Use R_NoSave over R_Slave for R 4.0+

[jmcphers]

This is the minimal change required to get RStudio building against R 4.0+. In R 4.0, the field R_Slave was removed from structRstart and replaced with R_NoEcho.

wch/r-source@f1ff49e#diff-178cb445c6da1cdb9d55c898b6118256R70

Since RStudio references this field, and we would like to be able to build against both R 4.0 and R 3.x (largely for ease of development and testing), we need to refer to the field by the appropriate name.

The irony of using --slave to determine whether we need to use R_Slave or R_NoEcho is not lost. R 4.0 (currently) only soft-deprecates --slave, whereas R_Slave has been simply removed.

Related to #5923. We will need to make another (significantly more invasive) change when R fully removes --slave.

[kevinushey]

Can we check for / report an error here if one occurs? (Basically signal for future-us that --slave is really gone now)

[jmcphers]

This will already happen elsewhere:

rstudio/src/cpp/CMakeLists.txt

Lines 469 to 478 in b22f63e

	# verify we got the required R version
	if(LIBR_FOUND AND RSTUDIO_VERIFY_R_VERSION)
	set(CMAKE_REQUIRED_INCLUDES ${LIBR_INCLUDE_DIRS})
	execute_process(
	COMMAND "${LIBR_EXECUTABLE}" "--vanilla" "--slave" "-e" "cat(as.character(getRversion()))"
	OUTPUT_VARIABLE LIBR_VERSION)
	if (LIBR_VERSION VERSION_LESS "3.0.1")
	message(FATAL_ERROR "Minimum R version (${RSTUDIO_R_MAJOR_VERSION_REQUIRED}.${RSTUDIO_R_MINOR_VERSION_REQUIRED}.${RSTUDIO_R_PATCH_VERSION_REQUIRED}) not found.")
	endif()
	endif()

If you're wondering why we don't piggyback on this check, it's because the r/ folder is a separate CMake project which has no visibility of the parent project's variables.

[Enchufa2]

Are you going to backport this to the v1.2 branch?

[yurivict]

Please make a release because the currently released 1.2.5042 isn't compatible with R-4.0.0.

[jmcphers]

We're working on it (should be within a few weeks).

[Jylpah]

I am receiving the same error with 1.3.959 so is this still open? WSL 2, Ubuntu 20.04LTS, 64bit

/usr/local/src/rstudio-1.3.959/src/cpp/r/session/REmbeddedPosix.cpp:107:8: error: ‘struct structRstart’ has no member named ‘R_Slave’
  107 |    Rp->R_Slave = FALSE ;
      |        ^~~~~~~
make[2]: *** [src/cpp/r/CMakeFiles/rstudio-r.dir/build.make:544: src/cpp/r/CMakeFiles/rstudio-r.dir/session/REmbeddedPosix.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:647: src/cpp/r/CMakeFiles/rstudio-r.dir/all] Error 2
make: *** [Makefile:152: all] Error 2

[kevinushey]

This should be fixed in the v1.3 patch release:

rstudio/src/cpp/r/session/REmbeddedPosix.cpp

Lines 109 to 113 in 21b2886

	#if R_VERSION < R_Version(4, 0, 0)
	Rp->R_Slave = FALSE;
	#else
	Rp->R_NoEcho = FALSE;
	#endif