# Copyright 2018 The Fuchsia Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

import("//build/rust/rustc_artifact.gni")

# Defines a Rust procedural macro
#
# Parameters
#
#   name
#     Name of the crate as defined in its manifest file. If not specified, it is
#     assumed to be the same as the target name.
#
#   version (optional)
#     Semver version of the crate as seen on crates.io.
#
#   edition
#     Edition of the Rust language to be used.
#     Options are "2015" and "2018". If unsure, choose "2018".
#
#   configs (optional)
#     A list of config labels applying to this target.
#
#   deps (optional)
#     List of rust_library GN targets on which this crate depends.
#     Third party crates can be included through paths like
#     "//third_party/rust_crates:<cratename>".
#
#   source_root (optional)
#     Location of the crate root (e.g. `src/main.rs` or `src/lib.rs`).
#     This defaults to `./src/main.rs` for binaries and `./src/lib.rs` for libraries,
#     and should only be changed when absolutely necessary
#     (such as in the case of generated code).
#
#   with_lto (optional)
#     Force LTO to be enabled/disabled for the binary. Values are "none", "thin" and
#     "fat". This value takes precedence over GN args or the default value for the
#     type of build (debug or release).
#
#   features (optional)
#     A list of conditional compilation flags to enable. This can be used to set features for crates
#     built in-tree which are also published to crates.io. This would be passed to rustc as
#     '--cfg feature=XXX'
#
# Example of usage:
#
#   rustc_macro("foo") {
#     deps = [
#       "//garnet/public/rust/bar",
#       "//third_party/rust_crates:serde",
#       "//third_party/rust_crates:slab",
#     ]
#   }
template("rustc_macro") {
  # Compiling procedural macros is... a bit awkward.
  #
  # Even though they're provided to crates that use them as if they were normal
  # external crates, they're actually '.so'/'.dylib's that are compiled for the host machine
  # and then linked into the compiler, so they and all their dependencies should
  # be built for the host target.
  #
  # Once this is done, the resulting artifacts are copied into the Fuchsia target
  # directories to act as if they had been built for Fuchsia. In order to avoid
  # conflicts, the outputs of the original (host) artifact are built with a
  # `_proc_macro` suffix added onto the end, which is removed when they're copied
  # into the final target directory.
  forward_variables_from(invoker, [ "visibility" ])

  proc_macro_target = "${target_name}_proc_macro"
  proc_macro_host_target = ":${proc_macro_target}($host_toolchain)"

  # The actual host-target build of the proc macro crate.
  if (is_host) {
    if (defined(invoker.name)) {
      rustc_artifact_name = invoker.name
    } else {
      rustc_artifact_name = target_name
    }
    rustc_artifact(proc_macro_target) {
      forward_variables_from(invoker,
                             [
                               "deps",
                               "version",
                               "edition",
                               "configs",
                               "source_root",
                               "testonly",
                               "with_lto",
                               "features",
                             ])
      name = rustc_artifact_name
      type = "proc-macro"
      output_filename_suffix = "_proc_macro"
    }
  } else {
    not_needed(invoker, "*")
  }

  # Copy the generated Cargo.toml
  copy_cargo_toml_target = "${target_name}_copy_cargo_toml"
  proc_macro_target_gen_dir =
      get_label_info(proc_macro_host_target, "target_gen_dir")
  copy(copy_cargo_toml_target) {
    visibility = [ ":${target_name}" ]
    forward_variables_from(invoker, [ "testonly" ])
    deps = [ proc_macro_host_target ]
    sources = [ "$proc_macro_target_gen_dir/$proc_macro_target/Cargo.toml" ]
    outputs = [ "$target_gen_dir/$target_name/Cargo.toml" ]
  }

  # Copy the generated _info.json which provides data about the target that
  # was built.
  copy_build_info_target = "${target_name}_info"
  copy(copy_build_info_target) {
    deps = [ ":${proc_macro_target}_info($host_toolchain)" ]
    proc_macro_target_out_dir =
        get_label_info(proc_macro_host_target, "target_out_dir")
    sources = [ "$proc_macro_target_out_dir/${proc_macro_target}_info.json" ]
    outputs = [ "$target_out_dir/${copy_build_info_target}.json" ]
  }

  # Group all of the `copy` targets into one group which is publicly exposed.
  group(target_name) {
    public_deps = [
      ":$copy_build_info_target",
      ":$copy_cargo_toml_target",
      proc_macro_host_target,
    ]
  }
}