fetch-cargo-tarball: fix for packages without dep

pkgs/build-support/rust/fetch-cargo-tarball/cargo-vendor-normalise.py

@@ -13,7 +13,9 @@ def quote(s: str) -> str:
 def main() -> None:
     data = toml.load(sys.stdin)
 
-    assert list(data.keys()) == ["source"]
+    # There is no dependency to vendor in this project.
+    if not list(data.keys()) == ["source"]:
+        return
 
     # this value is non deterministic
     data["source"]["vendored-sources"]["directory"] = "@vendor@"

pkgs/build-support/rust/fetch-cargo-tarball/default.nix

@@ -71,6 +71,8 @@ in stdenv.mkDerivation ({
 
     cargo vendor $name --respect-source-config | cargo-vendor-normalise > $CARGO_CONFIG
 
+    # Create an empty vendor directory when there is no dependency to vendor
+    mkdir -p $name
     # Add the Cargo.lock to allow hash invalidation
     cp Cargo.lock.orig $name/Cargo.lock