build_tools/bazel/template_rule.bzl

# Copyright 2020 Google LLC
#
# 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
#
#      https://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.

# Rule for simple expansion of template files. This performs a simple
# search over the template file for the keys in substitutions,
# and replaces them with the corresponding values.
#
# Typical usage:
#   load("//build_tools/bazel:template_rule.bzl", "template_rule")
#   template_rule(
#       name = "ExpandMyTemplate",
#       src = "my.template",
#       out = "my.txt",
#       substitutions = {
#         "$VAR1": "foo",
#         "$VAR2": "bar",
#       }
#   )
#
# Args:
#   name: The name of the rule.
#   template: The template file to expand
#   out: The destination of the expanded file
#   substitutions: A dictionary mapping strings to their substitutions

def template_rule_impl(ctx):
    ctx.actions.expand_template(
        template = ctx.file.src,
        output = ctx.outputs.out,
        substitutions = ctx.attr.substitutions,
    )

template_rule = rule(
    attrs = {
        "src": attr.label(
            mandatory = True,
            allow_single_file = True,
        ),
        "substitutions": attr.string_dict(mandatory = True),
        "out": attr.output(mandatory = True),
    },
    # output_to_genfiles is required for header files.
    output_to_genfiles = True,
    implementation = template_rule_impl,
)