Add toolchain_rule to enable rule authors to define toolchains.

Change-Id: I407240708f4aacc89ec5c00bf3e8ff46a1d6d6d6
PiperOrigin-RevId: 152692981

3 files changed, 66 insertions, 0 deletions

diff --git a/tools/platforms/BUILD b/tools/platforms/BUILD
index 4e788aea2c..a54e5e6ea9 100644
--- a/tools/platforms/BUILD
+++ b/tools/platforms/BUILD
@@ -6,6 +6,7 @@ filegroup(
     name = "package-srcs",
     srcs = [
         "platforms.BUILD",
+        "toolchains.bzl",
     ],
 )
 
diff --git a/tools/platforms/platforms.BUILD b/tools/platforms/platforms.BUILD
index 4e477e72a8..dd12d661f4 100644
--- a/tools/platforms/platforms.BUILD
+++ b/tools/platforms/platforms.BUILD
@@ -4,6 +4,8 @@ package(
     default_visibility = ["//visibility:public"],
 )
 
+export_files = ["toolchains.bzl"]
+
 # These match values in //src/main/java/com/google/build/lib/util:CPU.java
 constraint_setting(name = "cpu")
 
diff --git a/tools/platforms/toolchains.bzl b/tools/platforms/toolchains.bzl
new file mode 100644
index 0000000000..b931905eeb
--- /dev/null
+++ b/tools/platforms/toolchains.bzl
@@ -0,0 +1,63 @@
+# Copyright 2017 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.
+"""Useful functions and rules for defining toolchains."""
+
+_default_toolchain_rule_attrs = {
+    "exec_compatible_with": attr.label_list(
+        providers = [platform_common.ConstraintValueInfo]),
+    "target_compatible_with": attr.label_list(
+        providers = [platform_common.ConstraintValueInfo]),
+}
+
+def default_toolchain_rule_impl(ctx, override_attrs = {}):
+  """A default implementation for toolchain_rule.
+  This implementation creates a toolchain provider and adds all extra
+  attributes.
+
+  Args:
+    ctx: The rule context.
+    override_attrs: Any data in this dict will override the corresponding
+      attribute from the context. toolchain_rule implementations can use this
+      to customize the values set in the provider.
+
+  Returns:
+    The created toolchain provider.
+  """
+  toolchain_data = {}
+
+  # Collect the extra_args from ctx.attrs.
+  attr_names = ctx.attr._extra_attr_names
+  for name in attr_names:
+    if name in override_attrs:
+      attr = override_attrs[name]
+    else:
+      attr = getattr(ctx.attr, name)
+    toolchain_data[name] = attr
+
+  toolchain = platform_common.toolchain(
+      exec_compatible_with = ctx.attr.exec_compatible_with,
+      target_compatible_with = ctx.attr.target_compatible_with,
+      **toolchain_data)
+
+  return [toolchain]
+
+def toolchain_rule(implementation = default_toolchain_rule_impl, fragments = [], extra_attrs = {}):
+  return rule(
+      implementation = implementation,
+      attrs = _default_toolchain_rule_attrs + extra_attrs + {
+          # default_toolchain_rule_impl needs this to know what attributes are extra args.
+          "_extra_attr_names": attr.string_list(default = extra_attrs.keys()),
+      },
+      fragments = fragments,
+  )