pigweed/pw_toolchain_bazel/api.rst

.. _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.