cross stdenv adaptor: Support --host --build --target across the board

Packages get --host and --target by default, but can explicitly request
any subset to be passed as needed. See docs for more info.

rustc: Avoid hash breakage by using the old (ignored)
dontSetConfigureCross when not cross building

pkgs/stdenv/adapters.nix

@@ -56,8 +56,15 @@ rec {
 
   # Return a modified stdenv that adds a cross compiler to the
   # builds.
-  makeStdenvCross = stdenvOrig: cross: cc: let
-    stdenv = stdenvOrig.override {
+  makeStdenvCross = { stdenv
+                    , cc
+                    , buildPlatform, hostPlatform, targetPlatform
+                    } @ overrideArgs: let
+    stdenv = overrideArgs.stdenv.override {
+      # TODO(@Ericson2314): Cannot do this for now because then Nix thinks the
+      # resulting derivation should be built on the host platform.
+      #hostPlatform = buildPlatform;
+      #targetPlatform = hostPlatform;
       inherit cc;
 
       allowedRequisites = null;
@@ -70,7 +77,12 @@ rec {
     mkDerivation =
       { name ? "", buildInputs ? [], nativeBuildInputs ? []
       , propagatedBuildInputs ? [], propagatedNativeBuildInputs ? []
-      , selfNativeBuildInput ? false, ...
+      , configureFlags ? []
+      , # Target is not included by default because most programs don't care.
+        # Including it then would cause needless massive rebuilds.
+        configurePlatforms   ? args.crossAttrs.configurePlatforms   or [ "build" "host" ]
+      , selfNativeBuildInput ? args.crossAttrs.selfNativeBuildInput or false
+      , ...
       } @ args:
 
       let
@@ -93,16 +105,23 @@ rec {
         nativeInputsFromBuildInputs = stdenv.lib.filter hostAsNativeDrv buildInputsNotNull;
       in
         stdenv.mkDerivation (args // {
-          name = name + "-" + cross.config;
+          name = name + "-" + hostPlatform.config;
           nativeBuildInputs = nativeBuildInputs
             ++ nativeInputsFromBuildInputs
             ++ stdenv.lib.optional selfNativeBuildInput nativeDrv
               # without proper `file` command, libtool sometimes fails
               # to recognize 64-bit DLLs
-            ++ stdenv.lib.optional (cross.config  == "x86_64-w64-mingw32") pkgs.file
-            ++ stdenv.lib.optional (cross.config  == "aarch64-linux-gnu") pkgs.updateAutotoolsGnuConfigScriptsHook
+            ++ stdenv.lib.optional (hostPlatform.config == "x86_64-w64-mingw32") pkgs.file
+            ++ stdenv.lib.optional (hostPlatform.config == "aarch64-linux-gnu") pkgs.updateAutotoolsGnuConfigScriptsHook
             ;
 
+          # This parameter is sometimes a string and sometimes a list, yuck
+          configureFlags = let inherit (stdenv.lib) optional elem; in
+            (if stdenv.lib.isString configureFlags then [configureFlags] else configureFlags)
+            ++ optional (elem "build"  configurePlatforms) "--build=${buildPlatform.config}"
+            ++ optional (elem "host"   configurePlatforms) "--host=${hostPlatform.config}"
+            ++ optional (elem "target" configurePlatforms) "--target=${targetPlatform.config}";
+
           # Cross-linking dynamic libraries, every buildInput should
           # be propagated because ld needs the -rpath-link to find
           # any library needed to link the program dynamically at
@@ -111,7 +130,7 @@ rec {
           propagatedBuildInputs = propagatedBuildInputs ++ buildInputs;
           propagatedNativeBuildInputs = propagatedNativeBuildInputs;
 
-          crossConfig = cross.config;
+          crossConfig = hostPlatform.config;
         } // args.crossAttrs or {});
   };
 

pkgs/stdenv/cross/default.nix

@@ -31,12 +31,15 @@ in bootStages ++ [
     targetPlatform = crossSystem;
     inherit config overlays;
     selfBuild = false;
-    stdenv = buildPackages.makeStdenvCross
-      buildPackages.stdenv
-      crossSystem
-      (if crossSystem.useiOSCross or false
-       then buildPackages.darwin.ios-cross
-       else buildPackages.gccCrossStageFinal);
+    stdenv = buildPackages.makeStdenvCross {
+      inherit (buildPackages) stdenv;
+      buildPlatform = localSystem;
+      hostPlatform = crossSystem;
+      targetPlatform = crossSystem;
+      cc = if crossSystem.useiOSCross or false
+           then buildPackages.darwin.ios-cross
+           else buildPackages.gccCrossStageFinal;
+    };
   })
 
 ]