Add apparent_repo_name utility to modules.bzl

MODULE.bazel

@@ -37,3 +37,6 @@ use_repo(as_extension_test_ext, "bar", "foo")
 
 use_all_repos_test_ext = use_extension("//tests:modules_test.bzl", "use_all_repos_test_ext", dev_dependency = True)
 use_repo(use_all_repos_test_ext, "baz", "qux")
+
+apparent_repo_name_test_ext = use_extension("//tests:extensions/apparent_repo_name.bzl", "apparent_repo_name_test_ext", dev_dependency = True)
+use_repo(apparent_repo_name_test_ext, "apparent-repo-name-test")

WORKSPACE

@@ -81,3 +81,7 @@ load("//tests/directory:external_directory_tests.bzl", "external_directory_tests
 external_directory_tests(name = "external_directory_tests")
 
 register_unittest_toolchains()
+
+load("//tests:extensions/apparent_repo_name.bzl", "apparent_repo_name_test_macro")
+
+apparent_repo_name_test_macro()

docs/modules_doc.md

@@ -2,6 +2,48 @@
 
 Skylib module containing utilities for Bazel modules and module extensions.
 
+<a id="modules.apparent_repo_name"></a>
+
+## modules.apparent_repo_name
+
+<pre>
+modules.apparent_repo_name(<a href="#modules.apparent_repo_name-repository_ctx">repository_ctx</a>)
+</pre>
+
+Generates a repository's apparent name from a repository_ctx object.
+
+Useful when generating the default top level `BUILD` target for the
+repository.
+
+Example:
+```starlark
+_ALIAS_TARGET_TEMPLATE = """alias(
+    name = "{name}",
+    actual = "@{target_repo_name}",
+    visibility = ["//visibility:public"],
+)
+"""
+
+def _alias_repository_impl(repository_ctx):
+    repository_ctx.file("BUILD", _ALIAS_TARGET_TEMPLATE.format(
+        name = apparent_repo_name(rctx),
+        target = rctx.attr.target_repo_name,
+    ))
+```
+
+
+**PARAMETERS**
+
+
+| Name  | Description | Default Value |
+| :------------- | :------------- | :------------- |
+| <a id="modules.apparent_repo_name-repository_ctx"></a>repository_ctx |  a repository_ctx object   |  none |
+
+**RETURNS**
+
+An apparent repo name derived from repository_ctx.name
+
+
 <a id="modules.as_extension"></a>
 
 ## modules.as_extension

lib/modules.bzl

@@ -114,7 +114,47 @@ def _use_all_repos(module_ctx, reproducible = False):
         **extension_metadata_kwargs
     )
 
+def _apparent_repo_name(repository_ctx):
+    """Generates a repository's apparent name from a repository_ctx object.
+
+    Useful when generating the default top level `BUILD` target for the
+    repository.
+
+    Example:
+    ```starlark
+    _ALIAS_TARGET_TEMPLATE = \"\"\"alias(
+        name = "{name}",
+        actual = "@{target_repo_name}",
+        visibility = ["//visibility:public"],
+    )
+    \"\"\"
+
+    def _alias_repository_impl(repository_ctx):
+        repository_ctx.file("BUILD", _ALIAS_TARGET_TEMPLATE.format(
+            name = apparent_repo_name(rctx),
+            target = rctx.attr.target_repo_name,
+        ))
+    ```
+
+    Args:
+        repository_ctx: a repository_ctx object
+
+    Returns:
+        An apparent repo name derived from repository_ctx.name
+    """
+    repo_name = repository_ctx.name
+
+    # Bazed on this pattern from the Bazel source:
+    # com.google.devtools.build.lib.cmdline.RepositoryName.VALID_REPO_NAME
+    for i in range(len(repo_name) - 1, -1, -1):
+        c = repo_name[i]
+        if not (c.isalnum() or c in "_-."):
+            return repo_name[i + 1:]
+
+    return repo_name
+
 modules = struct(
     as_extension = _as_extension,
     use_all_repos = _use_all_repos,
+    apparent_repo_name = _apparent_repo_name,
 )

tests/extensions/apparent_repo_name.bzl

@@ -0,0 +1,22 @@
+"""Repo rule and module extension used to test modules.apparent_repo_name"""
+
+load("//lib:modules.bzl", "modules")
+
+def _apparent_repo_name_test_repo_impl(repository_ctx):
+    repo_name = modules.apparent_repo_name(repository_ctx)
+    test_file = "repo-name.bzl"
+    repository_ctx.file("WORKSPACE")
+    repository_ctx.file("BUILD", """exports_files(["%s"])""" % test_file)
+    repository_ctx.file(test_file, "REPO_NAME = \"%s\"" % repo_name)
+
+apparent_repo_name_test_repo = repository_rule(
+    _apparent_repo_name_test_repo_impl,
+)
+
+def apparent_repo_name_test_macro():
+    apparent_repo_name_test_repo(name = "apparent-repo-name-test")
+
+apparent_repo_name_test_ext = module_extension(
+    lambda _: apparent_repo_name_test_macro(),
+    doc = "Only used for testing modules.apparent_repo_name()",
+)

tests/modules_test.bzl

@@ -14,9 +14,13 @@
 
 """Test usage of modules.bzl."""
 
+load("@apparent-repo-name-test//:repo-name.bzl", "REPO_NAME")
 load("//lib:modules.bzl", "modules")
+load("//lib:unittest.bzl", "asserts", "unittest")
 load("//rules:build_test.bzl", "build_test")
 
+_is_bzlmod_enabled = str(Label("//tests:module_tests.bzl")).startswith("@@")
+
 def _repo_rule_impl(repository_ctx):
     repository_ctx.file("WORKSPACE")
     repository_ctx.file("BUILD", """exports_files(["hello"])""")
@@ -45,12 +49,34 @@ use_all_repos_test_ext = module_extension(
     doc = "Only used for testing modules.use_all_repos().",
 )
 
+def _apparent_repo_name_test(ctx):
+    """Unit tests for modules.apparent_repo_name."""
+    env = unittest.begin(ctx)
+
+    asserts.equals(
+        env,
+        "apparent-repo-name-test",
+        REPO_NAME,
+        msg = " ".join([
+            "Returns the original name unchanged under WORKSPACE, and",
+            "the apparent repo name under Bzlmod.",
+        ]),
+    )
+
+    return unittest.end(env)
+
+apparent_repo_name_test = unittest.make(_apparent_repo_name_test)
+
 # buildifier: disable=unnamed-macro
 def modules_test_suite():
     """Creates the tests for modules.bzl if Bzlmod is enabled."""
 
-    is_bzlmod_enabled = str(Label("//tests:module_tests.bzl")).startswith("@@")
-    if not is_bzlmod_enabled:
+    unittest.suite(
+        "modules_tests",
+        apparent_repo_name_test,
+    )
+
+    if not _is_bzlmod_enabled:
         return
 
     build_test(