binary-extensions: update Windows instructions

source/guides/packaging-binary-extensions.rst

@@ -258,48 +258,27 @@ Binary extensions for Windows
 Before it is possible to build a binary extension, it is necessary to ensure
 that you have a suitable compiler available. On Windows, Visual C is used to
 build the official CPython interpreter, and should be used to build compatible
-binary extensions.
+binary extensions.  To set up a build environment for binary extensions, install
+`Visual Studio Community Edition <https://www.visualstudio.com/en-us/downloads/download-visual-studio-vs.aspx>`__
+- any recent version is fine.
+
+One caveat: if you use Visual Studio 2019 or later, your extension will depend
+on an "extra" file, ``VCRUNTIME140_1.dll``, in addition to the
+``VCRUNTIME140.dll`` that all previous versions back to 2015 depend on. This
+will add an extra requirement to using your extension on versions of CPython
+that do not include this extra file. To avoid this, you can add the
+compile-time argument ``/d2FH4-``. Recent versions of Python may include this
+file.
+
+Building for Python prior to 3.5 is discouraged, because older versions of
+Visual Studio are no longer available from Microsoft. If you do need to build
+for older versions, you can set ``DISTUTILS_USE_SDK=1`` and ``MSSdk=1`` to
+force a the currently activated version of MSVC to be found, and you should
+exercise care when designing your extension not to malloc/free memory across
+different libraries, avoid relying on changed data structures, and so on. Tools
+for generating extension modules usually avoid these things for you.
 
-Python 2.7 used Visual Studio 2008, Python 3.3 and 3.4 used Visual Studio 2010,
-and Python 3.5+ uses Visual Studio 2015 or later. Unfortunately, older versions of
-Visual Studio are no longer easily available from Microsoft, so for versions
-of Python prior to 3.5, the compilers must be obtained differently if you do
-not already have a copy of the relevant version of Visual Studio.
 
-To set up a build environment for binary extensions, the steps are as follows:
-
-    For Python 2.7
-
-        1. Install "Visual C++ Compiler Package for Python 2.7",
-           which is available from
-           `Microsoft's website <https://www.microsoft.com/en-gb/download/details.aspx?id=44266>`__.
-        2. Use (a recent version of) setuptools in your setup.py (pip will
-           do this for you, in any case).
-        3. Done.
-
-    For Python 3.4
-
-        1. Install "Windows SDK for Windows 7 and .NET Framework 4" (v7.1),
-           which is available from
-           `Microsoft's website <https://www.microsoft.com/en-gb/download/details.aspx?id=8279>`__.
-        2. Work from an SDK command prompt (with the environment variables
-           set, and the SDK on PATH).
-        3. Set DISTUTILS_USE_SDK=1
-        4. Done.
-
-    For Python 3.5
-
-        1. Install `Visual Studio 2015 Community Edition
-           <https://www.visualstudio.com/en-us/downloads/download-visual-studio-vs.aspx>`__
-           (or any later version, when these are released).
-        2. Done.
-
-Note that from Python 3.5 onwards, Visual Studio works in a backward
-compatible way, which means that any future version of Visual Studio will
-be able to build Python extensions for all Python versions from 3.5 onwards.
-
-Building with the recommended compiler on Windows ensures that a compatible C library
-is used throughout the Python process.
 
 Binary extensions for Linux
 ---------------------------