[Rd] R/CRAN switch to UCRT on Windows

[Tomas Kalibera]

Please note an update concerning the support of UTF-8 as native encoding 
on Windows, which may at this point be of interest particularly to 
developers of packages with native code and to R users using R-devel 
(the development version of R) on Windows:

https://developer.r-project.org/Blog/public/2021/12/07/upcoming-changes-in-r-4.2-on-windows/

The key part is that CRAN will switch the incoming checks of R packages 
on Windows to a new toolchain targeting UCRT on Monday, December 13.

It may take up to several days for all systems to synchronize and during 
this time, it may be difficult to build R-devel on Windows from source 
or to install packages.  After the switch, the snapshot R-devel builds 
and binary package builds provided by CRAN will be built using the new 
toolchain for UCRT. These new builds will use UTF-8 as the native 
encoding on recent Windows.

These builds will be incompatible with the previous builds for MSVCRT 
and installed/binary packages will be incompatible as well. The 
recommended/simplest course of action for R-devel users is to uninstall 
the old build of R-devel, RTools, delete the old package libraries, and 
then install the new versions.

Checks of CRAN packages with the new toolchain have been running since 
March with results available on CRAN pages. By now, most packages are 
working, but some packages using native (C, C++, Fortran) code still 
have to be updated. The Winbuilder service and R-hub support the new 
toolchain, there is also support/example for using github actions. The 
builds of R-devel and CRAN (and recommended binary packages) with the 
new toolchain are available regularly since March.

I've created patches for CRAN (and required Bioconductor packages) which 
are installed automatically at package installation time by R. This 
feature will be also in R-devel after the switch and will be used 
temporarily to give package authors more time to fix their packages. Uwe 
Ligges, other CRAN team members and I have also been in touch with some 
package authors, providing advice how to fix their packages, when the 
issues required more explanation. I am prepared to help the remaining 
authors as well if needed.

Please see the blog post and materials linked from there for more 
details and feel free to ask questions.

Thanks
Tomas