rustPlatform.buildRustPackage: specify environment variables via the env attribute

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

@@ -15,6 +15,16 @@
   windows,
 }:
 
+let
+  interpolateString =
+    s:
+    if lib.isList s then
+      lib.concatMapStringsSep " " (s: "${s}") (lib.filter (s: s != null) s)
+    else if s == null then
+      ""
+    else
+      "${s}";
+in
 lib.extendMkDerivation {
   constructDrv = stdenv.mkDerivation;
 
@@ -23,6 +33,7 @@ lib.extendMkDerivation {
     "cargoUpdateHook"
     "cargoLock"
     "useFetchCargoVendor"
+    "RUSTFLAGS"
   ];
 
   extendDrvArgs =
@@ -77,11 +88,19 @@ lib.extendMkDerivation {
     assert lib.warnIf (args ? useFetchCargoVendor)
       "buildRustPackage: `useFetchCargoVendor` is non‐optional and enabled by default as of 25.05, remove it"
       true;
+    {
+      env = {
+        PKG_CONFIG_ALLOW_CROSS = if stdenv.buildPlatform != stdenv.hostPlatform then 1 else 0;
+        RUST_LOG = logLevel;
+        RUSTFLAGS =
+          lib.optionalString (
+            stdenv.hostPlatform.isDarwin && buildType == "debug"
+          ) "-C split-debuginfo=packed "
+          # Workaround the existing RUSTFLAGS specified as a list.
+          + interpolateString (args.RUSTFLAGS or "");
+      }
+      // args.env or { };
 
-    lib.optionalAttrs (stdenv.hostPlatform.isDarwin && buildType == "debug") {
-      RUSTFLAGS = "-C split-debuginfo=packed " + (args.RUSTFLAGS or "");
-    }
-    // {
       cargoDeps =
         if cargoVendorDir != null then
           null
@@ -143,12 +162,8 @@ lib.extendMkDerivation {
 
       patches = cargoPatches ++ patches;
 
-      PKG_CONFIG_ALLOW_CROSS = if stdenv.buildPlatform != stdenv.hostPlatform then 1 else 0;
-
       postUnpack = ''
         eval "$cargoDepsHook"
-
-        export RUST_LOG=${logLevel}
       ''
       + (args.postUnpack or "");