cargoSetupHook: pass host config flags

pkgs/build-support/rust/hooks/cargo-setup-hook.sh

@@ -27,7 +27,7 @@ cargoSetupPostUnpackHook() {
     cat ${tmp_config} >> .cargo/config
 
     cat >> .cargo/config <<'EOF'
-    @rustTarget@
+    @cargoConfig@
 EOF
 
     echo "Finished cargoSetupPostUnpackHook"

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

@@ -68,15 +68,37 @@ in {
         # The `.nativeDrv` stanza works like nativeBuildInputs and ensures cross-compiling has the right version available.
         diff = "${diffutils.nativeDrv or diffutils}/bin/diff";
 
-        # Target platform
+        # We want to specify the correct crt-static flag for both
-        rustTarget = ''
+        # the build and host platforms. This is important when the wanted
-          [target."${rust.toRustTarget stdenv.buildPlatform}"]
+        # value for crt-static does not match the defaults in the rustc target,
+        # like for pkgsMusl or pkgsCross.musl64; Upstream rustc still assumes
+        # that musl = static[1].
+        #
+        # By default, Cargo doesn't apply RUSTFLAGS when building build.rs
+        # if --target is passed, so the only good way to set crt-static for
+        # build.rs files is to use the unstable -Zhost-config Cargo feature.
+        # This allows us to specify flags that should be passed to rustc
+        # when building for the build platform. We also need to use
+        # -Ztarget-applies-to-host, because using -Zhost-config requires it.
+        #
+        # When doing this, we also have to specify the linker, or cargo
+        # won't pass a -C linker= argument to rustc.  This will make rustc
+        # try to use its default value of "cc", which won't be available
+        # when cross-compiling.
+        #
+        # [1]: https://github.com/rust-lang/compiler-team/issues/422
+        cargoConfig = ''
+          [host]
           "linker" = "${ccForBuild}"
-          ${lib.optionalString (stdenv.buildPlatform.config != stdenv.hostPlatform.config) ''
+          "rustflags" = [ "-C", "target-feature=${if stdenv.buildPlatform.isStatic then "+" else "-"}crt-static" ]
-            [target."${shortTarget}"]
+
-            "linker" = "${ccForHost}"
+          [target."${shortTarget}"]
-          ''}
+          "linker" = "${ccForHost}"
           "rustflags" = [ "-C", "target-feature=${if stdenv.hostPlatform.isStatic then "+" else "-"}crt-static" ]
+
+          [unstable]
+          host-config = true
+          target-applies-to-host = true
         '';
       };
     } ./cargo-setup-hook.sh) {};