rust: added hack to use correctly build cross-compiled standard crates

pkgs/build-support/rust/default.nix

@@ -1,4 +1,6 @@
-{ stdenv, cacert, git, cargo, rustc, cargo-vendor, fetchcargo, python3, buildPackages }:
+{ stdenv, cacert, git, cargo, cargo-vendor, fetchcargo, python3
+, buildPackages, pkgsCross, runCommand, rustc
+}:
 
 { name, cargoSha256 ? "unset"
 , src ? null
@@ -45,12 +47,34 @@ let
   cxxForHost="${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cxx";
   releaseDir = "target/${stdenv.hostPlatform.config}/${buildType}";
 
+  # NOTE: this is a HACK to use other rustc's standard crates
+  rustcCross = let
+    host = stdenv.hostPlatform.platform.name;
+  in runCommand "rustc-cross-${stdenv.hostPlatform.config}" {} ''
+    mkdir -p $out
+    cp -a ${buildPackages.rustc}/. $out/
+    chmod u+w -R $out
+
+    rm -rf $out/lib/rustlib/${stdenv.hostPlatform.config}
+    cp -a \
+      ${pkgsCross."${host}".rustc}/lib/rustlib/${stdenv.hostPlatform.config}/. \
+      $out/lib/rustlib/${stdenv.hostPlatform.config}/
+
+    chmod a-w -R $out
+  '';
+  isCross = stdenv.buildPlatform.config != stdenv.hostPlatform.config;
+
 in stdenv.mkDerivation (args // {
   inherit cargoDeps;
 
   patchRegistryDeps = ./patch-registry-deps;
 
-  nativeBuildInputs = [ cargo rustc git cacert ] ++ nativeBuildInputs;
+  nativeBuildInputs = [
+    cargo
+    (if isCross then rustcCross else rustc)
+    git
+    cacert
+  ] ++ nativeBuildInputs;
   inherit buildInputs;
 
   patches = cargoPatches ++ patches;