pw_toolchain_bazel/api.rst - pigweed/pigweed - Git at Google

 .. _module-pw_toolchain_bazel-api:

 =============
 API reference
 =============
 .. pigweed-module-subpage::
    :name: pw_toolchain_bazel

 .. py:class:: pw_cc_toolchain

   This rule is the core of a C/C++ toolchain definition. Critically, it is
   intended to fully specify the following:

   * Which tools to use for various compile/link actions.
   * Which `well-known features <https://bazel.build/docs/cc-toolchain-config-reference#wellknown-features>`_
     are supported.
   * Which flags to apply to various actions.

    .. py:attribute:: action_configs
       :type: List[label]

       List of :py:class:`pw_cc_action_config` labels that bind tools to the
       appropriate actions. This is how Bazel knows which binaries to use when
       compiling, linking, or taking other actions like embedding data using
       objcopy.

    .. py:attribute:: flag_sets
       :type: List[label]

       List of flag sets to unconditionally apply to the respective
       :py:class:`pw_cc_action_config`\s. The labels listed here will point to
       :py:class:`pw_cc_flag_set` rules.

    .. py:attribute:: extra_action_files
       :type: List[label]

       List of extra files to give to each different type of action. Replaces
       all_files, ar_files, compiler_files, ... in the existing toolchain
       definition.

    .. py:attribute:: toolchain_identifier
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: host_system_name
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: target_system_name
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: target_cpu
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: target_libc
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: compiler
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: abi_version
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: abi_libc_version
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: cc_target_os
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: builtin_sysroot
       :type: str

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

    .. py:attribute:: cxx_builtin_include_directories
       :type: List[str]

       See  `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

 .. py:class:: pw_cc_flag_set

    Declares an ordered set of flags bound to a set of actions.

    Flag sets can be attached to a :py:class:`pw_cc_toolchain` via
    :py:attr:`pw_cc_toolchain.flag_sets`\.

    Examples:

    .. code-block:: py

       pw_cc_flag_set(
           name = "warnings_as_errors",
           flags = ["-Werror"],
       )

       pw_cc_flag_set(
           name = "layering_check",
           flag_groups = [
               ":strict_module_headers",
               ":dependent_module_map_files",
           ],
       )

    .. inclusive-language: disable

    Note: In the vast majority of cases, alphabetical sorting is not desirable
    for the :py:attr:`pw_cc_flag_set.flags` and
    :py:attr:`pw_cc_flag_set.flag_groups` attributes.
    `Buildifier <https://github.com/bazelbuild/buildtools/blob/master/buildifier/README.md>`_
    shouldn't ever try to sort these, but in the off chance it starts to these
    members should be listed as exceptions in the ``SortableDenylist``.

    .. inclusive-language: enable

    .. py:attribute:: actions
       :type: List[str]

       A list of action names that this flag set applies to.

       .. inclusive-language: disable

       Valid choices are listed at
       `@rules_cc//cc:action_names.bzl <https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/cc/action_names.bzl>`_\.

       .. inclusive-language: enable

       It is possible for some needed action names to not be enumerated in this list,
       so there is not rigid validation for these strings. Prefer using constants
       rather than manually typing action names.

    .. py:attribute:: flags
       :type: List[str]

       Flags that should be applied to the specified actions.

       These are evaluated in order, with earlier flags appearing earlier in the
       invocation of the underlying tool. If you need expansion logic, prefer
       enumerating flags in a :py:class:`pw_cc_flag_group` or create a custom
       rule that provides ``FlagGroupInfo``.

       Note: :py:attr:`pw_cc_flag_set.flags` and
       :py:attr:`pw_cc_flag_set.flag_groups` are mutually exclusive.

    .. py:attribute:: flag_groups
       :type: List[label]

       Labels pointing to :py:class:`pw_cc_flag_group` rules.

       This is intended to be compatible with any other rules that provide
       ``FlagGroupInfo``. These are evaluated in order, with earlier flag groups
       appearing earlier in the invocation of the underlying tool.

       Note: :py:attr:`pw_cc_flag_set.flag_groups` and
       :py:attr:`pw_cc_flag_set.flags` are mutually exclusive.

 .. py:class:: pw_cc_flag_group

    Declares an (optionally parametric) ordered set of flags.

    :py:class:`pw_cc_flag_group` rules are expected to be consumed exclusively by
    :py:class:`pw_cc_flag_set` rules. Though simple lists of flags can be
    expressed by populating ``flags`` on a :py:class:`pw_cc_flag_set`,
    :py:class:`pw_cc_flag_group` provides additional power in the following two
    ways:

     1. Iteration and conditional expansion. Using
        :py:attr:`pw_cc_flag_group.iterate_over`,
        :py:attr:`pw_cc_flag_group.expand_if_available`\, and
        :py:attr:`pw_cc_flag_group.expand_if_not_available`\, more complex
        flag expressions can be made. This is critical for implementing things
        like the ``libraries_to_link`` feature, where library names are
        transformed into flags that end up in the final link invocation.

        Note: ``expand_if_equal``, ``expand_if_true``, and ``expand_if_false``
        are not yet supported.

     2. Flags are tool-independent. A :py:class:`pw_cc_flag_group` expresses
        ordered flags that may be reused across various
        :py:class:`pw_cc_flag_set` rules. This is useful for cases where multiple
        :py:class:`pw_cc_flag_set` rules must be created to implement a feature
        for which flags are slightly different depending on the action (e.g.
        compile vs link). Common flags can be expressed in a shared
        :py:class:`pw_cc_flag_group`, and the differences can be relegated to
        separate :py:class:`pw_cc_flag_group` instances.

    Examples:

    .. code-block:: py

       pw_cc_flag_group(
           name = "user_compile_flag_expansion",
           flags = ["%{user_compile_flags}"],
           iterate_over = "user_compile_flags",
           expand_if_available = "user_compile_flags",
       )

       # This flag_group might be referenced from various FDO-related
       # `pw_cc_flag_set` rules. More importantly, the flag sets pulling this in
       # may apply to different sets of actions.
       pw_cc_flag_group(
           name = "fdo_profile_correction",
           flags = ["-fprofile-correction"],
           expand_if_available = "fdo_profile_path",
       )

    .. py:attribute:: flags
       :type: List[str]

       List of flags provided by this rule.

       For extremely complex expressions of flags that require nested flag groups
       with multiple layers of expansion, prefer creating a custom rule in
       `Starlark <https://bazel.build/rules/language>`_ that provides
       ``FlagGroupInfo`` or ``FlagSetInfo``.


    .. py:attribute:: iterate_over
       :type: str

       Expands :py:attr:`pw_cc_flag_group.flags` for items in the named list.

       Toolchain actions have various variables accessible as names that can be
       used to guide flag expansions. For variables that are lists,
       :py:attr:`pw_cc_flag_group.iterate_over` must be used to expand the list into a series of flags.

       Note that :py:attr:`pw_cc_flag_group.iterate_over` is the string name of a
       build variable, and not an actual list. Valid options are listed in the
       `C++ Toolchain Configuration <https://bazel.build/docs/cc-toolchain-config-reference#cctoolchainconfiginfo-build-variables>`_
       reference.


       Note that the flag expansion stamps out the entire list of flags in
       :py:attr:`pw_cc_flag_group.flags` once for each item in the list.

       Example:

       .. code-block:: py

          # Expands each path in ``system_include_paths`` to a series of
          # ``-isystem`` includes.
          #
          # Example input:
          #     system_include_paths = ["/usr/local/include", "/usr/include"]
          #
          # Expected result:
          #     "-isystem /usr/local/include -isystem /usr/include"
          pw_cc_flag_group(
              name = "system_include_paths",
              flags = ["-isystem", "%{system_include_paths}"],
              iterate_over = "system_include_paths",
          )

    .. py:attribute:: expand_if_available
       :type: str

       Expands the expression in :py:attr:`pw_cc_flag_group.flags` if the
       specified build variable is set.

    .. py:attribute:: expand_if_not_available
       :type: str

       Expands the expression in :py:attr:`pw_cc_flag_group.flags` if the
       specified build variable is **NOT** set.

 .. py:class:: pw_cc_tool

    Declares a singular tool that can be bound to action configs.

    :py:class:`pw_cc_tool` rules are intended to be consumed exclusively by
    :py:class:`pw_cc_action_config` rules. These rules declare an underlying tool
    that can be used to fulfill various actions. Many actions may reuse a shared
    tool.

    Examples:

    .. code-block:: py

       # A project-provided tool.
       pw_cc_tool(
           name = "clang_tool",
           tool = "@llvm_toolchain//:bin/clang",
       )

       # A tool expected to be preinstalled on a user's machine.
       pw_cc_tool(
           name = "clang_tool",
           path = "/usr/bin/clang",
       )

    .. py:attribute:: tool
       :type: label

       The underlying tool that this rule represents.

       This attribute is a label rather than a simple file path. This means that
       the file must be referenced relative to the BUILD file that exports it.
       For example:

       .. code-block:: none

          @llvm_toolchain//:bin/clang
          ^              ^  ^

       Where:

       * ``@llvm_toolchain`` is the repository.
       * ``//`` is the directory of the BUILD file that exports the file of
         interest.
       * ``bin/clang`` is the path of the actual binary relative to the BUILD
         file of interest.

       Note: :py:attr:`pw_cc_tool.tool` and :py:attr:`pw_cc_tool.path` are
       mutually exclusive.

    .. py:attribute:: path
       :type: Path

       An absolute path to a binary to use for this tool.

       Relative paths are also supported, but they are relative to the
       :py:class:`pw_cc_toolchain` that uses this tool rather than relative to
       this :py:class:`pw_cc_tool` rule.

       Note: :py:attr:`pw_cc_tool.path` and :py:attr:`pw_cc_tool.tool` are
       mutually exclusive.

       .. admonition:: Note
          :class: warning

          This method of listing a tool is NOT recommended, and is provided as an
          escape hatch for edge cases. Prefer using :py:attr:`pw_cc_tool.tool`
          whenever possible.

    .. py:attribute:: execution_requirements
       :type: List[str]

       A list of strings that provide hints for execution environment
       compatibility (e.g. ``requires-darwin``).

    .. py:attribute:: requires_any_of
       :type: List[label]

       This tool may only be enabled when at least one of the constraints are
       met.

       If omitted, this tool will be enabled unconditionally.

    .. py:attribute:: additional_files
       :type: List[label]

       Additional files that are required for this tool to correctly operate.
       These files are propagated up to the :py:class:`pw_cc_toolchain` so you
       typically won't need to explicitly specify the ``*_files`` attributes
       on a :py:class:`pw_cc_toolchain`.


 .. py:class:: pw_cc_action_config

    Declares the configuration and selection of `pw_cc_tool` rules.

    Action configs are bound to a toolchain through `action_configs`, and are the
    driving mechanism for controlling toolchain tool invocation/behavior.

    Action configs define three key things:

    * Which tools to invoke for a given type of action.
    * Tool features and compatibility.
    * :py:class:`pw_cc_flag_set`\s that are unconditionally bound to a tool
      invocation.

    Examples:

    .. code-block:: py

       pw_cc_action_config(
           name = "ar",
           action_names = ["@pw_toolchain//actions:all_ar_actions"],
           implies = [
               "@pw_toolchain//features/legacy:archiver_flags",
               "@pw_toolchain//features/legacy:linker_param_file",
           ],
           tools = [":ar_tool"],
       )

       pw_cc_action_config(
           name = "clang",
           action_names = [
               "@pw_toolchain//actions:all_asm_actions",
               "@pw_toolchain//actions:all_c_compiler_actions",
           ]
           tools = [":clang_tool"],
       )

    .. py:attribute:: action_names
       :type: List[str]

       A list of action names to apply this action to.

       .. inclusive-language: disable

       Valid choices are listed at
       `@rules_cc//cc:action_names.bzl <https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/cc/action_names.bzl>`_\.

       .. inclusive-language: enable

       It is possible for some needed action names to not be enumerated in this list,
       so there is not rigid validation for these strings. Prefer using constants
       rather than manually typing action names.

    .. py:attribute:: enabled
       :type: bool

       Whether or not this action config is enabled by default.

       .. admonition:: Note

          This defaults to ``True`` since it's assumed that most listed action
          configs will be enabled and used by default. This is the opposite of
          Bazel's native default.

    .. py:attribute:: tools
       :type: List[label]

       The tool to use for the specified actions.

       A tool can be a :py:class:`pw_cc_tool`, or a binary.

       If multiple tools are specified, the first tool that has ``with_features``
       that satisfy the currently enabled feature set is used.

    .. py:attribute:: flag_sets
       :type: List[label]

       Labels that point to :py:class:`pw_cc_flag_set`\s that are unconditionally
       bound to the specified actions.

       .. admonition:: Note

          The flags in the :py:class:`pw_cc_flag_set` are only bound to matching
          action names. If an action is listed in this rule's
          :py:attr:`pw_cc_action_config.action_names`,
          but is NOT listed in the :py:class:`pw_cc_flag_set`\'s
          :py:attr:`pw_cc_flag_set.actions`, the flag will not be applied to that
          action.

    .. py:attribute:: implies
       :type: List[str]

       Names of features that should be automatically enabled when this tool is
       used.

       .. admonition:: Note
          :class: warning

          If this action config implies an unknown feature, this action config
          will silently be disabled. This behavior is native to Bazel itself, and
          there's no way to detect this and emit an error instead. For this
          reason, be very cautious when listing implied features!

 .. py:class:: pw_cc_feature

    Defines the implemented behavior of a C/C++ toolchain feature.


    This rule is effectively a wrapper for the ``feature`` constructor in
    `@rules_cc//cc:cc_toolchain_config_lib.bzl <https://github.com/bazelbuild/rules_cc/blob/main/cc/cc_toolchain_config_lib.bzl>`_.

    A feature is basically a dynamic flag set. There are a variety of
    dependencies and compatibility requirements that must be satisfied for the
    listed flag sets to be applied.

    A feature may be enabled or disabled through the following mechanisms:\

    * Via command-line flags, or a
      `.bazelrc file <https://bazel.build/run/bazelrc>`_\.
    * Through inter-feature relationships (enabling one feature may implicitly
      enable another).
    * Individual rules may elect to manually enable or disable features through
      the
      `builtin features attribute <https://bazel.build/reference/be/common-definitions#common.features>`_\.

    Because of the dynamic nature of toolchain features, it's generally best to
    avoid enumerating features as part of your toolchain with the following
    exceptions:

    * You want the flags to be controllable via Bazel's CLI. For example, adding
      ``-v`` to a compiler invocation is often too verbose to be useful for most
      workflows, but can be instrumental when debugging obscure errors. By
      expressing compiler verbosity as a feature, users may opt-in when
      necessary.
    * You need to carry forward Starlark toolchain behaviors. If you're migrating
      a complex Starlark-based toolchain definition to these rules, many of the
      workflows and flags were likely based on features. This rule exists to
      support those existing structures.

    For more details about how Bazel handles features, see the official Bazel
    documentation at
    https://bazel.build/docs/cc-toolchain-config-reference#features.

    Note: ``env_sets`` are not yet supported.

    Examples:

    .. code-block:: py

       # A feature that can be easily toggled to include extra compiler output to
       # help debug things like include search path ordering and showing all the
       # flags passed to the compiler.
       #
       # Add `--features=verbose_compiler_output` to your Bazel invocation to
       # enable.
       pw_cc_feature(
           name = "verbose_compiler_output",
           enabled = False,
           feature_name = "verbose_compiler_output",
           flag_sets = [":verbose_compiler_flags"],
       )

       # This feature signals a capability, and doesn't have associated flags.
       #
       # For a list of well-known features, see:
       #    https://bazel.build/docs/cc-toolchain-config-reference#wellknown-features
       pw_cc_feature(
           name = "link_object_files",
           enabled = True,
           feature_name = "supports_start_end_lib",
       )

    .. py:attribute:: feature_name
       :type: str

       The name of the feature that this rule implements.

       Feature names are used to express feature dependencies and compatibility.
       Because features are tracked by string names rather than labels, there's
       great flexibility in swapping out feature implementations or overriding
       the built-in legacy features that Bazel silently binds to every
       toolchain.

       :py:attr:`pw_cc_feature.feature_name` is used rather than ``name`` to
       distinguish between the rule name, and the intended final feature name.
       This allows similar rules to exist in the same package, even if slight
       differences are required.

       Example:

       .. code-block:: py

          pw_cc_feature(
              name = "sysroot_macos",
              feature_name = "sysroot",
              ...
          )

          pw_cc_feature(
              name = "sysroot_linux",
              feature_name = "sysroot",
              ...
          )

       While two features with the same :py:attr:`pw_cc_feature.feature_name` may
       not be bound to the same toolchain, they can happily live alongside each
       other in the same BUILD file.

    .. py:attribute:: enabled
       :type: bool

       Whether or not this feature is enabled by default.

    .. py:attribute:: flag_sets
       :type: List[label]

       Flag sets that, when expanded, implement this feature.

    .. py:attribute:: requires_any_of
       :type: List[label]

       A list of feature sets that define toolchain compatibility.

       If **at least one** of the listed :py:class:`pw_cc_feature`\s or
       :py:class:`pw_cc_feature_set`\s are satisfied (all features exist in the
       toolchain AND are currently enabled), this feature is deemed compatible
       and may be enabled.

       If this feature **cannot** be enabled (such as if, in the first example
       below, thin_lto didn't exist in the toolchain), it will throw an error.

       .. code-block:: py

          pw_cc_feature(
              name = "requires_thin_lto_and_opt",
              feature_name = "requires_thin_lto_and_opt",
              requires_any_of = [":thin_lto_requirements"]
          )

          pw_cc_feature(
              name = "requires_thin_lto_or_opt",
              feature_name = "requires_thin_lto_or_opt",
              requires_any_of = [":thin_lto", ":opt"]
          )

       .. admonition:: Note

          Even if :py:attr:`pw_cc_feature.requires_any_of` is satisfied, a
          feature is not enabled unless another mechanism (e.g. command-line
          flags, :py:attr:`pw_cc_feature.implies`, or
          :py:attr:`pw_cc_feature.enabled`\) signals that the feature should
          actually be enabled.

    .. py:attribute:: implies
       :type: List[label]

       List of features or action configs enabled along with this feature.

       .. admonition:: Note
          :class: warning

          If any of the named features cannot be enabled, this feature is
          silently disabled.

    .. py:attribute:: mutually_exclusive
       :type: List[Label]

       A list of feature or mutually exclusive categories that this feature is
       mutually exclusive with.

       .. admonition:: Note

          This feature cannot be enabled if another feature also provides the
          listed feature.


 .. py:class:: pw_cc_feature_set

    Defines a group of features.

    Semantically equivalent to "all of the specified features", and may be used
    wherever you can provide multiple features.

    Example:

    .. code-block:: py

       pw_cc_feature_set(
           name = "thin_lto_requirements",
           all_of = [
               ":thin_lto",
               ":opt",
           ],
       )

    .. py:attribute:: features
       :type: List[label]

       Features that must be enabled for this feature set to be deemed compatible
       with the current toolchain configuration.
	.. _module-pw_toolchain_bazel-api:

	=============
	API reference
	=============
	.. pigweed-module-subpage::
	:name: pw_toolchain_bazel

	.. py:class:: pw_cc_toolchain

	This rule is the core of a C/C++ toolchain definition. Critically, it is
	intended to fully specify the following:

	* Which tools to use for various compile/link actions.
	* Which `well-known features <https://bazel.build/docs/cc-toolchain-config-reference#wellknown-features>`_
	are supported.
	* Which flags to apply to various actions.

	.. py:attribute:: action_configs
	:type: List[label]

	List of :py:class:`pw_cc_action_config` labels that bind tools to the
	appropriate actions. This is how Bazel knows which binaries to use when
	compiling, linking, or taking other actions like embedding data using
	objcopy.

	.. py:attribute:: flag_sets
	:type: List[label]

	List of flag sets to unconditionally apply to the respective
	:py:class:`pw_cc_action_config`\s. The labels listed here will point to
	:py:class:`pw_cc_flag_set` rules.

	.. py:attribute:: extra_action_files
	:type: List[label]

	List of extra files to give to each different type of action. Replaces
	all_files, ar_files, compiler_files, ... in the existing toolchain
	definition.

	.. py:attribute:: toolchain_identifier
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: host_system_name
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: target_system_name
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: target_cpu
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: target_libc
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: compiler
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: abi_version
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: abi_libc_version
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: cc_target_os
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: builtin_sysroot
	:type: str

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:attribute:: cxx_builtin_include_directories
	:type: List[str]

	See `cc_common.create_cc_toolchain_config_info() <https://bazel.build/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info>`_\.

	.. py:class:: pw_cc_flag_set

	Declares an ordered set of flags bound to a set of actions.

	Flag sets can be attached to a :py:class:`pw_cc_toolchain` via
	:py:attr:`pw_cc_toolchain.flag_sets`\.

	Examples:

	.. code-block:: py

	pw_cc_flag_set(
	name = "warnings_as_errors",
	flags = ["-Werror"],
	)

	pw_cc_flag_set(
	name = "layering_check",
	flag_groups = [
	":strict_module_headers",
	":dependent_module_map_files",
	],
	)

	.. inclusive-language: disable

	Note: In the vast majority of cases, alphabetical sorting is not desirable
	for the :py:attr:`pw_cc_flag_set.flags` and
	:py:attr:`pw_cc_flag_set.flag_groups` attributes.
	`Buildifier <https://github.com/bazelbuild/buildtools/blob/master/buildifier/README.md>`_
	shouldn't ever try to sort these, but in the off chance it starts to these
	members should be listed as exceptions in the ``SortableDenylist``.

	.. inclusive-language: enable

	.. py:attribute:: actions
	:type: List[str]

	A list of action names that this flag set applies to.

	.. inclusive-language: disable

	Valid choices are listed at
	`@rules_cc//cc:action_names.bzl <https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/cc/action_names.bzl>`_\.

	.. inclusive-language: enable

	It is possible for some needed action names to not be enumerated in this list,
	so there is not rigid validation for these strings. Prefer using constants
	rather than manually typing action names.

	.. py:attribute:: flags
	:type: List[str]

	Flags that should be applied to the specified actions.

	These are evaluated in order, with earlier flags appearing earlier in the
	invocation of the underlying tool. If you need expansion logic, prefer
	enumerating flags in a :py:class:`pw_cc_flag_group` or create a custom
	rule that provides ``FlagGroupInfo``.

	Note: :py:attr:`pw_cc_flag_set.flags` and
	:py:attr:`pw_cc_flag_set.flag_groups` are mutually exclusive.

	.. py:attribute:: flag_groups
	:type: List[label]

	Labels pointing to :py:class:`pw_cc_flag_group` rules.

	This is intended to be compatible with any other rules that provide
	``FlagGroupInfo``. These are evaluated in order, with earlier flag groups
	appearing earlier in the invocation of the underlying tool.

	Note: :py:attr:`pw_cc_flag_set.flag_groups` and
	:py:attr:`pw_cc_flag_set.flags` are mutually exclusive.

	.. py:class:: pw_cc_flag_group

	Declares an (optionally parametric) ordered set of flags.

	:py:class:`pw_cc_flag_group` rules are expected to be consumed exclusively by
	:py:class:`pw_cc_flag_set` rules. Though simple lists of flags can be
	expressed by populating ``flags`` on a :py:class:`pw_cc_flag_set`,
	:py:class:`pw_cc_flag_group` provides additional power in the following two
	ways:

	1. Iteration and conditional expansion. Using
	:py:attr:`pw_cc_flag_group.iterate_over`,
	:py:attr:`pw_cc_flag_group.expand_if_available`\, and
	:py:attr:`pw_cc_flag_group.expand_if_not_available`\, more complex
	flag expressions can be made. This is critical for implementing things
	like the ``libraries_to_link`` feature, where library names are
	transformed into flags that end up in the final link invocation.

	Note: ``expand_if_equal``, ``expand_if_true``, and ``expand_if_false``
	are not yet supported.

	2. Flags are tool-independent. A :py:class:`pw_cc_flag_group` expresses
	ordered flags that may be reused across various
	:py:class:`pw_cc_flag_set` rules. This is useful for cases where multiple
	:py:class:`pw_cc_flag_set` rules must be created to implement a feature
	for which flags are slightly different depending on the action (e.g.
	compile vs link). Common flags can be expressed in a shared
	:py:class:`pw_cc_flag_group`, and the differences can be relegated to
	separate :py:class:`pw_cc_flag_group` instances.

	Examples:

	.. code-block:: py

	pw_cc_flag_group(
	name = "user_compile_flag_expansion",
	flags = ["%{user_compile_flags}"],
	iterate_over = "user_compile_flags",
	expand_if_available = "user_compile_flags",
	)

	# This flag_group might be referenced from various FDO-related
	# `pw_cc_flag_set` rules. More importantly, the flag sets pulling this in
	# may apply to different sets of actions.
	pw_cc_flag_group(
	name = "fdo_profile_correction",
	flags = ["-fprofile-correction"],
	expand_if_available = "fdo_profile_path",
	)

	.. py:attribute:: flags
	:type: List[str]

	List of flags provided by this rule.

	For extremely complex expressions of flags that require nested flag groups
	with multiple layers of expansion, prefer creating a custom rule in
	`Starlark <https://bazel.build/rules/language>`_ that provides
	``FlagGroupInfo`` or ``FlagSetInfo``.


	.. py:attribute:: iterate_over
	:type: str

	Expands :py:attr:`pw_cc_flag_group.flags` for items in the named list.

	Toolchain actions have various variables accessible as names that can be
	used to guide flag expansions. For variables that are lists,
	:py:attr:`pw_cc_flag_group.iterate_over` must be used to expand the list into a series of flags.

	Note that :py:attr:`pw_cc_flag_group.iterate_over` is the string name of a
	build variable, and not an actual list. Valid options are listed in the
	`C++ Toolchain Configuration <https://bazel.build/docs/cc-toolchain-config-reference#cctoolchainconfiginfo-build-variables>`_
	reference.



	Note that the flag expansion stamps out the entire list of flags in
	:py:attr:`pw_cc_flag_group.flags` once for each item in the list.

	Example:

	.. code-block:: py

	# Expands each path in ``system_include_paths`` to a series of
	# ``-isystem`` includes.
	#
	# Example input:
	# system_include_paths = ["/usr/local/include", "/usr/include"]
	#
	# Expected result:
	# "-isystem /usr/local/include -isystem /usr/include"
	pw_cc_flag_group(
	name = "system_include_paths",
	flags = ["-isystem", "%{system_include_paths}"],
	iterate_over = "system_include_paths",
	)

	.. py:attribute:: expand_if_available
	:type: str

	Expands the expression in :py:attr:`pw_cc_flag_group.flags` if the
	specified build variable is set.

	.. py:attribute:: expand_if_not_available
	:type: str

	Expands the expression in :py:attr:`pw_cc_flag_group.flags` if the
	specified build variable is NOT set.

	.. py:class:: pw_cc_tool

	Declares a singular tool that can be bound to action configs.

	:py:class:`pw_cc_tool` rules are intended to be consumed exclusively by
	:py:class:`pw_cc_action_config` rules. These rules declare an underlying tool
	that can be used to fulfill various actions. Many actions may reuse a shared
	tool.

	Examples:

	.. code-block:: py

	# A project-provided tool.
	pw_cc_tool(
	name = "clang_tool",
	tool = "@llvm_toolchain//:bin/clang",
	)

	# A tool expected to be preinstalled on a user's machine.
	pw_cc_tool(
	name = "clang_tool",
	path = "/usr/bin/clang",
	)

	.. py:attribute:: tool
	:type: label

	The underlying tool that this rule represents.

	This attribute is a label rather than a simple file path. This means that
	the file must be referenced relative to the BUILD file that exports it.
	For example:

	.. code-block:: none

	@llvm_toolchain//:bin/clang
	^ ^ ^

	Where:

	* ``@llvm_toolchain`` is the repository.
	* ``//`` is the directory of the BUILD file that exports the file of
	interest.
	* ``bin/clang`` is the path of the actual binary relative to the BUILD
	file of interest.

	Note: :py:attr:`pw_cc_tool.tool` and :py:attr:`pw_cc_tool.path` are
	mutually exclusive.

	.. py:attribute:: path
	:type: Path

	An absolute path to a binary to use for this tool.

	Relative paths are also supported, but they are relative to the
	:py:class:`pw_cc_toolchain` that uses this tool rather than relative to
	this :py:class:`pw_cc_tool` rule.

	Note: :py:attr:`pw_cc_tool.path` and :py:attr:`pw_cc_tool.tool` are
	mutually exclusive.

	.. admonition:: Note
	:class: warning

	This method of listing a tool is NOT recommended, and is provided as an
	escape hatch for edge cases. Prefer using :py:attr:`pw_cc_tool.tool`
	whenever possible.

	.. py:attribute:: execution_requirements
	:type: List[str]

	A list of strings that provide hints for execution environment
	compatibility (e.g. ``requires-darwin``).

	.. py:attribute:: requires_any_of
	:type: List[label]

	This tool may only be enabled when at least one of the constraints are
	met.

	If omitted, this tool will be enabled unconditionally.

	.. py:attribute:: additional_files
	:type: List[label]

	Additional files that are required for this tool to correctly operate.
	These files are propagated up to the :py:class:`pw_cc_toolchain` so you
	typically won't need to explicitly specify the ``*_files`` attributes
	on a :py:class:`pw_cc_toolchain`.


	.. py:class:: pw_cc_action_config

	Declares the configuration and selection of `pw_cc_tool` rules.

	Action configs are bound to a toolchain through `action_configs`, and are the
	driving mechanism for controlling toolchain tool invocation/behavior.

	Action configs define three key things:

	* Which tools to invoke for a given type of action.
	* Tool features and compatibility.
	* :py:class:`pw_cc_flag_set`\s that are unconditionally bound to a tool
	invocation.

	Examples:

	.. code-block:: py

	pw_cc_action_config(
	name = "ar",
	action_names = ["@pw_toolchain//actions:all_ar_actions"],
	implies = [
	"@pw_toolchain//features/legacy:archiver_flags",
	"@pw_toolchain//features/legacy:linker_param_file",
	],
	tools = [":ar_tool"],
	)

	pw_cc_action_config(
	name = "clang",
	action_names = [
	"@pw_toolchain//actions:all_asm_actions",
	"@pw_toolchain//actions:all_c_compiler_actions",
	]
	tools = [":clang_tool"],
	)

	.. py:attribute:: action_names
	:type: List[str]

	A list of action names to apply this action to.

	.. inclusive-language: disable

	Valid choices are listed at
	`@rules_cc//cc:action_names.bzl <https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/cc/action_names.bzl>`_\.

	.. inclusive-language: enable

	It is possible for some needed action names to not be enumerated in this list,
	so there is not rigid validation for these strings. Prefer using constants
	rather than manually typing action names.

	.. py:attribute:: enabled
	:type: bool

	Whether or not this action config is enabled by default.

	.. admonition:: Note

	This defaults to ``True`` since it's assumed that most listed action
	configs will be enabled and used by default. This is the opposite of
	Bazel's native default.

	.. py:attribute:: tools
	:type: List[label]

	The tool to use for the specified actions.

	A tool can be a :py:class:`pw_cc_tool`, or a binary.

	If multiple tools are specified, the first tool that has ``with_features``
	that satisfy the currently enabled feature set is used.

	.. py:attribute:: flag_sets
	:type: List[label]

	Labels that point to :py:class:`pw_cc_flag_set`\s that are unconditionally
	bound to the specified actions.

	.. admonition:: Note

	The flags in the :py:class:`pw_cc_flag_set` are only bound to matching
	action names. If an action is listed in this rule's
	:py:attr:`pw_cc_action_config.action_names`,
	but is NOT listed in the :py:class:`pw_cc_flag_set`\'s
	:py:attr:`pw_cc_flag_set.actions`, the flag will not be applied to that
	action.

	.. py:attribute:: implies
	:type: List[str]

	Names of features that should be automatically enabled when this tool is
	used.

	.. admonition:: Note
	:class: warning

	If this action config implies an unknown feature, this action config
	will silently be disabled. This behavior is native to Bazel itself, and
	there's no way to detect this and emit an error instead. For this
	reason, be very cautious when listing implied features!

	.. py:class:: pw_cc_feature

	Defines the implemented behavior of a C/C++ toolchain feature.


	This rule is effectively a wrapper for the ``feature`` constructor in
	`@rules_cc//cc:cc_toolchain_config_lib.bzl <https://github.com/bazelbuild/rules_cc/blob/main/cc/cc_toolchain_config_lib.bzl>`_.

	A feature is basically a dynamic flag set. There are a variety of
	dependencies and compatibility requirements that must be satisfied for the
	listed flag sets to be applied.

	A feature may be enabled or disabled through the following mechanisms:\

	* Via command-line flags, or a
	`.bazelrc file <https://bazel.build/run/bazelrc>`_\.
	* Through inter-feature relationships (enabling one feature may implicitly
	enable another).
	* Individual rules may elect to manually enable or disable features through
	the
	`builtin features attribute <https://bazel.build/reference/be/common-definitions#common.features>`_\.

	Because of the dynamic nature of toolchain features, it's generally best to
	avoid enumerating features as part of your toolchain with the following
	exceptions:

	* You want the flags to be controllable via Bazel's CLI. For example, adding
	``-v`` to a compiler invocation is often too verbose to be useful for most
	workflows, but can be instrumental when debugging obscure errors. By
	expressing compiler verbosity as a feature, users may opt-in when
	necessary.
	* You need to carry forward Starlark toolchain behaviors. If you're migrating
	a complex Starlark-based toolchain definition to these rules, many of the
	workflows and flags were likely based on features. This rule exists to
	support those existing structures.

	For more details about how Bazel handles features, see the official Bazel
	documentation at
	https://bazel.build/docs/cc-toolchain-config-reference#features.

	Note: ``env_sets`` are not yet supported.

	Examples:

	.. code-block:: py

	# A feature that can be easily toggled to include extra compiler output to
	# help debug things like include search path ordering and showing all the
	# flags passed to the compiler.
	#
	# Add `--features=verbose_compiler_output` to your Bazel invocation to
	# enable.
	pw_cc_feature(
	name = "verbose_compiler_output",
	enabled = False,
	feature_name = "verbose_compiler_output",
	flag_sets = [":verbose_compiler_flags"],
	)

	# This feature signals a capability, and doesn't have associated flags.
	#
	# For a list of well-known features, see:
	# https://bazel.build/docs/cc-toolchain-config-reference#wellknown-features
	pw_cc_feature(
	name = "link_object_files",
	enabled = True,
	feature_name = "supports_start_end_lib",
	)

	.. py:attribute:: feature_name
	:type: str

	The name of the feature that this rule implements.

	Feature names are used to express feature dependencies and compatibility.
	Because features are tracked by string names rather than labels, there's
	great flexibility in swapping out feature implementations or overriding
	the built-in legacy features that Bazel silently binds to every
	toolchain.

	:py:attr:`pw_cc_feature.feature_name` is used rather than ``name`` to
	distinguish between the rule name, and the intended final feature name.
	This allows similar rules to exist in the same package, even if slight
	differences are required.

	Example:

	.. code-block:: py

	pw_cc_feature(
	name = "sysroot_macos",
	feature_name = "sysroot",
	...
	)

	pw_cc_feature(
	name = "sysroot_linux",
	feature_name = "sysroot",
	...
	)

	While two features with the same :py:attr:`pw_cc_feature.feature_name` may
	not be bound to the same toolchain, they can happily live alongside each
	other in the same BUILD file.

	.. py:attribute:: enabled
	:type: bool

	Whether or not this feature is enabled by default.

	.. py:attribute:: flag_sets
	:type: List[label]

	Flag sets that, when expanded, implement this feature.

	.. py:attribute:: requires_any_of
	:type: List[label]

	A list of feature sets that define toolchain compatibility.

	If at least one of the listed :py:class:`pw_cc_feature`\s or
	:py:class:`pw_cc_feature_set`\s are satisfied (all features exist in the
	toolchain AND are currently enabled), this feature is deemed compatible
	and may be enabled.

	If this feature cannot be enabled (such as if, in the first example
	below, thin_lto didn't exist in the toolchain), it will throw an error.

	.. code-block:: py

	pw_cc_feature(
	name = "requires_thin_lto_and_opt",
	feature_name = "requires_thin_lto_and_opt",
	requires_any_of = [":thin_lto_requirements"]
	)

	pw_cc_feature(
	name = "requires_thin_lto_or_opt",
	feature_name = "requires_thin_lto_or_opt",
	requires_any_of = [":thin_lto", ":opt"]
	)

	.. admonition:: Note

	Even if :py:attr:`pw_cc_feature.requires_any_of` is satisfied, a
	feature is not enabled unless another mechanism (e.g. command-line
	flags, :py:attr:`pw_cc_feature.implies`, or
	:py:attr:`pw_cc_feature.enabled`\) signals that the feature should
	actually be enabled.

	.. py:attribute:: implies
	:type: List[label]

	List of features or action configs enabled along with this feature.

	.. admonition:: Note
	:class: warning

	If any of the named features cannot be enabled, this feature is
	silently disabled.

	.. py:attribute:: mutually_exclusive
	:type: List[Label]

	A list of feature or mutually exclusive categories that this feature is
	mutually exclusive with.

	.. admonition:: Note

	This feature cannot be enabled if another feature also provides the
	listed feature.


	.. py:class:: pw_cc_feature_set

	Defines a group of features.

	Semantically equivalent to "all of the specified features", and may be used
	wherever you can provide multiple features.

	Example:

	.. code-block:: py

	pw_cc_feature_set(
	name = "thin_lto_requirements",
	all_of = [
	":thin_lto",
	":opt",
	],
	)

	.. py:attribute:: features
	:type: List[label]

	Features that must be enabled for this feature set to be deemed compatible
	with the current toolchain configuration.