feat: support mapping Zig mode to Bazel compilation mode

zig/config/mode/BUILD.bazel

@@ -1,31 +1,90 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
+
 config_setting(
-    name = "debug",
+    name = "_debug_explicit",
     flag_values = {
         "//zig/settings:mode": "debug",
     },
 )
 
 config_setting(
-    name = "release_safe",
+    name = "_debug_auto_dbg",
+    flag_values = {
+        "//zig/settings:mode": "auto",
+    },
+    values = {
+        "compilation_mode": "dbg",
+    },
+)
+
+config_setting(
+    name = "_debug_auto_fastbuild",
+    flag_values = {
+        "//zig/settings:mode": "auto",
+    },
+    values = {
+        "compilation_mode": "fastbuild",
+    },
+)
+
+selects.config_setting_group(
+    name = "debug",
+    match_any = [
+        ":_debug_explicit",
+        ":_debug_auto_dbg",
+        ":_debug_auto_fastbuild",
+    ],
+)
+
+config_setting(
+    name = "_release_safe_explicit",
     flag_values = {
         "//zig/settings:mode": "release_safe",
     },
 )
 
+selects.config_setting_group(
+    name = "release_safe",
+    match_any = [":_release_safe_explicit"],
+)
+
 config_setting(
-    name = "release_small",
+    name = "_release_small_explicit",
     flag_values = {
         "//zig/settings:mode": "release_small",
     },
 )
 
+selects.config_setting_group(
+    name = "release_small",
+    match_any = [":_release_small_explicit"],
+)
+
 config_setting(
-    name = "release_fast",
+    name = "_release_fast_explicit",
     flag_values = {
         "//zig/settings:mode": "release_fast",
     },
 )
 
+config_setting(
+    name = "_release_fast_auto_opt",
+    flag_values = {
+        "//zig/settings:mode": "auto",
+    },
+    values = {
+        "compilation_mode": "opt",
+    },
+)
+
+selects.config_setting_group(
+    name = "release_fast",
+    match_any = [
+        ":_release_fast_explicit",
+        ":_release_fast_auto_opt",
+    ],
+)
+
 # Execute `bazel run //util:update_filegroups` to update this target.
 filegroup(
     name = "all_files",

zig/private/settings.bzl

@@ -73,7 +73,13 @@ MODE_ARGS = {
     "release_fast": ["-O", "ReleaseFast"],
 }
 
-MODE_VALUES = ["debug", "release_safe", "release_small", "release_fast"]
+MODE_VALUES = ["auto", "debug", "release_safe", "release_small", "release_fast"]
+
+COMPILATION_MODE_TO_MODE = {
+    "dbg": "debug",
+    "fastbuild": "debug",
+    "opt": "release_fast",
+}
 
 THREADED_ARGS = {
     "multi": ["-fno-single-threaded"],
@@ -85,12 +91,23 @@ THREADED_VALUES = ["multi", "single"]
 def _is_exec_configuration(ctx):
     return ctx.genfiles_dir.path.find("-exec") != -1
 
+def _resolve_mode(ctx, mode):
+    if mode != "auto":
+        return mode
+
+    compilation_mode = ctx.var["COMPILATION_MODE"]
+    if compilation_mode not in COMPILATION_MODE_TO_MODE:
+        fail("Unrecognized Bazel compilation mode {}.".format(compilation_mode))
+
+    return COMPILATION_MODE_TO_MODE[compilation_mode]
+
 def _settings_impl(ctx):
     args = []
 
     is_exec_configuration = _is_exec_configuration(ctx)
 
     mode = ctx.attr.host_mode[BuildSettingInfo].value if is_exec_configuration else ctx.attr.mode[BuildSettingInfo].value
+    mode = _resolve_mode(ctx, mode)
     args.extend(MODE_ARGS[mode])
 
     threaded = ctx.attr.host_threaded[BuildSettingInfo].value if is_exec_configuration else ctx.attr.threaded[BuildSettingInfo].value

zig/private/zig_configure.bzl

@@ -10,6 +10,8 @@ Transitions a target and its dependencies to a different configuration.
 Settings like the build mode, e.g. `ReleaseSafe`, or the target platform,
 can be set on the command-line on demand,
 e.g. using `--@rules_zig//zig/settings:mode=release_safe`.
+By default, target build mode is `auto`: Bazel `dbg` and `fastbuild`
+compilation modes map to Zig `Debug`, and Bazel `opt` maps to Zig `ReleaseFast`.
 
 However, you may wish to always build a given target
 in a particular configuration,
@@ -208,12 +210,12 @@ def _make_attrs(*, executable):
             default = -1,
         ),
         "mode": attr.string(
-            doc = "The build mode setting, corresponds to the `-O` Zig compiler flag.",
+            doc = "The build mode setting, corresponds to the `-O` Zig compiler flag. `auto` follows Bazel's `--compilation_mode`.",
             mandatory = False,
             values = MODE_VALUES,
         ),
         "host_mode": attr.string(
-            doc = "The build mode setting for the host configuration, corresponds to the `-O` Zig compiler flag.",
+            doc = "The build mode setting for the host configuration, corresponds to the `-O` Zig compiler flag. `auto` follows Bazel's `--compilation_mode`.",
             mandatory = False,
             values = MODE_VALUES,
         ),

zig/settings/BUILD.bazel

@@ -33,7 +33,7 @@ bool_flag(
 
 string_flag(
     name = "mode",
-    build_setting_default = "debug",
+    build_setting_default = "auto",
     values = MODE_VALUES,
     visibility = ["//zig/config/mode:__pkg__"],
 )

zig/tests/config_test.bzl

@@ -19,23 +19,32 @@ _string_value = rule(
     },
 )
 
-def _define_config_settings_test(*, flag, value):
+_COMPILATION_MODE = "//command_line_option:compilation_mode"
+
+def _define_config_settings_test(*, flag, value, expected_value = None, compilation_mode = None):
     def _test_impl(ctx):
         env = analysistest.begin(ctx)
 
         actual_value = analysistest.target_under_test(env)[_ValueInfo].value
-        asserts.equals(env, value, actual_value, "Unexpected value for {}".format(flag))
+        asserts.equals(env, expected_value or value, actual_value, "Unexpected value for {}".format(flag))
 
         return analysistest.end(env)
 
+    config_settings = {flag: value}
+    if compilation_mode:
+        config_settings[_COMPILATION_MODE] = compilation_mode
+
     return analysistest.make(
         _test_impl,
-        config_settings = {flag: value},
+        config_settings = config_settings,
     )
 
 _SETTINGS_MODE = canonical_label("@//zig/settings:mode")
 _SETTINGS_THREADED = canonical_label("@//zig/settings:threaded")
 
+_mode_auto_dbg_test = _define_config_settings_test(flag = _SETTINGS_MODE, value = "auto", expected_value = "debug", compilation_mode = "dbg")
+_mode_auto_fastbuild_test = _define_config_settings_test(flag = _SETTINGS_MODE, value = "auto", expected_value = "debug", compilation_mode = "fastbuild")
+_mode_auto_opt_test = _define_config_settings_test(flag = _SETTINGS_MODE, value = "auto", expected_value = "release_fast", compilation_mode = "opt")
 _mode_debug_test = _define_config_settings_test(flag = _SETTINGS_MODE, value = "debug")
 _mode_release_safe_test = _define_config_settings_test(flag = _SETTINGS_MODE, value = "release_safe")
 _mode_release_small_test = _define_config_settings_test(flag = _SETTINGS_MODE, value = "release_small")
@@ -71,6 +80,9 @@ def config_test_suite(name):
     unittest.suite(
         name,
         # mode
+        partial.make(_mode_auto_dbg_test, target_under_test = mode_name, size = "small"),
+        partial.make(_mode_auto_fastbuild_test, target_under_test = mode_name, size = "small"),
+        partial.make(_mode_auto_opt_test, target_under_test = mode_name, size = "small"),
         partial.make(_mode_debug_test, target_under_test = mode_name, size = "small"),
         partial.make(_mode_release_safe_test, target_under_test = mode_name, size = "small"),
         partial.make(_mode_release_small_test, target_under_test = mode_name, size = "small"),