rstudio: Fix recompilation issue with custom package set

This attempts to fix the issue described at
https://github.com/NixOS/nixpkgs/pull/22219#issuecomment-291801133.
Any change to the custom packages passed to RStudio causes this to
completely rebuild RStudio, which is completely unnecessary and also a
bit of a hindrance as it's a fairly slow build.

This rolls back most of that old PR, and instead implements something
more like rWrapper.  Existing configurations with the old useRPackages
will break.

pkgs/development/r-modules/README.md

@@ -55,28 +55,23 @@ available.
 
 ## RStudio
 
-RStudio by default will not use the libraries installed like above.
-You must override its R version with your custom R environment, and
-set `useRPackages` to `true`, like below:
+RStudio uses a standard set of packages and ignores any custom R
+environments or installed packages you may have.  To create a custom
+environment, see `rstudioWrapper`, which functions similarly to
+`rWrapper`:
 
 ```nix
 {
     packageOverrides = super: let self = super.pkgs; in
     {
 
-        rEnv = super.rWrapper.override {
+        rstudioEnv = super.rstudioWrapper.override {
             packages = with self.rPackages; [
-                devtools
+                dplyr
                 ggplot2
                 reshape2
-                yaml
-                optparse
                 ];
         };
-        rstudioEnv = super.rstudio.override {
-            R = rEnv;
-            useRPackages = true;
-        };
     };
 }
 ```

pkgs/development/r-modules/wrapper-rstudio.nix

@@ -0,0 +1,33 @@
+{ stdenv, R, rstudio, makeWrapper, recommendedPackages, packages }:
+
+stdenv.mkDerivation rec {
+
+  name = rstudio.name + "-wrapper";
+
+  buildInputs = [makeWrapper R rstudio] ++ recommendedPackages ++ packages;
+
+  unpackPhase = ":";
+
+  # rWrapper points R to a specific set of packages by using a wrapper
+  # (as in https://nixos.org/nixpkgs/manual/#r-packages) which sets
+  # R_LIBS_SITE.  Ordinarily, it would be possible to make RStudio use
+  # this same set of packages by simply overriding its version of R
+  # with the wrapped one, however, RStudio internally overrides
+  # R_LIBS_SITE.  The below works around this by turning R_LIBS_SITE
+  # into an R file (fixLibsR) which achieves the same effect, then
+  # uses R_PROFILE_USER to load this code at startup in RStudio.
+  fixLibsR = "fix_libs.R";
+  installPhase = ''
+    mkdir $out
+    echo "# Autogenerated by wrapper-rstudio.nix from R_LIBS_SITE" > $out/${fixLibsR}
+    echo -n ".libPaths(c(.libPaths(), \"" >> $out/${fixLibsR}
+    echo -n $R_LIBS_SITE | sed -e 's/:/", "/g' >> $out/${fixLibsR}
+    echo -n "\"))" >> $out/${fixLibsR}
+    echo >> $out/${fixLibsR}
+    makeWrapper ${rstudio}/bin/rstudio $out/bin/rstudio --set R_PROFILE_USER $out/${fixLibsR}
+  '';
+  
+  meta = {
+    platforms = stdenv.lib.platforms.unix;
+  };
+}

pkgs/development/r-modules/wrapper.nix

@@ -1,19 +1,12 @@
 { stdenv, R, makeWrapper, recommendedPackages, packages }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   name = R.name + "-wrapper";
 
   buildInputs = [makeWrapper R] ++ recommendedPackages ++ packages;
 
   unpackPhase = ":";
 
-  # This filename is used in 'installPhase', but needs to be
-  # referenced elsewhere.  This will be relative to this package's
-  # path.
-  passthru = {
-    fixLibsR = "fix_libs.R";
-  };
-  
   installPhase = ''
     mkdir -p $out/bin
     cd ${R}/bin
@@ -21,17 +14,6 @@ stdenv.mkDerivation rec {
       makeWrapper ${R}/bin/$exe $out/bin/$exe \
         --prefix "R_LIBS_SITE" ":" "$R_LIBS_SITE"
     done
-    # RStudio (and perhaps other packages) overrides the R_LIBS_SITE
-    # which the wrapper above applies, and as a result packages
-    # installed in the wrapper (as in the method described in
-    # https://nixos.org/nixpkgs/manual/#r-packages) aren't visible.
-    # The below turns R_LIBS_SITE into some R startup code which can
-    # correct this.
-    echo "# Autogenerated by wrapper.nix from R_LIBS_SITE" > $out/${passthru.fixLibsR}
-    echo -n ".libPaths(c(.libPaths(), \"" >> $out/${passthru.fixLibsR}
-    echo -n $R_LIBS_SITE | sed -e 's/:/", "/g' >> $out/${passthru.fixLibsR}
-    echo -n "\"))" >> $out/${passthru.fixLibsR}
-    echo >> $out/${passthru.fixLibsR}
   '';
 
   meta = {