Skip hub alias for workspace-root members

Author: isaacparker0

rs/extensions.bzl

@@ -484,6 +484,12 @@ alias(
     for package in cargo_metadata["packages"]:
         package_dir = _manifest_package_dir(package["manifest_path"], repo_root)
         bazel_package = paths.join(workspace_package, package_dir).removesuffix("/")
+        if not bazel_package:
+            # The alias relies on Bazel's `//foo/bar` -> `//foo/bar:bar`
+            # shorthand to pick a target name. When the workspace member is
+            # at the bazel workspace root, there's no path component to
+            # derive a name from.
+            continue
         hub_contents.append("""
 alias(
     name = "{name}-{version}",

test/BUILD.bazel

@@ -333,6 +333,7 @@ filegroup(
         "@binaries//:protoc-gen-prost-extra",
         "@binaries//:protoc-gen-prost__protoc-gen-prost",
         "@binary_only//:b3sum__b3sum",
+        "@root_package//:_workspace_deps",
     ] + select({
         "@platforms//os:windows": [],
         "//conditions:default": [

test/Cargo.lock

@@ -0,0 +1,8 @@
+[package]
+name = "root_pkg"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[lib]
+path = "lib.rs"

test/Cargo.toml

@@ -184,6 +184,24 @@ TESTS = [
 
 [use_repo(crate, test) for test in TESTS]
 
+# Workspace member whose Cargo.toml lives at the bazel workspace root.
+crate.from_cargo(
+    name = "root_package",
+    cargo_lock = "//:Cargo.lock",
+    cargo_toml = "//:Cargo.toml",
+    debug = True,
+    platform_triples = [
+        "aarch64-unknown-linux-gnu",
+        "aarch64-apple-darwin",
+        "aarch64-pc-windows-gnullvm",
+        "x86_64-unknown-linux-gnu",
+        "x86_64-apple-darwin",
+        "x86_64-pc-windows-gnullvm",
+    ],
+)
+
+use_repo(crate, "root_package")
+
 crate.annotation(
     crate = "rustls",
     # Test for https://github.com/bazelbuild/rules_rust/issues/3626