old_contents: """ Rule for building Boost from sources. """
  
  load(
      "//tools/build_defs:framework.bzl",
      "CC_EXTERNAL_RULE_ATTRIBUTES",
      "cc_external_rule_impl",
      "create_attrs",
  )
  load("//tools/build_defs:detect_root.bzl", "detect_root")
  
  def _boost_build(ctx):
      attrs = create_attrs(
          ctx.attr,
          configure_name = "BuildBoost",
          create_configure_script = _create_configure_script,
          make_commands = ["./b2 install {} --prefix=.".format(" ".join(ctx.attr.user_options))],
      )
      return cc_external_rule_impl(ctx, attrs)
  
  def _create_configure_script(configureParameters):
      ctx = configureParameters.ctx
      root = detect_root(ctx.attr.lib_source)
  
      return "\n".join([
          "cd $INSTALLDIR",
          "cp -R $EXT_BUILD_ROOT/{}/. .".format(root),
          "./bootstrap.sh",
      ])
  
  def _attrs():
      attrs = dict(CC_EXTERNAL_RULE_ATTRIBUTES)
      attrs.update({
          # any additional flags to pass to b2
          "user_options": attr.string_list(mandatory = False),
      })
      return attrs
  
  """ Rule for building Boost. Invokes bootstrap.sh and then b2 install.
    Attributes:
      boost_srcs - target with the boost sources
  """
  boost_build = rule(
      attrs = _attrs(),
      fragments = ["cpp"],
      output_to_genfiles = True,
      implementation = _boost_build,
  )
  
new_contents: """ Rule for building Boost from sources. """
  
  load(
      "//tools/build_defs:framework.bzl",
      "CC_EXTERNAL_RULE_ATTRIBUTES",
      "cc_external_rule_impl",
      "create_attrs",
  )
  load("//tools/build_defs:detect_root.bzl", "detect_root")
  
  def _boost_build(ctx):
      attrs = create_attrs(
          ctx.attr,
          configure_name = "BuildBoost",
          create_configure_script = _create_configure_script,
          make_commands = ["./b2 install {} --prefix=.".format(" ".join(ctx.attr.user_options))],
      )
      return cc_external_rule_impl(ctx, attrs)
  
  def _create_configure_script(configureParameters):
      ctx = configureParameters.ctx
      root = detect_root(ctx.attr.lib_source)
  
      return "\n".join([
          "cd $INSTALLDIR",
          "##copy_dir_contents_to_dir## $$EXT_BUILD_ROOT$$/{}/. .".format(root),
          "./bootstrap.sh",
      ])
  
  def _attrs():
      attrs = dict(CC_EXTERNAL_RULE_ATTRIBUTES)
      attrs.update({
          # any additional flags to pass to b2
          "user_options": attr.string_list(mandatory = False),
      })
      return attrs
  
  """ Rule for building Boost. Invokes bootstrap.sh and then b2 install.
    Attributes:
      boost_srcs - target with the boost sources
  """
  boost_build = rule(
      attrs = _attrs(),
      fragments = ["cpp"],
      output_to_genfiles = True,
      implementation = _boost_build,
      toolchains = [
          "@rules_foreign_cc//tools/build_defs/shell_toolchain/toolchains:shell_commands",
      ],
  )
  
subject: Boost rule to use shell toolchain, fixes #202 (#203)