diff --git a/tools/migration/BUILD b/tools/migration/BUILD
new file mode 100644
index 00000000..06b4f278
--- /dev/null
+++ b/tools/migration/BUILD
@@ -0,0 +1,45 @@
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+py_binary(
+    name = "legacy_fields_migrator",
+    srcs = ["legacy_fields_migrator.py"],
+    deps = [
+        ":legacy_fields_migration_lib",
+        "//third_party/com/github/bazelbuild/bazel/src/main/protobuf:crosstool_config_py_pb2",
+        "@io_abseil_py//absl:app",
+        "@io_abseil_py//absl/flags",
+    ],
+)
+
+py_library(
+    name = "legacy_fields_migration_lib",
+    srcs = ["legacy_fields_migration_lib.py"],
+    deps = [
+        "//third_party/com/github/bazelbuild/bazel/src/main/protobuf:crosstool_config_py_pb2",
+    ],
+)
+
+py_test(
+    name = "legacy_fields_migration_lib_test",
+    srcs = ["legacy_fields_migration_lib_test.py"],
+    deps = [
+        ":legacy_fields_migration_lib",
+        "//third_party/com/github/bazelbuild/bazel/src/main/protobuf:crosstool_config_py_pb2",
+    ],
+)
diff --git a/tools/migration/legacy_fields_migration_lib.py b/tools/migration/legacy_fields_migration_lib.py
new file mode 100644
index 00000000..ac0ae8d4
--- /dev/null
+++ b/tools/migration/legacy_fields_migration_lib.py
@@ -0,0 +1,217 @@
+"""Module providing migrate_legacy_fields function.
+
+migrate_legacy_fields takes parsed CROSSTOOL proto and migrates it (inplace) to
+use only the features.
+
+Tracking issue: https://github.com/bazelbuild/bazel/issues/5187
+
+Since C++ rules team is working on migrating CROSSTOOL from text proto into
+Starlark, we advise CROSSTOOL owners to wait for the CROSSTOOL -> Starlark
+migrator before they invest too much time into fixing their pipeline. Tracking
+issue for the Starlark effort is
+https://github.com/bazelbuild/bazel/issues/5380.
+"""
+
+from third_party.com.github.bazelbuild.bazel.src.main.protobuf import crosstool_config_pb2
+
+ALL_COMPILE_ACTIONS = [
+    "assemble", "preprocess-assemble", "linkstamp-compile", "c-compile",
+    "c++-compile", "c++-header-parsing", "c++-module-compile",
+    "c++-module-codegen", "lto-backend", "clif-match"
+]
+
+ALL_CXX_COMPILE_ACTIONS = [
+    action for action in ALL_COMPILE_ACTIONS if action != "c-compile"
+]
+
+ALL_LINK_ACTIONS = [
+    "c++-link-executable", "c++-link-dynamic-library",
+    "c++-link-nodeps-dynamic-library"
+]
+
+# Map converting from LinkingMode to corresponding feature name
+LINKING_MODE_TO_FEATURE_NAME = {
+    "FULLY_STATIC": "fully_static_link",
+    "MOSTLY_STATIC": "static_linking_mode",
+    "DYNAMIC": "dynamic_linking_mode",
+    "MOSTLY_STATIC_LIBRARIES": "static_linking_mode_nodeps_library",
+}
+
+
+def migrate_legacy_fields(crosstool):
+  """Migrates parsed crosstool (inplace) to not use legacy fields."""
+  crosstool.ClearField("default_toolchain")
+  for toolchain in crosstool.toolchain:
+    # clear noop fields first
+    toolchain.ClearField("debian_extra_requires")
+    toolchain.ClearField("gcc_plugin_compiler_flag")
+    toolchain.ClearField("ar_flag")
+    toolchain.ClearField("ar_thin_archives_flag")
+    toolchain.ClearField("gcc_plugin_header_directory")
+    toolchain.ClearField("mao_plugin_header_directory")
+    toolchain.ClearField("supports_normalizing_ar")
+    toolchain.ClearField("supports_thin_archives")
+    toolchain.ClearField("supports_incremental_linker")
+    toolchain.ClearField("supports_dsym")
+    toolchain.ClearField("default_python_top")
+    toolchain.ClearField("default_python_version")
+    toolchain.ClearField("python_preload_swigdeps")
+
+    _ = [_migrate_expand_if_all_available(f) for f in toolchain.feature]
+    _ = [_migrate_expand_if_all_available(ac) for ac in toolchain.action_config]
+
+    # Create default_link_flags feature for linker_flag
+    flag_sets = _extract_legacy_link_flag_sets_for(toolchain)
+    if flag_sets:
+      if _contains_feature(toolchain, "default_link_flags"):
+        continue
+      feature = _prepend_feature(toolchain)
+      feature.name = "default_link_flags"
+      feature.enabled = True
+      _add_flag_sets(feature, flag_sets)
+
+    # Create default_compile_flags feature for compiler_flag, cxx_flag
+    flag_sets = _extract_legacy_compile_flag_sets_for(toolchain)
+    if flag_sets and not _contains_feature(toolchain, "default_compile_flags"):
+      feature = _prepend_feature(toolchain)
+      feature.enabled = True
+      feature.name = "default_compile_flags"
+      _add_flag_sets(feature, flag_sets)
+
+    toolchain.ClearField("compilation_mode_flags")
+
+    # Unfiltered cxx flags have to have their own special feature.
+    # "unfiltered_compile_flags" is a well-known (by Bazel) feature name that is
+    # excluded from nocopts filtering.
+    if toolchain.unfiltered_cxx_flag:
+      # If there already is a feature named unfiltered_compile_flags, the
+      # crosstool is already migrated for unfiltered_compile_flags
+      if _contains_feature(toolchain, "unfiltered_compile_flags"):
+        for f in toolchain.feature:
+          if f.name == "unfiltered_compile_flags":
+            feature = f
+            break
+      else:
+        feature = toolchain.feature.add()
+        feature.name = "unfiltered_compile_flags"
+        feature.enabled = True
+
+      _add_flag_sets(
+          feature, [[None, ALL_COMPILE_ACTIONS, toolchain.unfiltered_cxx_flag]])
+      toolchain.ClearField("unfiltered_cxx_flag")
+
+
+def _add_flag_sets(feature, flag_sets):
+  """Add flag sets into a feature."""
+  for flag_set in flag_sets:
+    with_feature = flag_set[0]
+    actions = flag_set[1]
+    flags = flag_set[2]
+    flag_set = feature.flag_set.add()
+    if with_feature is not None:
+      flag_set.with_feature.add().feature[:] = [with_feature]
+    flag_set.action[:] = actions
+    flag_group = flag_set.flag_group.add()
+    flag_group.flag[:] = flags
+  return feature
+
+
+def _extract_legacy_compile_flag_sets_for(toolchain):
+  """Get flag sets for default_compile_flags feature."""
+  result = []
+  if toolchain.compiler_flag:
+    result.append([None, ALL_COMPILE_ACTIONS, toolchain.compiler_flag])
+    toolchain.ClearField("compiler_flag")
+  if toolchain.cxx_flag:
+    result.append([None, ALL_CXX_COMPILE_ACTIONS, toolchain.cxx_flag])
+    toolchain.ClearField("cxx_flag")
+
+  # Migrate compiler_flag/cxx_flag from compilation_mode_flags
+  for cmf in toolchain.compilation_mode_flags:
+    mode = crosstool_config_pb2.CompilationMode.Name(cmf.mode).lower()
+    # coverage mode has been a noop since a while
+    if mode == "coverage":
+      continue
+
+    if (cmf.compiler_flag or
+        cmf.cxx_flag) and not _contains_feature(toolchain, mode):
+      feature = toolchain.feature.add()
+      feature.name = mode
+
+    if cmf.compiler_flag:
+      result.append([mode, ALL_COMPILE_ACTIONS, cmf.compiler_flag])
+
+    if cmf.cxx_flag:
+      result.append([mode, ALL_CXX_COMPILE_ACTIONS, cmf.cxx_flag])
+
+  return result
+
+
+def _extract_legacy_link_flag_sets_for(toolchain):
+  """Get flag sets for default_link_flags feature."""
+  result = []
+
+  # Migrate linker_flag
+  if toolchain.linker_flag:
+    result.append([None, ALL_LINK_ACTIONS, toolchain.linker_flag])
+  toolchain.ClearField("linker_flag")
+
+  # Migrate linker_flags from compilation_mode_flags
+  for cmf in toolchain.compilation_mode_flags:
+    mode = crosstool_config_pb2.CompilationMode.Name(cmf.mode).lower()
+    # coverage mode has beed a noop since a while
+    if mode == "coverage":
+      continue
+
+    if cmf.linker_flag and not _contains_feature(toolchain, mode):
+      feature = toolchain.feature.add()
+      feature.name = mode
+
+    if cmf.linker_flag:
+      result.append([mode, ALL_LINK_ACTIONS, cmf.linker_flag])
+
+  # Migrate linker_flags from linking_mode_flags
+  for lmf in toolchain.linking_mode_flags:
+    mode = crosstool_config_pb2.LinkingMode.Name(lmf.mode)
+    mode = LINKING_MODE_TO_FEATURE_NAME.get(mode)
+    # if the feature is already there, we don't migrate, lmf is not used
+    if _contains_feature(toolchain, mode):
+      continue
+
+    # dynamic_linking_mode is also a marker feature it has a meaning
+    # even when empty
+    if lmf.linker_flag or mode == "dynamic_linking_mode":
+      feature = toolchain.feature.add()
+      feature.name = mode
+
+    if lmf.linker_flag:
+      result.append([mode, ALL_LINK_ACTIONS, lmf.linker_flag])
+  toolchain.ClearField("linking_mode_flags")
+
+  return result
+
+
+def _prepend_feature(toolchain):
+  """Create a new feature and make it be the first in the toolchain."""
+  features = toolchain.feature
+  toolchain.ClearField("feature")
+  new_feature = toolchain.feature.add()
+  toolchain.feature.extend(features)
+  return new_feature
+
+
+def _contains_feature(toolchain, name):
+  """Returns True when toolchain contains a feature with a given name."""
+  return any(feature.name == name for feature in toolchain.feature)
+
+
+def _migrate_expand_if_all_available(message):
+  """Move expand_if_all_available fields to flag_groups."""
+  for flag_set in message.flag_set:
+    if flag_set.expand_if_all_available:
+      for flag_group in flag_set.flag_group:
+        new_vars = (
+            flag_group.expand_if_all_available[:] +
+            flag_set.expand_if_all_available[:])
+        flag_group.expand_if_all_available[:] = new_vars
+      flag_set.ClearField("expand_if_all_available")
diff --git a/tools/migration/legacy_fields_migration_lib_test.py b/tools/migration/legacy_fields_migration_lib_test.py
new file mode 100644
index 00000000..c0165ce1
--- /dev/null
+++ b/tools/migration/legacy_fields_migration_lib_test.py
@@ -0,0 +1,422 @@
+import unittest
+from google.protobuf import text_format
+from third_party.com.github.bazelbuild.bazel.src.main.protobuf import crosstool_config_pb2
+from tools.migration.legacy_fields_migration_lib import ALL_COMPILE_ACTIONS
+from tools.migration.legacy_fields_migration_lib import ALL_CXX_COMPILE_ACTIONS
+from tools.migration.legacy_fields_migration_lib import ALL_LINK_ACTIONS
+from tools.migration.legacy_fields_migration_lib import migrate_legacy_fields
+
+
+def assert_has_feature(self, toolchain, name):
+  self.assertTrue(any(feature.name == name for feature in toolchain.feature))
+
+
+def make_crosstool(string):
+  crosstool = crosstool_config_pb2.CrosstoolRelease()
+  text_format.Merge("major_version: '123' minor_version: '456'", crosstool)
+  toolchain = crosstool.toolchain.add()
+  text_format.Merge(string, toolchain)
+  return crosstool
+
+
+def migrate_to_string(crosstool):
+  migrate_legacy_fields(crosstool)
+  return to_string(crosstool)
+
+
+def to_string(crosstool):
+  return text_format.MessageToString(crosstool)
+
+
+class LegacyFieldsMigrationLibTest(unittest.TestCase):
+
+  def test_deletes_unused_fields(self):
+    crosstool = make_crosstool("""
+          debian_extra_requires: 'debian-1'
+          gcc_plugin_compiler_flag: 'gcc_plugin_compiler_flag-1'
+          ar_flag: 'ar_flag-1'
+          ar_thin_archives_flag: 'ar_thin_archives_flag-1'
+          gcc_plugin_header_directory: 'gcc_plugin_header_directory-1'
+          mao_plugin_header_directory: 'mao_plugin_header_directory-1'
+          default_python_top: 'default_python_top-1'
+          default_python_version: 'default_python_version-1'
+          python_preload_swigdeps: false
+          supports_normalizing_ar: false
+          supports_thin_archives: false
+          supports_incremental_linker: false
+          supports_dsym: false
+      """)
+    output = migrate_to_string(crosstool)
+    self.assertNotIn("debian_extra_requires", output)
+    self.assertNotIn("gcc_plugin_compiler_flag", output)
+    self.assertNotIn("ar_flag", output)
+    self.assertNotIn("ar_thin_archives_flag", output)
+    self.assertNotIn("gcc_plugin_header_directory", output)
+    self.assertNotIn("mao_plugin_header_directory", output)
+    self.assertNotIn("supports_normalizing_ar", output)
+    self.assertNotIn("supports_thin_archives", output)
+    self.assertNotIn("supports_incremental_linker", output)
+    self.assertNotIn("supports_dsym", output)
+    self.assertNotIn("default_python_top", output)
+    self.assertNotIn("default_python_version", output)
+    self.assertNotIn("python_preload_swigdeps", output)
+
+  def test_deletes_default_toolchains(self):
+    crosstool = make_crosstool("")
+    crosstool.default_toolchain.add()
+    self.assertEqual(len(crosstool.default_toolchain), 1)
+    migrate_legacy_fields(crosstool)
+    self.assertEqual(len(crosstool.default_toolchain), 0)
+
+  def test_migrate_compiler_flags(self):
+    crosstool = make_crosstool("""
+        compiler_flag: 'clang-flag-1'
+    """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(len(output.compiler_flag), 0)
+    self.assertEqual(output.feature[0].name, "default_compile_flags")
+    self.assertEqual(output.feature[0].flag_set[0].action, ALL_COMPILE_ACTIONS)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["clang-flag-1"])
+
+  def test_migrate_cxx_flags(self):
+    crosstool = make_crosstool("""
+        cxx_flag: 'clang-flag-1'
+    """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(len(output.cxx_flag), 0)
+    self.assertEqual(output.feature[0].name, "default_compile_flags")
+    self.assertEqual(output.feature[0].flag_set[0].action,
+                     ALL_CXX_COMPILE_ACTIONS)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["clang-flag-1"])
+
+  def test_compiler_flag_come_before_cxx_flags(self):
+    crosstool = make_crosstool("""
+        compiler_flag: 'clang-flag-1'
+        cxx_flag: 'clang-flag-2'
+    """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "default_compile_flags")
+    self.assertEqual(output.feature[0].flag_set[0].action, ALL_COMPILE_ACTIONS)
+    self.assertEqual(output.feature[0].flag_set[1].action,
+                     ALL_CXX_COMPILE_ACTIONS)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["clang-flag-1"])
+    self.assertEqual(output.feature[0].flag_set[1].flag_group[0].flag,
+                     ["clang-flag-2"])
+
+  def test_migrate_linker_flags(self):
+    crosstool = make_crosstool("""
+        linker_flag: 'linker-flag-1'
+    """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(len(output.linker_flag), 0)
+    self.assertEqual(output.feature[0].name, "default_link_flags")
+    self.assertEqual(output.feature[0].flag_set[0].action, ALL_LINK_ACTIONS)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["linker-flag-1"])
+
+  def test_compilation_mode_flags(self):
+    crosstool = make_crosstool("""
+        compiler_flag: "compile-flag-1"
+        cxx_flag: "cxx-flag-1"
+        linker_flag: "linker-flag-1"
+        compilation_mode_flags {
+          mode: OPT
+          compiler_flag: "opt-flag-1"
+          cxx_flag: "opt-flag-2"
+          linker_flag: "opt-flag-3"
+        }
+    """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(len(output.compilation_mode_flags), 0)
+    assert_has_feature(self, output, "opt")
+
+    self.assertEqual(output.feature[0].name, "default_compile_flags")
+    self.assertEqual(output.feature[1].name, "default_link_flags")
+
+    # flag set for compiler_flag fields
+    self.assertEqual(len(output.feature[0].flag_set[0].with_feature), 0)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["compile-flag-1"])
+
+    # flag set for cxx_flag fields
+    self.assertEqual(len(output.feature[0].flag_set[1].with_feature), 0)
+    self.assertEqual(output.feature[0].flag_set[1].flag_group[0].flag,
+                     ["cxx-flag-1"])
+
+    # flag set for compiler_flag from compilation_mode_flags
+    self.assertEqual(len(output.feature[0].flag_set[2].with_feature), 1)
+    self.assertEqual(output.feature[0].flag_set[2].with_feature[0].feature[0],
+                     "opt")
+    self.assertEqual(output.feature[0].flag_set[2].flag_group[0].flag,
+                     ["opt-flag-1"])
+
+    # flag set for cxx_flag from compilation_mode_flags
+    self.assertEqual(len(output.feature[0].flag_set[3].with_feature), 1)
+    self.assertEqual(output.feature[0].flag_set[3].with_feature[0].feature[0],
+                     "opt")
+    self.assertEqual(output.feature[0].flag_set[3].flag_group[0].flag,
+                     ["opt-flag-2"])
+
+    # default_link_flags, flag set for linker_flag
+    self.assertEqual(len(output.feature[1].flag_set[0].with_feature), 0)
+    self.assertEqual(output.feature[1].flag_set[0].flag_group[0].flag,
+                     ["linker-flag-1"])
+
+    # default_link_flags, flag set for linker_flag from
+    # compilation_mode_flags
+    self.assertEqual(len(output.feature[1].flag_set[1].with_feature), 1)
+    self.assertEqual(output.feature[1].flag_set[1].with_feature[0].feature[0],
+                     "opt")
+    self.assertEqual(output.feature[1].flag_set[1].flag_group[0].flag,
+                     ["opt-flag-3"])
+
+  def test_linking_mode_flags(self):
+    crosstool = make_crosstool("""
+        linker_flag: "linker-flag-1"
+        compilation_mode_flags {
+          mode: DBG
+          linker_flag: "dbg-flag-1"
+        }
+        linking_mode_flags {
+          mode: MOSTLY_STATIC
+          linker_flag: "mostly-static-flag-1"
+        }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(len(output.compilation_mode_flags), 0)
+    self.assertEqual(len(output.linking_mode_flags), 0)
+
+    # flag set for linker_flag
+    self.assertEqual(len(output.feature[0].flag_set[0].with_feature), 0)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["linker-flag-1"])
+
+    # flag set for compilation_mode_flags
+    self.assertEqual(len(output.feature[0].flag_set[1].with_feature), 1)
+    self.assertEqual(output.feature[0].flag_set[1].with_feature[0].feature[0],
+                     "dbg")
+    self.assertEqual(output.feature[0].flag_set[1].flag_group[0].flag,
+                     ["dbg-flag-1"])
+
+    # flag set for linking_mode_flags
+    self.assertEqual(len(output.feature[0].flag_set[2].with_feature), 1)
+    self.assertEqual(output.feature[0].flag_set[2].with_feature[0].feature[0],
+                     "static_linking_mode")
+    self.assertEqual(output.feature[0].flag_set[2].flag_group[0].flag,
+                     ["mostly-static-flag-1"])
+
+  def test_coverage_compilation_mode_ignored(self):
+    crosstool = make_crosstool("""
+    compilation_mode_flags {
+      mode: COVERAGE
+      compiler_flag: "coverage-flag-1"
+      cxx_flag: "coverage-flag-2"
+      linker_flag: "coverage-flag-3"
+    }
+    """)
+    output = migrate_to_string(crosstool)
+    self.assertNotIn("compilation_mode_flags", output)
+    self.assertNotIn("coverage-flag-1", output)
+    self.assertNotIn("coverage-flag-2", output)
+    self.assertNotIn("coverage-flag-3", output)
+    self.assertNotIn("COVERAGE", output)
+
+  def test_dynamic_linking_mode_is_added_even_when_empty(self):
+    crosstool = make_crosstool("""
+    linking_mode_flags {
+      mode: DYNAMIC
+    }
+    """)
+    output = migrate_to_string(crosstool)
+    self.assertNotIn("linking_mode_flags", output)
+    self.assertNotIn("DYNAMIC", output)
+    self.assertIn("name: \"dynamic_linking_mode\"", output)
+
+  def test_linking_mode_features_are_not_added_when_present(self):
+    crosstool = make_crosstool("""
+    linking_mode_flags {
+      mode: DYNAMIC
+      linker_flag: 'dynamic-flag'
+    }
+    linking_mode_flags {
+      mode: FULLY_STATIC
+      linker_flag: 'fully-static-flag'
+    }
+    linking_mode_flags {
+      mode: MOSTLY_STATIC
+      linker_flag: 'mostly-static-flag'
+    }
+    linking_mode_flags {
+      mode: MOSTLY_STATIC_LIBRARIES
+      linker_flag: 'mostly-static-libraries-flag'
+    }
+    feature { name: "static_linking_mode" }
+    feature { name: "dynamic_linking_mode" }
+    feature { name: "static_linking_mode_nodeps_library" }
+    feature { name: "fully_static_link" }
+    """)
+    output = migrate_to_string(crosstool)
+    self.assertNotIn("linking_mode_flags", output)
+    self.assertNotIn("DYNAMIC", output)
+    self.assertNotIn("MOSTLY_STATIC", output)
+    self.assertNotIn("MOSTLY_STATIC_LIBRARIES", output)
+    self.assertNotIn("MOSTLY_STATIC_LIBRARIES", output)
+    self.assertNotIn("dynamic-flag", output)
+    self.assertNotIn("fully-static-flag", output)
+    self.assertNotIn("mostly-static-flag", output)
+    self.assertNotIn("mostly-static-libraries-flag", output)
+
+  def test_unfiltered_compile_flags_is_not_added_when_already_present(self):
+    crosstool = make_crosstool("""
+            unfiltered_cxx_flag: 'unfiltered-flag-1'
+            feature { name: 'something_else' }
+            feature { name: 'unfiltered_compile_flags' }
+            feature { name: 'something_else_2' }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "something_else")
+    self.assertEqual(output.feature[1].name, "unfiltered_compile_flags")
+    self.assertEqual(output.feature[1].flag_set[0].action, ALL_COMPILE_ACTIONS)
+    self.assertEqual(output.feature[1].flag_set[0].flag_group[0].flag,
+                     ["unfiltered-flag-1"])
+    self.assertEqual(output.feature[2].name, "something_else_2")
+
+  def test_unfiltered_compile_flags_is_added_at_the_end(self):
+    crosstool = make_crosstool("""
+            feature { name: 'something_else' }
+            unfiltered_cxx_flag: 'unfiltered-flag-1'
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "something_else")
+    self.assertEqual(output.feature[1].name, "unfiltered_compile_flags")
+    self.assertEqual(output.feature[1].flag_set[0].action, ALL_COMPILE_ACTIONS)
+    self.assertEqual(output.feature[1].flag_set[0].flag_group[0].flag,
+                     ["unfiltered-flag-1"])
+
+  def test_default_link_flags_is_added_first(self):
+    crosstool = make_crosstool("""
+          linker_flag: 'linker-flag-1'
+          feature { name: 'something_else' }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "default_link_flags")
+    self.assertEqual(output.feature[0].enabled, True)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["linker-flag-1"])
+
+  def test_default_link_flags_is_not_added_when_already_present(self):
+    crosstool = make_crosstool("""
+            linker_flag: 'linker-flag-1'
+            feature { name: 'something_else' }
+            feature { name: 'default_link_flags' }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "something_else")
+    self.assertEqual(output.feature[1].name, "default_link_flags")
+
+  def test_default_compile_flags_is_not_added_when_no_reason_to(self):
+    crosstool = make_crosstool("""
+          feature { name: 'something_else' }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "something_else")
+    self.assertEqual(len(output.feature), 1)
+
+  def test_default_compile_flags_is_first(self):
+    crosstool = make_crosstool("""
+          compiler_flag: 'compiler-flag-1'
+          feature { name: 'something_else' }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "default_compile_flags")
+    self.assertEqual(output.feature[0].enabled, True)
+    self.assertEqual(output.feature[0].flag_set[0].flag_group[0].flag,
+                     ["compiler-flag-1"])
+
+  def test_default_compile_flags_not_added_when_present(self):
+    crosstool = make_crosstool("""
+          compiler_flag: 'compiler-flag-1'
+          feature { name: 'something_else' }
+          feature { name: 'default_compile_flags' }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.feature[0].name, "something_else")
+    self.assertEqual(output.feature[1].name, "default_compile_flags")
+    self.assertEqual(len(output.feature[1].flag_set), 0)
+
+  def test_migrate_expand_if_all_available_from_flag_sets(self):
+    crosstool = make_crosstool("""
+        action_config {
+          action_name: 'something'
+          config_name: 'something'
+          flag_set {
+            expand_if_all_available: 'foo'
+            flag_group {
+              flag: '%{foo}'
+            }
+            flag_group {
+              expand_if_all_available: 'bar'
+              flag: 'bar'
+            }
+          }
+        }
+        feature {
+          name: 'something_else'
+          flag_set {
+            action: 'c-compile'
+            expand_if_all_available: 'foo'
+            flag_group {
+              flag: '%{foo}'
+            }
+            flag_group {
+              expand_if_all_available: 'bar'
+              flag: 'bar'
+            }
+          }
+        }
+        """)
+    migrate_legacy_fields(crosstool)
+    output = crosstool.toolchain[0]
+    self.assertEqual(output.action_config[0].action_name, "something")
+    self.assertEqual(len(output.action_config[0].flag_set), 1)
+    self.assertEqual(
+        len(output.action_config[0].flag_set[0].expand_if_all_available), 0)
+    self.assertEqual(len(output.action_config[0].flag_set[0].flag_group), 2)
+    self.assertEqual(
+        output.action_config[0].flag_set[0].flag_group[0]
+        .expand_if_all_available, ["foo"])
+    self.assertEqual(
+        output.action_config[0].flag_set[0].flag_group[1]
+        .expand_if_all_available, ["bar", "foo"])
+
+    self.assertEqual(output.feature[0].name, "something_else")
+    self.assertEqual(len(output.feature[0].flag_set), 1)
+    self.assertEqual(
+        len(output.feature[0].flag_set[0].expand_if_all_available), 0)
+    self.assertEqual(len(output.feature[0].flag_set[0].flag_group), 2)
+    self.assertEqual(
+        output.feature[0].flag_set[0].flag_group[0].expand_if_all_available,
+        ["foo"])
+    self.assertEqual(
+        output.feature[0].flag_set[0].flag_group[1].expand_if_all_available,
+        ["bar", "foo"])
+
+
+if __name__ == "__main__":
+  unittest.main()
diff --git a/tools/migration/legacy_fields_migrator.py b/tools/migration/legacy_fields_migrator.py
new file mode 100644
index 00000000..10ebacd2
--- /dev/null
+++ b/tools/migration/legacy_fields_migrator.py
@@ -0,0 +1,53 @@
+"""Script migrating legacy CROSSTOOL fields into features.
+
+This script migrates the CROSSTOOL to use only the features to describe C++
+command lines. It is intended to be added as a last step of CROSSTOOL generation
+pipeline. Since it doesn't retain comments, we assume CROSSTOOL owners will want
+to migrate their pipeline manually.
+"""
+
+# Tracking issue: https://github.com/bazelbuild/bazel/issues/5187
+#
+# Since C++ rules team is working on migrating CROSSTOOL from text proto into
+# Starlark, we advise CROSSTOOL owners to wait for the CROSSTOOL -> Starlark
+# migrator before they invest too much time into fixing their pipeline. Tracking
+# issue for the Starlark effort is
+# https://github.com/bazelbuild/bazel/issues/5380.
+
+from absl import app
+from absl import flags
+from google.protobuf import text_format
+from third_party.com.github.bazelbuild.bazel.src.main.protobuf import crosstool_config_pb2
+from tools.migration.legacy_fields_migration_lib import migrate_legacy_fields
+
+flags.DEFINE_string("input", None, "Input CROSSTOOL file to be migrated")
+flags.DEFINE_string("output", None,
+                    "Output path where to write migrated CROSSTOOL.")
+
+
+def main(unused_argv):
+  crosstool = crosstool_config_pb2.CrosstoolRelease()
+
+  input_filename = flags.FLAGS.input
+  output_filename = flags.FLAGS.output
+
+  if not input_filename:
+    raise app.UsageError("ERROR input unspecified")
+  if not output_filename:
+    raise app.UsageError("ERROR output unspecified")
+
+  f = open(input_filename, "r")
+  input_text = f.read()
+  text_format.Merge(input_text, crosstool)
+  f.close()
+
+  output_text = migrate_legacy_fields(crosstool)
+
+  f = open(output_filename, "w")
+  output_text = text_format.MessageToString(crosstool)
+  f.write(output_text)
+  f.close()
+
+
+if __name__ == "__main__":
+  app.run(main)