# 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",
]
}
}
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