Retain module map names as Labels

With the exception of special module map names, all of them are effectively labels. This commit ensures that they are retained as `Label`s, not label string and thus:
* memory usage is reduced for module maps from external repos, whose module map name previously resulted in a newly retained string;
* module map names now use properly formatted external repo labels as names, which makes it more obvious what they are;
* the roundtrip in header module compilation flows is avoided. This actually fixes a bug where the missing `@@` prefix for label strings resulted in incorrect or even failing `Label` calls due to main repo labels being resolved relative to the rules_cc repo and external repo labels failing the validation of the `Label` constructor.

Author: fmeum

cc/private/cc_info.bzl

@@ -148,22 +148,22 @@ _ModuleMapInfo = provider(
     "ModuleMapInfo",
     fields = {
         "file": "The module map file.",
-        "name": "The name of the module.",
+        "identifier": "The identifier of the module, either a label or a string.",
     },
 )
 
-def create_module_map(*, file, name):
+def create_module_map(*, file, identifier):
     """
     Creates a module map struct.
 
     Args:
         file: The module map file.
-        name: The name of the module.
+        identifier: The identifier of the module.
     Returns:
         A module map struct.
     """
     check_private_api()
-    return _ModuleMapInfo(file = file, name = name)
+    return _ModuleMapInfo(file = file, identifier = identifier)
 
 def create_separate_module_map(module_map):
     """
@@ -174,7 +174,11 @@ def create_separate_module_map(module_map):
     Returns:
         A module map struct.
     """
-    return _ModuleMapInfo(file = module_map.file, name = module_map.name + ".sep")
+    if type(module_map.identifier) == type(""):
+        identifier = module_map.identifier + ".sep"
+    else:
+        identifier = module_map.identifier.same_package_label(module_map.identifier.name + ".sep")
+    return _ModuleMapInfo(file = module_map.file, identifier = identifier)
 
 def create_linking_context(
         *,

cc/private/compile/cc_compilation_helper.bzl

@@ -212,10 +212,16 @@ _ModuleMapInfo = provider(
     ],
 )
 
+def _stringify_module_map_identifier(label_or_string):
+    unambiguous_canonical_label_string = str(label_or_string)
+    if unambiguous_canonical_label_string.startswith("@@"):
+        return unambiguous_canonical_label_string[2:]
+    return unambiguous_canonical_label_string
+
 def _module_map_struct_to_module_map_content(parameters, tree_expander):
     lines = []
     module_map = parameters.module_map
-    lines.append("module \"%s\" {" % module_map.name)
+    lines.append("module \"%s\" {" % _stringify_module_map_identifier(module_map.identifier))
     lines.append("  export *")
 
     def expanded(artifacts):
@@ -281,10 +287,10 @@ def _module_map_struct_to_module_map_content(parameters, tree_expander):
 
     dependency_module_maps = parameters.dependency_module_maps.to_list()
     for dep in dependency_module_maps:
-        lines.append("  use \"" + dep.name + "\"")
+        lines.append("  use \"" + _stringify_module_map_identifier(dep.identifier) + "\"")
 
     if parameters.separate_module_headers:
-        separate_name = module_map.name + ".sep"
+        separate_name = _stringify_module_map_identifier(module_map.identifier) + ".sep"
         lines.append("  use \"" + separate_name + "\"")
         lines.append("}")
         lines.append("module \"" + separate_name + "\" {")
@@ -298,14 +304,14 @@ def _module_map_struct_to_module_map_content(parameters, tree_expander):
             added_paths.add(header.path)
 
         for dep in dependency_module_maps:
-            lines.append("  use \"" + dep.name + "\"")
+            lines.append("  use \"" + _stringify_module_map_identifier(dep.identifier) + "\"")
 
     lines.append("}")
 
     if parameters.extern_dependencies:
         for dep in dependency_module_maps:
             lines.append(
-                "extern module \"" + dep.name + "\" \"" +
+                "extern module \"" + _stringify_module_map_identifier(dep.identifier) + "\" \"" +
                 parameters.leading_periods + dep.file.path + "\"",
             )
 
@@ -508,7 +514,7 @@ def _init_cc_compilation_context(
         if not module_map:
             module_map = create_module_map(
                 file = actions.declare_file(label.name + ".cppmap"),
-                name = label.workspace_name + "//" + label.package + ":" + label.name,
+                identifier = label,
             )
 
         # There are different modes for module compilation:

cc/private/compile/compile.bzl

@@ -1149,7 +1149,7 @@ def _create_cc_compile_actions(
 
     if _should_provide_header_modules(feature_configuration, private_headers, public_headers):
         cpp_module_map = cc_compilation_context._module_map
-        module_map_label = Label(cpp_module_map.name)
+        module_map_label = Label(cpp_module_map.identifier)
         modules = _create_module_action(
             action_construction_context = action_construction_context,
             cc_compilation_context = cc_compilation_context,
@@ -2106,7 +2106,7 @@ def _create_module_action(
         additional_compilation_inputs,
         additional_include_scanning_roots,
         outputs):
-    module_map_label = Label(cpp_module_map.name)
+    module_map_label = Label(cpp_module_map.identifier)
     return _create_pic_nopic_compile_source_actions(
         action_construction_context = action_construction_context,
         cc_compilation_context = cc_compilation_context,

cc/private/compile/compile_build_variables.bzl

@@ -324,7 +324,7 @@ def get_specific_compile_build_variables(
     result = {}
 
     if feature_configuration.is_enabled("module_maps") and cpp_module_map:
-        result[_VARS.MODULE_NAME] = cpp_module_map.name
+        result[_VARS.MODULE_NAME] = str(cpp_module_map.identifier)
         result[_VARS.MODULE_MAP_FILE] = cpp_module_map.file
         result[_VARS.DEPENDENT_MODULE_MAP_FILES] = direct_module_maps
 

cc/private/rules_impl/cc_toolchain_provider_helper.bzl

@@ -230,7 +230,7 @@ def get_cc_toolchain_provider(ctx, attributes):
 
     module_map = None
     if attributes.module_map != None and attributes.module_map_artifact != None:
-        module_map = cc_common.create_module_map(file = attributes.module_map_artifact, name = "crosstool")
+        module_map = cc_common.create_module_map(file = attributes.module_map_artifact, identifier = "crosstool")
 
     cc_compilation_context = cc_common.create_compilation_context(module_map = module_map)
 

cc/private/rules_impl/objc_intermediate_artifacts.bzl

@@ -54,13 +54,13 @@ def _swift_module_map(ctx):
             ctx,
             ".modulemaps/module.modulemap",
         ),
-        name = module_name,
+        identifier = module_name,
     )
 
 def _internal_module_map(ctx):
     return cc_common.create_module_map(
         file = _declare_file_with_extension(ctx, ".internal.cppmap"),
-        name = str(ctx.label),
+        identifier = ctx.label,
     )
 
 def _create_closure_struct(ctx, archive_file_name_suffix, enforce_always_link):