It seems that when installing rtools 42 the mingw compiler is added to the PATH even if setting the windows-path-include-mingw variable. This contradicts what's documented here:

This argument is now defunct on Rtools40 and later, which never add the mingw directory to the path.

I wonder if we could re-add support for that flag for rtools 42. In my use case, having mingw in the path causes problems when compiling other libraries.

Here's a link showing that mingw is in the PATH: https://github.com/dfalbel/testrtools/runs/6798819503?check_suite_focus=true#step:4:12

FWIW the Rtools42 documentation states that one of the differences compared to rtools40 is that R adds the compiler toolchain to the PATH (??) when installed:

When R is installed from the binary installer, PATH to the compiler toolchain and build tools is set automatically (by R) based on the default location, information in the registry, or the R_CUSTOM_TOOLS_PATH variable.