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A general description is here.
Make files and installer scripts for Windows have been tailored to ‘Rtools43’, an update of the ‘Rtools42’ toolchain. It is based on ‘gcc’ 12 and newer version of MinGW-W64, binutils and libraries. At this point, R-devel can still be built using Rtools42 without changes, but when R-devel is installed via the installer, it will by default look for Rtools43. Pre-release builds of Rtools43 are available from <URL: https://www.r-project.org/nosvn/winutf8/ucrt3/>.