# Copyright 2021 The Chromium Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

# This file provides the ability for our C++ toolchain to successfully
# link binaries containing arbitrary Rust code.
#
# By "arbitrary Rust code" I mean .rlib archives full of Rust code, which
# is actually a static archive.
#
# Those static libraries don't link as-is into a final executable because
# they're designed for downstream processing by further invocations of rustc
# which link into a final binary. That final invocation of rustc knows how
# to do two things:
# * Find the Rust standard library.
# * Remap some generic allocator symbols to the specific allocator symbols
#   in use.
# This file does both those things. Any C++ target containing Rust .rlibs
# should simply depend on :std within this file and it will be taken care of.
# In practice, this will in future be taken care of by a standard template
# used for each Rust source set, so that a typical user of Rust need not
# think about it.
#
# This is obviously a bit fragile - rustc might do other magic in future.
# But, linking with a final C++ toolchain is something often needed, and
# https://github.com/rust-lang/rust/issues/64191 aims to make this
# officially possible.

import("//build/config/compiler/compiler.gni")
import("//build/config/rust.gni")

if (toolchain_has_rust) {
  # List of Rust stdlib rlibs which are present in the official
  # Rust toolchain we are using from the Android team. This is usually
  # a version or two behind nightly.
  # See //docs/security/rust-toolchain.md#building-on-non_linux-platforms
  # for how to maintain this list.
  stdlib_files = [
    "std",  # List first because it makes depfiles more debuggable (see below)
    "addr2line",
    "adler",
    "alloc",
    "cfg_if",
    "compiler_builtins",
    "core",
    "getopts",
    "gimli",
    "hashbrown",
    "libc",
    "memchr",
    "miniz_oxide",
    "object",
    "panic_abort",
    "panic_unwind",
    "proc_macro",
    "rustc_demangle",
    "std_detect",
    "test",
    "unicode_width",
    "unwind",
  ]

  # Different Rust toolchains may add or remove files relative to the above
  # list. That can be specified in gn args for anyone using (for instance)
  # nightly or some other experimental toolchain, prior to it becoming official.
  stdlib_files -= removed_rust_stdlib_libs
  stdlib_files += added_rust_stdlib_libs

  if (!use_unverified_rust_toolchain) {
    # rlib files which are distributed alongside Rust's prebuilt stdlib, but we
    # don't need to pass to the C++ linker because they're used for specialized
    # purposes.
    skip_stdlib_files = [
      "profiler_builtins",
      "rustc_std_workspace_alloc",
      "rustc_std_workspace_core",
      "rustc_std_workspace_std",
    ]
  }

  action("find_stdlib") {
    # Specifics of what we're doing here.
    #
    # We are using prebuilt Rust rlibs supplied along with the toolchain.
    # The Rust standard library consists of rlibs with roughly all the names
    # above.
    #
    # However, their filenames are not predictable, and therefore we can't
    # have ninja rules which depend upon them. (gn offers a facility to
    # build rules dynamically, but it's frowned upon because a script needs
    # to run each time).
    #
    # Instead therefore we copy these unpredictable .rlib paths to apredictable
    # location. That's what this script does. Furthermore, it generates a
    # .d file in order to teach Ninja that it only needs to do this copying
    # once, unless the source .rlibs change.
    #
    # The script accepts the list of known libraries and will raise an
    # exception if the list on disk differs. (Either 'Found stdlib rlib
    # that wasn't expected' or 'We failed to find all expected stdlib
    # rlibs').
    #
    # The script does one final job, which is to check that the rustc
    # version matches that in the gn arg 'rustc_version'. This is
    # technically orthogonal to the stdlib-finding job that we do here,
    # but it's something we want to be sure of running during any
    # typical Rust build, and this target happens to be depended upon
    # almost everywhere, so it's a good fit.
    script = "find_std_rlibs.py"
    depfile = "$target_out_dir/stdlib.d"
    out_libdir = rebase_path(target_out_dir, root_build_dir)
    out_depfile = rebase_path(depfile, root_build_dir)
    args = [
      "--rust-bin-dir",
      rebase_path("${rust_sysroot}/bin", root_build_dir),
      "--output",
      out_libdir,
      "--depfile",
      out_depfile,

      # Due to limitations in Ninja's handling of .d files, we have to pick
      # *the first* of our outputs. To make diagnostics more obviously
      # related to the Rust standard library, we ensure libstd.rlib is first.
      "--depfile-target",
      stdlib_files[0],

      "--expected-rustc-version",
      rustc_version,
    ]
    if (!use_unverified_rust_toolchain) {
      args += [
        "--stdlibs",
        string_join(",", stdlib_files),
        "--skip",
        string_join(",", skip_stdlib_files),
      ]
    }
    if (rust_abi_target != "") {
      args += [
        "--target",
        rust_abi_target,
      ]
    }

    outputs = []
    foreach(lib, stdlib_files) {
      outputs += [ "$target_out_dir/lib$lib.rlib" ]
    }
  }

  config("rust_stdlib_config") {
    ldflags = []
    out_libdir = rebase_path(target_out_dir, root_build_dir)
    foreach(lib, stdlib_files) {
      this_file = "$out_libdir/lib$lib.rlib"
      ldflags += [ this_file ]
    }
  }

  source_set("remap_alloc") {
    sources = [
      "immediate_crash.h",
      "remap_alloc.cc",
    ]
  }

  group("std") {
    assert(
        enable_rust,
        "Some C++ target is including Rust code even though enable_rust=false")
    all_dependent_configs = [ ":rust_stdlib_config" ]
    deps = [
      ":find_stdlib",
      ":remap_alloc",
    ]
  }
}