# 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.

# 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 (optional)
#     Edition of the Rust language to be used.
#     Options are "2015" and "2018". Defaults to "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).
#
# 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"

  # The actual host-target build of the proc macro crate.
  if (is_host) {
    if (defined(invoker.name)) {
      package_name = invoker.name
    } else {
      package_name = target_name
    }
    crate_name = string_replace(package_name, "-", "_")

    if (!defined(invoker.source_root)) {
      source_root = "src/lib.rs"
    } else {
      source_root = invoker.source_root
    }

    rust_proc_macro(proc_macro_target) {
      configs = []
      configs = invoker.configs

      not_needed(invoker,
                 [
                   "version",
                   "non_rust_deps",
                   "force_opt",
                 ])
      crate_root = source_root
      output_name = crate_name
      if (defined(invoker.non_rust_deps)) {
        data_deps = invoker.non_rust_deps
      }

      if (defined(invoker.edition) && invoker.edition == "2015") {
        configs -= [ "//build/config:rust_edition_2018" ]
        configs += [ "//build/config:rust_edition_2015" ]
      }

      forward_variables_from(invoker,
                             [
                               "deps",
                               "testonly",
                               "visibility",
                             ])
    }
  } else {
    not_needed(invoker, "*")
  }

  # redirect so that users don't need to be aware of host_toolchain requirement
  group(target_name) {
    public_deps = [ ":${proc_macro_target}($host_toolchain)" ]
  }
}