Support import std in Bazel with configurable std_module

BUILD.bazel

@@ -10,6 +10,16 @@ licenses(["notice"])  # MIT
 # Configuration Flags
 # ==============================================================================
 
+label_flag(
+    name = "std_module",
+    build_setting_default = ":empty_std_module",
+)
+
+config_setting(
+    name = "config_std_module_empty",
+    flag_values = {":std_module": ":empty_std_module"},
+)
+
 bool_flag(
     name = "no_exceptions",
     build_setting_default = False,
@@ -192,14 +202,21 @@ cc_library(
     includes = ["include"],
 )
 
+cc_library(
+    name = "empty_std_module",
+)
+
 quill_module(
     name = "quill_module",
     copts = select({
         "@rules_cc//cc/compiler:gcc": ["-Wno-gnu-zero-variadic-macro-arguments"],
         "@rules_cc//cc/compiler:clang": ["-Wno-gnu-zero-variadic-macro-arguments"],
         "//conditions:default": [],
     }),
-    defines = ["QUILL_USE_MODULE="],
+    defines = ["QUILL_USE_MODULE="] + select({
+        ":config_std_module_empty": [],
+        "//conditions:default": ["QUILL_IMPORT_STD"],
+    }),
     # rules_cc 0.2.22 uses the same module metadata path for PIC and non-PIC outputs.
     features = [
         "cpp_modules",
@@ -208,5 +225,8 @@ quill_module(
     module_interfaces = ["src/quill.cc"],
     # Prevent users without --experimental_cpp_modules support from accidentally building this
     tags = ["manual"],
-    deps = [":quill"],
+    deps = [":quill"] + select({
+        ":config_std_module_empty": [],
+        "//conditions:default": [":std_module"],
+    }),
 )

CHANGELOG.md

@@ -111,6 +111,8 @@
   Use `register_*_family()` when there is no initial metadata pointer to register.
 - Added the opt-in `QUILL_IMPORT_STD` option for named module builds on toolchains supporting
   `import std`; standard library headers remain the default.
+- Added the `--@quill//:std_module` Bazel flag to configure the standard library module target
+  for `quill_module`, automatically enabling `QUILL_IMPORT_STD`.
 - CMake and Bazel module targets now define `QUILL_USE_MODULE` automatically for consumers, while
   retaining compatibility with an explicit `#define QUILL_USE_MODULE` in source code.
 - Uncaught exceptions from `Frontend` logger creation and sink lookup or creation no longer leave

docs/installing.rst

@@ -158,6 +158,12 @@ whole build and pass ``--experimental_cpp_modules`` (for Clang, use
        deps = ["@quill//:quill_module"],
    )
 
+To enable experimental ``import std`` support with Bazel, configure the ``--@quill//:std_module``
+flag to point to your toolchain's standard library module target (for example, in ``.bazelrc`` via
+``build --@quill//:std_module=@std_module``). When configured, Quill automatically defines
+``QUILL_IMPORT_STD`` and adds the target as a dependency. If omitted, ``@quill//:std_module``
+defaults to an empty target, falling back to standard library headers.
+
 The selected Clang toolchain must also provide a matching ``clang-scan-deps`` beside the
 compiler executable. On Ubuntu 24.04, install ``clang-18`` and ``clang-tools-18``, and set
 ``CC=/usr/lib/llvm-18/bin/clang`` and ``CXX=/usr/lib/llvm-18/bin/clang++``.