buildRustPackage: factor out build phase to cargoBuildHook

- API change: remove the `target` argument of `buildRustPackage`, the
  target should always be in sync with the C/C++ compiler that is used.

- Gathering of binaries has moved from `buildPhase` to `installPhase`,
  this simplifies the hook and orders this functionality logically
  with the installation logic.

pkgs/build-support/rust/hooks/default.nix

@@ -1,5 +1,6 @@
 { buildPackages
 , callPackage
+, cargo
 , diffutils
 , lib
 , makeSetupHook
@@ -16,9 +17,24 @@ let
   shortTarget = if targetIsJSON then
       (lib.removeSuffix ".json" (builtins.baseNameOf "${target}"))
     else target;
-  ccForBuild="${buildPackages.stdenv.cc}/bin/${buildPackages.stdenv.cc.targetPrefix}cc";
-  ccForHost="${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc";
+  ccForBuild = "${buildPackages.stdenv.cc}/bin/${buildPackages.stdenv.cc.targetPrefix}cc";
+  cxxForBuild = "${buildPackages.stdenv.cc}/bin/${buildPackages.stdenv.cc.targetPrefix}c++";
+  ccForHost = "${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc";
+  cxxForHost = "${stdenv.cc}/bin/${stdenv.cc.targetPrefix}c++";
+  rustBuildPlatform = rust.toRustTarget stdenv.buildPlatform;
+  rustTargetPlatform = rust.toRustTarget stdenv.hostPlatform;
+  rustTargetPlatformSpec = rust.toRustTargetSpec stdenv.hostPlatform;
 in {
+  cargoBuildHook = callPackage ({ }:
+    makeSetupHook {
+      name = "cargo-build-hook.sh";
+      deps = [ cargo ];
+      substitutions = {
+        inherit ccForBuild ccForHost cxxForBuild cxxForHost
+          rustBuildPlatform rustTargetPlatform rustTargetPlatformSpec;
+      };
+    } ./cargo-build-hook.sh) {};
+
   cargoSetupHook = callPackage ({ }:
     makeSetupHook {
       name = "cargo-setup-hook.sh";