BLD: Change cyg2win32 to use the cygpath utility.

The old function only handled the default `/cygdrive` prefix. This can be customized to different values: `/` and `/mnt` are both common. `/proc/cygdrive` does the same thing, regardless of user customization. `cygpath` handles all of these. There's also a C function if you'd prefer to avoid the `fork()` call, which tends to be slow on Cygwin. Edit: Fix the cyg2win32 function to have correct types. It returned bytes earlier; this should return a string. Edit: Fix docsrting to follow numpydoc.
numpy · DWesl · Feb 1, 2021 · Feb 1, 2021 · Feb 1, 2021 · Feb 1, 2021
commit e30548dce7b35e6ee06555d5894722d42bba6a0d
diff --git a/numpy/distutils/misc_util.py b/numpy/distutils/misc_util.py
@@ -377,9 +377,40 @@ def blue_text(s):
 #########################
 
 def cyg2win32(path):
-    if sys.platform=='cygwin' and path.startswith('/cygdrive'):
-        path = path[10] + ':' + os.path.normcase(path[11:])
-    return path
+    """Convert a path from Cygwin-native to Windows-native.
+
+    Uses the builting cygpath script to do the actual conversion.
+    Falls back to returning the original path if this fails.
+
+    Handles the default "/cygdrive" prefix as well as "/proc/cygdrive"
+    portable prefix and custom cygdrive prefixes such as "/" or "/mnt".
+
+    Parameters
+    ----------
+    path : str
+       The path to convert
+
+    Returns
+    -------
+    converted_path : str
+        The converted path
+
+    Notes
+    -----
+    Documentation for cygpath utility:
+    https://cygwin.com/cygwin-ug-net/cygpath.html
+    Documentation for the C function it wraps:
+    https://cygwin.com/cygwin-api/func-cygwin-conv-path.html
+
+    """
+    if sys.platform != "cygwin":
+        return path
+    try:
+        return subprocess.check_output(
+            ["/usr/bin/cygpath", "--windows", path], universal_newlines=True
+        )
+    except subprocess.CalledProcessError:
+        return path
 
 def mingw32():
     """Return true when using mingw32 environment.