feat: detect bzlmod automatically rather than requiring the user sets the flag.

Thanks to @aherrmann for pointing out this is possible: https://github.com/aherrmann/demo-bazel-detect-bzlmod-config-setting

docs/utils.md

@@ -155,6 +155,18 @@ That approach, however, incurs a cost in the user's WORKSPACE.
 True if the Bazel version being used is greater than or equal to 6 (including pre-releases and RCs)
 
 
+<a id="is_bzlmod_enabled"></a>
+
+## is_bzlmod_enabled
+
+<pre>
+is_bzlmod_enabled()
+</pre>
+
+Detect the value of the --enable_bzlmod flag
+
+
+
 <a id="is_external_label"></a>
 
 ## is_external_label

lib/BUILD.bazel

@@ -1,6 +1,7 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load("//lib/private:stamping.bzl", "stamp_build_setting")
+load("//lib:utils.bzl", "is_bzlmod_enabled")
 
 # Ensure that users building their own rules can dep on our bzl_library targets for their stardoc
 package(default_visibility = ["//visibility:public"])
@@ -16,11 +17,8 @@ exports_files(
 # Macro that creates targets enabling the use of `--stamp` in Starlark rules
 stamp_build_setting(name = "stamp")
 
-# Add a flag indicating that bzlmod is enabled so that we can use select()
-# to skip bzlmod-incompatible targets. Note that Bazel does not currently
-# support declaring a config_setting directly on --enable_bzlmod, so we
-# must use set this flag instead.
-
+# Add a config indicating that bzlmod is enabled so that we can use select()
+# to skip bzlmod-incompatible targets.
 config_setting(
     name = "bzlmod",
     flag_values = {
@@ -30,7 +28,7 @@ config_setting(
 
 bool_flag(
     name = "flag_bzlmod",
-    build_setting_default = False,
+    build_setting_default = True if is_bzlmod_enabled() else False,
 )
 
 toolchain_type(

lib/private/utils.bzl

@@ -202,6 +202,10 @@ def _is_bazel_6_or_greater():
     # native.bazel_version only works in repository rules.
     return "apple_binary" not in dir(native)
 
+def is_bzlmod_enabled():
+    """Detect the value of the --enable_bzlmod flag"""
+    return str(Label("@//:BUILD.bazel")).startswith("@@")
+
 def _maybe_http_archive(**kwargs):
     """Adapts a maybe(http_archive, ...) to look like an http_archive.
 
@@ -245,6 +249,7 @@ utils = struct(
     file_exists = _file_exists,
     glob_directories = _glob_directories,
     is_bazel_6_or_greater = _is_bazel_6_or_greater,
+    is_bzlmod_enabled = is_bzlmod_enabled,
     is_external_label = _is_external_label,
     maybe_http_archive = _maybe_http_archive,
     path_to_workspace_root = _path_to_workspace_root,

lib/utils.bzl

@@ -6,6 +6,7 @@ default_timeout = utils.default_timeout
 file_exists = utils.file_exists
 glob_directories = utils.glob_directories
 is_bazel_6_or_greater = utils.is_bazel_6_or_greater
+is_bzlmod_enabled = utils.is_bzlmod_enabled
 is_external_label = utils.is_external_label
 maybe_http_archive = utils.maybe_http_archive
 path_to_workspace_root = utils.path_to_workspace_root