Use arm64 MSVC on arm64 Windows

distutils/_msvccompiler.py

@@ -33,7 +33,7 @@
     LibError,
     LinkError,
 )
-from .util import get_platform
+from .util import get_host_platform, get_platform
 
 
 def _find_vc2015():
@@ -178,17 +178,43 @@ def _find_exe(exe, paths=None):
     return exe
 
 
-# A map keyed by get_platform() return values to values accepted by
-# 'vcvarsall.bat'. Always cross-compile from x86 to work with the
-# lighter-weight MSVC installs that do not include native 64-bit tools.
-PLAT_TO_VCVARS = {
+_vcvars_names = {
     'win32': 'x86',
-    'win-amd64': 'x86_amd64',
-    'win-arm32': 'x86_arm',
-    'win-arm64': 'x86_arm64',
+    'win-amd64': 'amd64',
+    'win-arm32': 'arm',
+    'win-arm64': 'arm64',
 }
 
 
+def _get_vcvars_spec(host_platform, platform):
+    """
+    Given a host platform and platform, determine the spec for vcvarsall.
+
+    Uses the native MSVC host if the host platform would need expensive
+    emulation for x86.
+
+    >>> _get_vcvars_spec('win-arm64', 'win32')
+    'arm64_x86'
+    >>> _get_vcvars_spec('win-arm64', 'win-amd64')
+    'arm64_amd64'
+
+    Otherwise, always cross-compile from x86 to work with the
+    lighter-weight MSVC installs that do not include native 64-bit tools.
+
+    >>> _get_vcvars_spec('win32', 'win32')
+    'x86'
+    >>> _get_vcvars_spec('win-arm32', 'win-arm32')
+    'x86_arm'
+    >>> _get_vcvars_spec('win-amd64', 'win-arm64')
+    'x86_arm64'
+    """
+    if host_platform != 'win-arm64':
+        host_platform = 'win32'
+    vc_hp = _vcvars_names[host_platform]
+    vc_plat = _vcvars_names[platform]
+    return vc_hp if vc_hp == vc_plat else f'{vc_hp}_{vc_plat}'
+
+
 class MSVCCompiler(CCompiler):
     """Concrete class that implements an interface to Microsoft Visual C++,
     as defined by the CCompiler abstract class."""
@@ -242,13 +268,12 @@ def initialize(self, plat_name=None):
         if plat_name is None:
             plat_name = get_platform()
         # sanity check for platforms to prevent obscure errors later.
-        if plat_name not in PLAT_TO_VCVARS:
+        if plat_name not in _vcvars_names:
             raise DistutilsPlatformError(
-                f"--plat-name must be one of {tuple(PLAT_TO_VCVARS)}"
+                f"--plat-name must be one of {tuple(_vcvars_names)}"
             )
 
-        # Get the vcvarsall.bat spec for the requested platform.
-        plat_spec = PLAT_TO_VCVARS[plat_name]
+        plat_spec = _get_vcvars_spec(get_host_platform(), get_platform())
 
         vc_env = _get_vc_env(plat_spec)
         if not vc_env: