Fix linking objc binaries in the root package

Fixes #24625

src/main/starlark/builtins_bzl/common/objc/compilation_support.bzl

@@ -28,6 +28,9 @@ load(":common/xcode/providers.bzl", "XcodeVersionInfo")
 
 objc_internal = _builtins.internal.objc_internal
 
+def _join_path(*path):
+    return "/".join([p for p in path if p != ""])
+
 def _build_variable_extensions(ctx, arc_enabled):
     extensions = {}
     if hasattr(ctx.attr, "pch") and ctx.attr.pch != None:
@@ -642,9 +645,13 @@ def _register_j2objc_dead_code_removal_actions(common_variables, deps, build_con
     replace_libs = {}
     for j2objc_archive in common_variables.objc_provider.j2objc_library.to_list():
         pruned_j2objc_archive = ctx.actions.declare_shareable_artifact(
-            ctx.label.package + "/_j2objc_pruned/" + ctx.label.name + "/" +
-            j2objc_archive.short_path[:-len(j2objc_archive.extension)].strip(".") +
-            "_pruned." + j2objc_archive.extension,
+            _join_path(
+                ctx.label.package,
+                "_j2objc_pruned",
+                ctx.label.name,
+                j2objc_archive.short_path[:-len(j2objc_archive.extension)].strip(".") +
+                "_pruned." + j2objc_archive.extension,
+            ),
             build_config.bin_dir,
         )
         replace_libs[j2objc_archive] = pruned_j2objc_archive
@@ -699,7 +706,7 @@ def _register_obj_filelist_action(ctx, build_config, obj_files):
     This File is suitable to signal symbols to archive in a libtool archiving invocation.
     """
     obj_list = ctx.actions.declare_shareable_artifact(
-        ctx.label.package + "/" + ctx.label.name + "-linker.objlist",
+        _join_path(ctx.label.package, ctx.label.name + "-linker.objlist"),
         build_config.bin_dir,
     )
 
@@ -737,7 +744,7 @@ def _register_binary_strip_action(
         strip_safe = True
 
     stripped_binary = ctx.actions.declare_shareable_artifact(
-        ctx.label.package + "/" + name,
+        _join_path(ctx.label.package, name),
         build_config.bin_dir,
     )
     args = ctx.actions.args()
@@ -778,12 +785,12 @@ def _linkstamp_map(ctx, linkstamps, output, build_config):
     # create linkstamps_map - mapping from linkstamps to object files
     linkstamps_map = {}
 
-    stamp_output_dir = ctx.label.package + "/_objs/" + output.basename + "/"
+    stamp_output_dir = _join_path(ctx.label.package, "_objs", output.basename)
     for linkstamp in linkstamps.to_list():
         linkstamp_file = linkstamp.file()
-        stamp_output_path = (
-            stamp_output_dir +
-            linkstamp_file.short_path[:-len(linkstamp_file.extension)].rstrip(".") + ".o"
+        stamp_output_path = _join_path(
+            stamp_output_dir,
+            linkstamp_file.short_path[:-len(linkstamp_file.extension)].rstrip(".") + ".o",
         )
         stamp_output_file = ctx.actions.declare_shareable_artifact(
             stamp_output_path,
@@ -840,12 +847,12 @@ def _register_configuration_specific_link_actions(
     if (ctx.fragments.cpp.objc_enable_binary_stripping() and
         ctx.fragments.cpp.compilation_mode() == "opt"):
         binary = ctx.actions.declare_shareable_artifact(
-            ctx.label.package + "/" + name + "_unstripped",
+            _join_path(ctx.label.package, name + "_unstripped"),
             build_config.bin_dir,
         )
     else:
         binary = ctx.actions.declare_shareable_artifact(
-            ctx.label.package + "/" + name,
+            _join_path(ctx.label.package, name),
             build_config.bin_dir,
         )
 

src/test/java/com/google/devtools/build/lib/rules/objc/ObjcStarlarkTest.java

@@ -293,6 +293,33 @@ def my_rule_impl(ctx):
         .contains("test_starlark/apple_starlark/liblib.a");
   }
 
+  @Test
+  public void testStarlarkLinkBinaryInRootPackage() throws Exception {
+    scratch.file("a.m");
+    addAppleBinaryStarlarkRule(scratch);
+    scratch.file(
+        "BUILD",
+        """
+        load("//test_starlark:apple_binary_starlark.bzl", "apple_binary_starlark")
+
+        package(default_visibility = ["//visibility:public"])
+
+        objc_library(
+            name = "lib",
+            srcs = ["a.m"],
+        )
+
+        apple_binary_starlark(
+            name = "bin",
+            platform_type = "macos",
+            deps = [":lib"],
+        )
+        """);
+
+    ConfiguredTarget binaryTarget = getConfiguredTarget("//:bin");
+    assertThat(getConfiguredTarget("//:bin")).isNotNull();
+  }
+
   @Test
   public void testObjcRuleCanDependOnArbitraryStarlarkRuleThatProvidesCcInfo() throws Exception {
     scratch.file("test_starlark/rule/BUILD");