rust: use {build,host}Platform.{gcc.arch,rustc.flags}

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

@@ -27,6 +27,18 @@ let
   rustBuildPlatform = rust.toRustTarget stdenv.buildPlatform;
   rustTargetPlatform = rust.toRustTarget stdenv.hostPlatform;
   rustTargetPlatformSpec = rust.toRustTargetSpec stdenv.hostPlatform;
+
+  rustFlagsFor = plat: [
+    # https://doc.rust-lang.org/rustc/codegen-options/index.html#target-feature
+    "-C" "target-feature=${if plat.isStatic then "+" else "-"}crt-static"
+  ] ++ lib.optionals (plat ? gcc.arch) [
+    # https://doc.rust-lang.org/rustc/codegen-options/index.html#target-cpu
+    #"-C" "target-cpu=${plat.gcc.arch}"
+  ] ++ lib.optionals (plat ? gcc.tune) [
+    # https://doc.rust-lang.org/rustc/codegen-options/index.html#tune-cpu
+    #"-Z" "tune-cpu=${plat.gcc.tune}"  # nightly builds only; uncomment when stabilized
+  ] ++ (plat.rustc.flags or []);
+
 in {
   cargoBuildHook = callPackage ({ }:
     makeSetupHook {
@@ -76,14 +88,37 @@ in {
         # inputs do not cause us to find the wrong `diff`.
         diff = "${lib.getBin buildPackages.diffutils}/bin/diff";
 
+        # We want to specify the correct crt-static flag for both
+        # the build and host platforms. This is important when the wanted
+        # value for crt-static does not match the defaults in the rustc target,
+        # like for pkgsMusl or pkgsCross.musl64; Upstream rustc still assumes
+        # that musl = static[1].
+        #
+        # By default, Cargo doesn't apply RUSTFLAGS when building build.rs
+        # if --target is passed, so the only good way to set crt-static for
+        # build.rs files is to use the unstable -Zhost-config Cargo feature.
+        # This allows us to specify flags that should be passed to rustc
+        # when building for the build platform. We also need to use
+        # -Ztarget-applies-to-host, because using -Zhost-config requires it.
+        #
+        # When doing this, we also have to specify the linker, or cargo
+        # won't pass a -C linker= argument to rustc.  This will make rustc
+        # try to use its default value of "cc", which won't be available
+        # when cross-compiling.
+        #
+        # [1]: https://github.com/rust-lang/compiler-team/issues/422
         cargoConfig = ''
-          [target."${rust.toRustTarget stdenv.buildPlatform}"]
+          [host]
           "linker" = "${ccForBuild}"
-          ${lib.optionalString (stdenv.buildPlatform.config != stdenv.hostPlatform.config) ''
-            [target."${shortTarget}"]
-            "linker" = "${ccForHost}"
-          ''}
-          "rustflags" = [ "-C", "target-feature=${if stdenv.hostPlatform.isStatic then "+" else "-"}crt-static" ]
+          "rustflags" = [ ${lib.concatMapStringsSep ", " (f: "\"${builtins.toString f}\"") (rustFlagsFor stdenv.buildPlatform)} ]
+
+          [target."${shortTarget}"]
+          "linker" = "${ccForHost}"
+          "rustflags" = [ ${lib.concatMapStringsSep ", " (f: "\"${builtins.toString f}\"") (rustFlagsFor stdenv.hostPlatform)} ]
+
+          [unstable]
+          host-config = true
+          target-applies-to-host = true
         '';
       };
     } ./cargo-setup-hook.sh) {};