rust/rs_bindings_from_cc.gni

# Copyright 2022 The Chromium Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

import("//build/config/clang/clang.gni")
import("//build/config/rust.gni")
import("//build/config/sysroot.gni")
import("//build/rust/mixed_static_library.gni")

# Template to generate and build Rust bindings for a set of C++ headers using
# Crubit's `rs_bindings_from_cc` tool.
#
# This template expands to a `mixed_static_library` named "<target>_rs_api" and
# containing the Rust side of the bindings (as well as internal C++ thunks
# needed to support the bindings).
#
# The generated out/.../gen/.../<target>_rs_api.rs is machine-generated, but
# should be fairly readable (inspecting it might be useful to discover the
# imported bindings and their shape).
#
# Parameters:
#
#   bindings_target:
#     The C++ target (e.g. a `source_set`) that Rust bindings should be
#     generated for.
#
#   public_headers:
#     The .h files to generate bindings for.
#
#     Implementation note: This doesn't just take *all* the headers of the
#     `bindings_target`, because typically only a *subset* of headers provides
#     the *public* API that bindings are needed for.
#
#     TODO(crbug.com/40226863): Internal headers should still to be included in
#     the targets_and_args metadata...
#
#   deps:
#     Other `rs_bindings_from_cc` targets that the bindings need to depend on
#     (e.g. because APIs in the `public_headers` refer to `struct`s declared in
#     those other targets. Note how in the usage example below bindings for
#     `struct Goat` are provided by `goat_rs_api`, and that therefore the
#     bindings for the `TeleportGoat` provided by `teleport_rs_api` depend on
#     `goat_rs_api`).
#
#     Oftentimes `deps` can be a copy of the `public_deps` of the
#     `bindings_target`, but depending on targets with the suffix "_rs_api".
#     Still, there are scenarios where `deps` don't parallel *all* entries from
#     `public_deps`:
#       * `public_deps` that don't expose Rust APIs (i.e. there are no
#         "..._rs_api" targets to depend on).
#       * `public_deps` that Crubit bindings don't depend on (dependencies that
#          don't provide re-exportable C++ APIs, or that only provide items
#          that are ignored by Crubit - e.g. `#define`s).
#
# Usage example:
#
#   BUILD.gn:
#       import("//build/rust/rs_bindings_from_cc.gni")
#       import("//build/rust/rust_executable.gni")
#
#       rust_executable("my_target") {
#         crate_root = "main.rs"
#         sources = [ "main.rs" ]
#         deps = [ ":teleport_rs_api" ]
#       ]
#
#       # This will generate "teleport_rs_api" target that provides Rust
#       # bindings for the "teleport.h" header from the ":teleport" source
#       # set.
#       rs_bindings_from_cc("teleport_rs_api") {
#         bindings_target = ":teleport"
#         public_headers = ["teleport.h"]
#         deps = [ ":goat_rs_api" ]  # Parallel's `public_deps` of ":teleport".
#       }
#
#       source_set("teleport") {
#         sources = [ "teleport.h", ... ]
#         public_deps = [ ":goat" ]
#       }
#
#       rs_bindings_from_cc("goat_rs_api") {
#         bindings_target = ":goat"
#         public_headers = ["goat.h"]
#       }
#       source_set("goat") {
#         sources = [ "goat.h", ... ]
#       }
#
#   teleport.h:
#     #include "goat.h"
#     void TeleportGoat(const Goat& goat_to_teleport);
#
#   goat.h:
#     struct Goat { ... };
#
#   main.rs:
#     fn main() {
#       let g: goat_rs_api::Goat = ...;
#       teleport_rs_api::TeleportGoat(&g);
#     }
#
# Debugging and implementation notes:
#
# - Consider running the build while CRUBIT_DEBUG environment variable is set.
#   This will generate additional `.ir` file and log extra information from
#   the `run_rs_bindings_from_cc.py` script (e.g. full cmdlines for invoking
#   `rs_bindings_from_cc`).
#
template("rs_bindings_from_cc") {
  # Mandatory parameter: bindings_target.
  assert(defined(invoker.bindings_target),
         "Must specify the C target to make bindings for.")
  _bindings_target = invoker.bindings_target

  # Mandatory/unavoidable parameter: target_name
  _lib_target_name = target_name
  _base_target_name = get_label_info(_bindings_target, "name")
  assert(_lib_target_name == "${_base_target_name}_rs_api",
         "The convention is that bindings for `foo` are named `foo_rs_api`")

  # Mandatory parameter: public_headers.
  assert(defined(invoker.public_headers),
         "Must specify the public C headers to make bindings for.")
  _rebased_public_headers = []
  foreach(hdr, invoker.public_headers) {
    _rebased_public_headers += [ rebase_path(hdr) ]
  }

  # Optional parameter: testonly.
  _testonly = false
  if (defined(invoker.testonly)) {
    _testonly = invoker.testonly
  }

  # Optional parameter: visibility.
  if (defined(invoker.visibility)) {
    _visibility = invoker.visibility
  }

  # Optional parameter: deps.
  #
  # TODO(crbug.com/40226863): Can we somehow assert that `_deps` only contains
  # some "..._rs_api" targets crated via
  # `mixed_static_library($_lib_target_name)` below?  foreach(dep, _deps) {
  # assert something }
  _deps = []
  if (defined(invoker.deps)) {
    _deps = invoker.deps
  }

  # Various names and paths that are shared across multiple targets defined
  # in the template here.
  _gen_bindings_target_name = "${_lib_target_name}_gen_bindings"
  _gen_metadata_target_name = "${_lib_target_name}_gen_metadata"
  _metadata_target_name = "${_lib_target_name}_metadata"
  _metadata_path = "${target_gen_dir}/${_lib_target_name}_meta.json"
  _rs_out_path = "${target_gen_dir}/${_lib_target_name}.rs"
  _cc_out_path = "${target_gen_dir}/${_lib_target_name}_impl.cc"

  # Calculating the --targets_and_args snippet for the *current* target
  # and putting it into GN's `metadata`.
  group(_metadata_target_name) {
    testonly = _testonly
    visibility = [
      ":${_gen_metadata_target_name}",
      ":${_lib_target_name}",
    ]
    deps = []

    metadata = {
      # The data below corresponds to a single-target entry inside
      # `--targets_and_args` cmdline argument of `rs_bindings_from_cc`.
      crubit_target_and_args = [
        {
          # The `get_label_info` call below expands ":foo_rs_api" into
          # something like "//dir/bar/baz:foo_rs_api".  Crubit assumes that
          # there is a colon + uses the after-colon-suffix as the name of the
          # crate.
          t = get_label_info(":${_lib_target_name}", "label_no_toolchain")
          h = _rebased_public_headers
        },
      ]
    }
  }

  # Gathering --targets-and-args data from *all* transitive dependencies and
  # putting them into the file at `_metadata_path`.
  generated_file(_gen_metadata_target_name) {
    testonly = _testonly
    visibility = [ ":${_gen_bindings_target_name}" ]

    deps = [ ":${_metadata_target_name}" ]
    deps += _deps

    testonly = _testonly
    outputs = [ _metadata_path ]
    output_conversion = "json"
    data_keys = [ "crubit_target_and_args" ]

    # `walk_keys` are used to limit how deep the transitive dependency goes.
    # This is important, because Crubit doesn't care about all the `deps` or
    # `public_deps` of the `_bindings_target`.  (See also the doc comment about
    # `rs_bindings_from_cc.deps` parameter at the top of this file.)
    walk_keys = [ "crubit_metadata_deps" ]
  }

  # Exposing the generated Rust bindings.
  mixed_static_library(_lib_target_name) {
    testonly = _testonly
    if (defined(_visibility)) {
      visibility = _visibility
    }

    sources = [ _cc_out_path ]
    deps = _deps
    deps += [
      ":${_gen_bindings_target_name}",
      ":${_metadata_target_name}",
      "//third_party/crubit:deps_of_rs_api_impl",
      _bindings_target,
    ]

    # Chromium already covers `chromium/src/` and `out/Release/gen` in the
    # include path, but we need to explicitly add `out/Release` below.  This
    # is needed, because `--public_headers` passed to Crubit use paths relative
    # to the `out/Release` directory.  See also b/239238801.
    include_dirs = [ root_build_dir ]

    rs_sources = [ _rs_out_path ]
    rs_crate_name = _lib_target_name
    rs_crate_root = _rs_out_path
    rs_deps = _deps
    rs_deps += [
      ":${_gen_bindings_target_name}",
      "//third_party/crubit:deps_of_rs_api",
    ]

    metadata = {
      crubit_metadata_deps = _deps + [ ":${_metadata_target_name}" ]
    }
  }

  # Invoking Crubit's `rs_bindings_from_cc` tool to generate Rust bindings.
  action(_gen_bindings_target_name) {
    testonly = _testonly
    if (defined(_visibility)) {
      visibility = _visibility
    }

    script = "//build/rust/run_rs_bindings_from_cc.py"
    inputs = [ "//third_party/rust-toolchain/bin/rs_bindings_from_cc" ]
    sources = invoker.public_headers
    outputs = [
      _rs_out_path,
      _cc_out_path,
    ]

    deps = [ ":${_gen_metadata_target_name}" ]
    args = [
      # Target-specific outputs:
      "--rs_out",
      rebase_path(_rs_out_path),
      "--cc_out",
      rebase_path(_cc_out_path),

      # Target-specific inputs:
      "--public_headers",
      string_join(",", _rebased_public_headers),
      "--targets_and_args_from_gn",
      rebase_path(_metadata_path),
    ]

    # Several important compiler flags come from default_compiler_configs
    configs = default_compiler_configs
    if (defined(invoker.configs)) {
      configs += invoker.configs
    }
    args += [
      "--",
      "{{defines}}",
      "{{include_dirs}}",
      "{{cflags}}",

      # This path contains important C headers (e.g. stddef.h) and {{cflags}}
      # does not include it. Normally this path is implicitly added by clang but
      # it does not happen for libclang.
      #
      # Add it last so includes from deps and configs take precedence.
      "-isystem" + rebase_path(
              clang_base_path + "/lib/clang/" + clang_version + "/include",
              root_build_dir),

      # Passes C comments through as rustdoc attributes.
      "-fparse-all-comments",
    ]
  }
}