Delay imports of non-stdlib dependencies until time of use

This is a bit of a hack, but we do this to ensure that when setuptools
loads entrypoint based hooks it cannot (will not?) crash.

The issue is that setuptools plugins are autoloaded, whether any given
project uses them at all or not. So if setuptools-rust is installed,
setuptools always tries to use it, and crashes if setuptools-rust is
broken.

Of course, setuptools-rust can't be broken, because it's a wonderful
project.

BUT.

As it happens, third-party vendors providing setuptools-rust can get
into a situation where multiple packages need to be installed, including
setuptools-rust, and also build yet other packages from source. In the
middle of this, setuptools-rust itself could be installed but in
"half-configured" state, i.e. its dependencies were queued for
afterwards due to complicated dependency graph magic.

In such a scenario, it should be nominally all right to have an inert
package installed, since if nothing actually uses setuptools-rust it
doesn't need to *work* yet. And in fact, it is all right, as long as
setuptools can import the autoloaded plugin hooks (and do nothing with
them).

Bug: https://bugs.gentoo.org/933553

Author: eli-schwartz

setuptools_rust/extension.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import json
 import os
 import re
@@ -14,11 +16,13 @@
     NewType,
     Optional,
     Sequence,
+    TYPE_CHECKING,
     Union,
     cast,
 )
 
-from semantic_version import SimpleSpec
+if TYPE_CHECKING:
+    from semantic_version import SimpleSpec
 
 from ._utils import format_called_process_error
 
@@ -185,6 +189,7 @@ def get_rust_version(self) -> Optional[SimpleSpec]:  # type: ignore[no-any-unimp
         if self.rust_version is None:
             return None
         try:
+            from semantic_version import SimpleSpec
             return SimpleSpec(self.rust_version)
         except ValueError:
             raise SetupError(

setuptools_rust/rustc_info.py

@@ -1,15 +1,19 @@
+from __future__ import annotations
+
 import subprocess
 from setuptools.errors import PlatformError
 from functools import lru_cache
-from typing import Dict, List, NewType, Optional
+from typing import Dict, List, NewType, Optional, TYPE_CHECKING
 
-from semantic_version import Version
+if TYPE_CHECKING:
+    from semantic_version import Version
 
 
 def get_rust_version() -> Optional[Version]:  # type: ignore[no-any-unimported]
     try:
         # first line of rustc -Vv is something like
         # rustc 1.61.0 (fe5b13d68 2022-05-18)
+        from semantic_version import Version
         return Version(_rust_version().split(" ")[1])
     except (subprocess.CalledProcessError, OSError):
         return None