     """
 
     def __init__(
@@ -64,6 +68,7 @@ def __init__(
         script=False,
         native=False,
         optional=False,
+        py_limited_api=False,
     ):
         if isinstance(target, dict):
             name = "; ".join("%s=%s" % (key, val) for key, val in target.items())
@@ -83,6 +88,7 @@ def __init__(
         self.script = script
         self.native = native
         self.optional = optional
+        self.py_limited_api = py_limited_api
 
         if features is None:
             features = []