[R-package] small cleanup after R 3.x drop (#7043)

R-package/README.md

@@ -128,6 +128,14 @@ If you want to force `LightGBM` to use MSYS2 (for any R version), pass `--use-ms
 Rscript build_r.R --use-msys2
 ```
 
+**MinGW**
+
+If you want to force `LightGBM` to use [MinGW](https://www.mingw-w64.org/) (for any R version), pass `--use-mingw` to the installation script.
+
+```shell
+Rscript build_r.R --use-mingw
+```
+
 #### Mac OS Preparation
 
 You can perform installation either with **Apple Clang** or **gcc**. In case you prefer **Apple Clang**, you should install **OpenMP** (details for installation can be found in [Installation Guide](https://github.com/microsoft/LightGBM/blob/master/docs/Installation-Guide.rst#apple-clang)) first. In case you prefer **gcc**, you need to install it (details for installation can be found in [Installation Guide](https://github.com/microsoft/LightGBM/blob/master/docs/Installation-Guide.rst#gcc)) and set some environment variables to tell R to use `gcc` and `g++`. If you install these from Homebrew, your versions of `g++` and `gcc` are most likely in `/usr/local/bin`, as shown below.

R-package/configure.win

@@ -11,7 +11,6 @@ CXX17=`"${R_EXE}" CMD config CXX17`
 CXX17STD=`"${R_EXE}" CMD config CXX17STD`
 CXX="${CXX17} ${CXX17STD}"
 CXXFLAGS=`"${R_EXE}" CMD config CXX17FLAGS`
-CXX_STD="CXX17"
 CPPFLAGS=`"${R_EXE}" CMD config CPPFLAGS`
 
 # LightGBM-specific flags
@@ -98,9 +97,6 @@ then
 fi
 
 # Generate Makevars.win from Makevars.win.in
-sed -e \
-    "s/@CXX_STD@/$CXX_STD/" \
-    < src/Makevars.win.in > src/Makevars.win
 sed -e \
     "s/@LGB_CPPFLAGS@/$LGB_CPPFLAGS/" \
     < src/Makevars.win.in > src/Makevars.win

R-package/src/Makevars.win.in

-CXX_STD = @CXX_STD@
+CXX_STD = CXX17
 
 PKGROOT=.
 

R-package/src/install.libs.R

@@ -15,8 +15,6 @@ if (.Machine$sizeof.pointer != 8L) {
   stop("LightGBM only supports 64-bit R, please check the version of R and Rtools.")
 }
 
-R_ver <- as.double(R.Version()$major) + as.double(R.Version()$minor) / 10.0
-
 # Get some paths
 source_dir <- file.path(R_PACKAGE_SOURCE, "src", fsep = "/")
 build_dir <- file.path(source_dir, "build", fsep = "/")
@@ -162,11 +160,7 @@ if (use_mingw) {
   # Rtools 4.0 moved from MinGW to MSYS toolchain. If user tries
   # Visual Studio install but that fails, fall back to the toolchain
   # supported in Rtools
-  if (R_ver >= 4.0) {
   windows_toolchain <- "MSYS2"
-  } else {
-    windows_toolchain <- "MinGW"
-  }
 }
 windows_build_tool <- WINDOWS_BUILD_TOOLS[[windows_toolchain]][["build_tool"]]
 windows_makefile_generator <- WINDOWS_BUILD_TOOLS[[windows_toolchain]][["makefile_generator"]]

R-package/tests/testthat/helper.R

@@ -39,7 +39,7 @@ data.table::setDTthreads(1L)
 # [description]
 #    test that every element of 'x' is in 'y'
 #
-#    testthat::expect_in() was added in {testthat} v3.1.19
+#    testthat::expect_in() was added in {testthat} v3.1.19.
 #    This is here to support a similar interface on older {testthat} versions.
 .expect_in <- function(x, y) {
   if (exists("expect_in")) {