fix: set VSCMD_ARG_TGT_ARCH based on targeted architecture (#1876)

* set VSCMD_ARG_TGT_ARCH based on targetted architecture

* only set VSCMD_ARG_TGT_ARCH if not already set

* add unit tests to check setuptools correctly identifies windows arch

* only run tests on windows

* arm64 fails on azure pipelines (only)

* Update windows.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Apply suggestions from code review

Co-authored-by: Joe Rickerby <joerick@mac.com>

* update test_env_set to validate FatalError is raised on env collision

* remove noqa ARG001 (no longer needed)

---------

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Joe Rickerby <joerick@mac.com>

cibuildwheel/windows.py

@@ -152,6 +152,16 @@ def setup_setuptools_cross_compile(
     # set the platform name
     map_plat = {"32": "win32", "64": "win-amd64", "ARM64": "win-arm64"}
     plat_name = map_plat[python_configuration.arch]
+
+    # Set environment variable so that setuptools._distutils.get_platform()
+    # identifies the target, not the host
+    vscmd_arg_tgt_arch = {"32": "x86", "64": "x64", "ARM64": "arm64"}
+    current_tgt_arch = vscmd_arg_tgt_arch[python_configuration.arch]
+    if (env.get("VSCMD_ARG_TGT_ARCH") or current_tgt_arch) != current_tgt_arch:
+        msg = f"VSCMD_ARG_TGT_ARCH must be set to {current_tgt_arch!r}, got {env['VSCMD_ARG_TGT_ARCH']!r}. If you're setting up MSVC yourself (e.g. using vcvarsall.bat or msvc-dev-cmd), make sure to target the right architecture. Alternatively, run cibuildwheel without configuring MSVC, and let the build backend handle it."
+        raise errors.FatalError(msg)
+    env["VSCMD_ARG_TGT_ARCH"] = current_tgt_arch
+
     # (This file must be default/locale encoding, so we can't pass 'encoding')
     distutils_cfg.write_text(
         textwrap.dedent(